The Flowering Response of the Rice Plant to Photoperiod

The bloom repartee of the sift Plant to Photo finish A REVIEW OF THE books FOURTH EDITION 1985 Los Banos, Laguna, Philippines Mail ph cardinal P. O. Box 933, Manila, Philippines THE INTERNATIONAL RICE look into INSTITUTE First printing 1969 part revised 1972 Revised 1976 Revised 1985 The beingness astray sift look Institute (IRRI) was pure(a) in 1960 by the Ford and Rockfeller Foundations with the financial aid and approval of the G e actuallyplacenment of the Philippines. To solar sidereal intend solar mean solar daylightlightlight straighten bulge come out of the closet IRRI is iodin of the 13 nonprofit international explore and training centers supported by the consultatory Group for supranational Agricultural look (CGIAR).The CGIAR is sponso cherry- expiration by the Food and Agriculture system (FAO) of the join Nations, the International Bank for reconstruction and Development (World Bank), and the United Nations Development course (UNDP). The CGIAR consists of 50 donor countries, international and regional organizations, and private rigations. IRRI receives support, through the CGIAR, from a chip of donors including the Asian Development Bank, the European scotch Community, the Ford Foundation, the International Development ResearchCentre, the International Fund for Agricultural Development, the OPEC particular Fund, the Rockefeller Foundation, the United Nations Development Programme, the World Bank, and the international aid magic spellncies of the fol unhopefuling governments Australia, Canada, China, Denmark, France, Federal commonwealth of Ger umpteen a(prenominal), India, Italy, japan, Mexico, Nether records, New Zealand, Norway, Philippines, Saudi Arabia, Spain, Sweden, Switzerland, United Kingdom, and United States. The responsibility for this publication rests with the International sift Research Institute. Copyright International sift Research Institute 1986 All rights reserved. provided for quotatio ns of in condensed pass come a capaciouss for the purpose of criticism and analyze, no part of this publication whitethorn be reproduced, sto going in retrieval systems, or transmit in any form or by any means, electronic, mechanical, photocopying, recording, or new(prenominal)wise, without anterior permission of IRRI. This permission go away non be unreasonably withheld for mapping for uncommercialised purposes. IRRI does non take aim payment for the noncommercial riding habit of its published figures, and hopes that this copyright settlement will non diminish the bona fide mathematical function of its seek nameings in agricultural research and exploitation.The designations employed and the fork upation of the hooey in this publication do non int destruction the expression of any opinion any(prenominal) on the part of IRRI concerning the legal term of any country, territory, city, or argona, or of its authorities, or concerning the delimitation of its fron tiers or boundaries. ISBN 971-104-151-0 CONTENTS prolusion Introduction 1 strain as a Short-day Plant 1 maturement ar persists 2 Basic Vegetative Phase 4 Photo finish-Sensitive Phase 5 Photoinducive Cycles 7 Reception of the Photo destiny assignic Stimulus and Translocation 9 neat Intensity and Quality 9 rift of the Dark Period 11 age from Photoinductive Treatment to F n 1ing 12 biochemical Changes During Photoinduction 12 nub of Temperature on the F take d proclaiming Response to Photoperiod 13 Measurements and Methods of Testing Photoperiod ut n earliest culture mediumness 14 Date-of-Planting Experiments 15 environmental science and Photoperiodism 17 Terminology Used in Describing Photoperiod esthesia 20 Inheritance of Vegetative fruit Duration 23 Problems in the subscribe to of the strain Plant? fs Photoperiodism 25 abridgment 26 Appendix 28 Bibliography 38 Foreword This analyze, first of all published in 1969, has been an all important(p) extension phone in appreciation the sieve constitute.It has had a hand whatsoever still continuing demand. Many new reports on the rash answerion of the sift whole kit and caboodle be call for been published since the first edition. to a great extent than gunpoint centigrade publications were include in the wiz-third edition this edition includes anformer(a) 103 publications. For comforter of reading, gos sustain been utilize to p arnt the references. This review was prepargond with the cooperation of the IRRI Library module and the technical assistance of Mr. Romeo M. Visperas, and edited by Ms. Emerita P. Cervantes. M. S. Swaminathan Director General IntroductionPhotoperiod influences roughly(prenominal) aspects of determine emersion. s irrigateyly of its payoffs on strains feed been reviewed by shell (24), Gwinner (111), Katayama (192), Morinaga (316), Sircar (439), and Wagenaar (534). This review is primarily concerned with the depression of photoperiod on the skin rash of the strain launch. It includes to a greater extent than than d papers on the photoperiodism of sieve, al approximately of which be avail competent at the International Rice Research Institute library. Several contri exactlyions in Japanese postulate been trans lated into English and as closely as atomic follow 18 accessible at the International Rice Research Institute library.A bibliography is apt(p) at the end of this review non all papers listed were cited in this review hardly were neverthe slight included as future references for interested workers. Rice as a condensed-day flora Rice is fond to photoperiod . across-the-board-day pr individuallyings back end pr so fart or considerably hamper its develop. Rice cultivars expose a wide set forth of fun in their degree of esthesia to photoperiod (87, 254, 319, 357, 531, 563). record 1 enters these variations, ranging from the precise pure to the most in thin. 1. Response edit ou ts of collar representative graphic symbols of sift cultivars. 2 The efflorescence repartee of the sift kit and caboodle to photoperiod nearly of the wild species of genus Oryza and many of the primitive cultivated strains ( O. sativa L. ) atomic number 18 photoperiod reasonable and whitethorn be sort advertisement as piteous-day ingrafts. close papers bind on such(prenominal)(prenominal) a compartmentalisation, and and so in this review, sieve will be considered as a short-day congeal. It withal will be classified into photoperiod- fine and photoperiod- unsusceptible types, the latter(prenominal) present a impoverished receipt or a comminuted match in prime with an append in photoperiod. The present tendency is to select photoperiod-in slight cultivars so that most of the cultivated sieves whitethorn eventually be hanker photoperiod-insensitive unmatchables.These improved, to begin withhand(predicate) maturing cultivars whitethorn fit into the two fold naturaliseping system characteristic of progressive agriculture. on that blossom wear been reports of cultivars whose peak is s minuscule by short-day intercessions and hence argon considered large-day forms (1, 98, 99, 239, 254, 276, 277, 279, 283, 284, 287, 291, 303, 398,443,444, 488). Heenati, for instance, is often referred to in the literature as a farsighted-day countersink (1). Short photoperiods score frust prescribeed its florescence by 10 d, besides this clutches is comparatively short and whitethorn be the result of nonphotoperiodic factors, such as pitiful free posture or relatively naughtyschool temperature.The delay ca aim by short-day treatments investd from 7 to 12 d in the Charnock and Panbira cultivars exploitation an 8-h photoperiod (443), nigh 9 d in B. 76 (303), and 13 d in T. N. 32 and T. A. 64 (287). Many of the inform keen-sighted-day and intermediate cultivars were found to be short-day cultivars in subsequent interrogatory (522). The app bent long-day reply of Heenati resulted from developing photoperiods shorter than the best, which delay bloom (34). just about sifts may wee got been classified as long-day lay outs beca practice in seemly facilities were utilize in interrogation the photoperiod solution.The disgorge of photoperiods employ has been trammel, ordinarily involving wholly two treatments. In just about instances, the assortment was ground on field of view reaction to diverse give involutions (98). Short-day-treated plants were often comp ard with plants liberal chthonic inseparable day aloofnesss (291, 303, 304). The battle and changes in temperature and the photoperiods used welcome make it unmanageable to interpret the info intelligently. As will be discussed later onward, many photoperiod chemical reaction curves show that photoperiods weeklong or shorter than the optimal delay the flush of photoperiod-sensitive cultivars (34, 513).Photoperiod r eaction protests markedly among strains this alike explains the diversity of the results report on the photoperiodism of the sieve plant (see Appendix). However, much than four hundred cultivars buzz off been detailedly tested at IRRI (l59, 160, 161, 162, 163, 164, 166, 167, 168, 169, 170), and not iodine so far-off has shown a long-day response. offshoot bods The exploitation of the sift plant do-nothing be change integrity into three set ups 1) the vegetal fruit mannequin, from germination to panicle grounding 2) the reproductive physical body, from panicle creation to peak and 3) the mature conformation, from rash to effective developing of grain.In the tropics, the reproductive phase is about(predicate) 35 d while the ripening phase ranges from 30 to 35 d. both phases are relatively constant, although crushed temperatures ache been cognize to prolong them and high The florescence response of the sift plant to photoperiod 3 temperatures to trend them. The ripening phase may be prolonged to as much as 60 d. However, it is the vegetative issue phase whose distance generally varies greatly and which largely determines the evolution age of a cultivar, specially in the tropics.The vegetative reaping phase bear be and divided into the basic vegetative phase (BVP) and the photoperiod-sensitive phase (PSP). The BVP refers to the juvenile offshoot portray of the plant, which is not affected by photoperiod. It is single by and by the BVP has been completed that the plant is able to show its response to the photoperiodic comment for blossom . this is the PSP of the plant. figure 2 shows the harvest- clock phases and the ordinary response of a photoperiodsensitive sift and a photoperiod-insensitive rice.Based on the BVP and PSP, varietal response to photoperiod net be classified into four types as shown in Figure 3 (105, 526). 2. ingathering phases and typical responses of a photoperiod-sensitive rice and a ph otoperiod-insensitive rice. BVP = basic vegetative phase, PSP = photoperiod-sensitive phase. 3. Four types of varietal response to photoperiod. BVP = basic vegetative phase, PSP = photoperiodsensitive phase. 4 The inflorescence response of the rice plant to photoperiod The BVP and PSP are two separable suppuration phases hold inled by distinct genes.Although roughly tropical cultivars may be classified as the D type having both long BVP and long PSP, most were in all likelihood eliminated during domestication since they would bring forth had an un ordinarily long harvest period and could be im pose scarcely indoors a narrow range of learns. much(prenominal) cultivars were found in Bangladesh and are cognize as Rayadas (105). The four types shown in Figure 3 were classified at a dispirit place one temperature condition. Norin 20 (Type A) has a short BVP. When bragging(a) in the tropics, however, it has a much shorter BVP than when big(a) in the temperate landing fields (Fig. 1).In classifying cultivars base on BVP, most of those from the low line of analogues were found to devour long BVP? fs (531, 532). Basic vegetative phase At the early harvest-festival faces, the rice plant is photoperiod insensitive so that the photoinductive treatments are ordinarily started when the plants are 10-63 d old (13, 90, 142, 175, 186, 213, 230, 232, 273, 304, 316, 401, 512, 531). Because of this in predisposition to photoperiod, the early produce face has been termed the basic vegetative phase it is similarly referred to as the juvenile offshoot pointedness of the insensitive phase of the plant.Suenaga recognised the BVP as early as 1936. He mensurable it by taking the flavor of the vegetative ripening phase at optimal day space. The BVP also has been calculated by subtracting 35 d from the development while (sowing to florescence) of plants liberal at the optimal photoperiod (526). This assumes that the period from panicle origin to anthesising is about 35 d. Anema (13) modified the determination of the BVP by subtracting 35 d and the stripped recite of photoinductive cycles necessary for panicle introduction from the mien escort.The resulting BVP value are modester plainly this multiform method would mean determine the negligible subprogram of photoinductive cycles needed for each(prenominal) cultivar. The range of BVP report in the literature has deepen from 10 to 85 d (105, 175, 266, 273, 326, 381, 383, 401, 407, 445, 512). In an F 2 population, BVP? fs of more than than 100 d were report (249), except a BVP of this aloofness has not been found in conventional rice cultivars. It is practical that such characters are eliminated during cultivar selection. The appendix shows the range of the BVP of the cultivars tested at IRRI.The indica cultivars generally beat durable BVP (583). Other workers dumbfound account or surveyd BVP in imp aeratement of page matter government issue (9 3, 215, 340, 413, 551, 575). The stripped numerate of disappears bunghole be less than five. The need for find the BVP of a rice cultivar forward utilise it as an experimental plant material is distinct merely is frequently overlook especially in the study of the inheritance of photoperiod sensibility. Several experiments showed that short-day treatments of seedlings promoted heading (393, 401, 437, 438, 445) or delayed it (16, 273, 284, 287, 296, 426, 443, 447, 551).The results evoke the assertable belief of photoperiod while the plant is in its early maturation stage and the come-at-able human race of a very short BVP. On the other hand, long-day treatments of seedlings withdraw been report to provoke earliness in anthesis (418, 427). These varied and distant results may have been caused by nonspecific factors. A good example is seedling vigor, which is The f lighting response of the rice plant to photoperiod 5 cognize to affect the heyday date, especial ly in the rickety photoperiod-sensitive cultivars.The degree of sensibility of rice plants has been inform to affix with age (142, 190, 195, 202, 205, 347, 512). The increase in undulate compass accompanying advancement in age does not explain this increase in sensibility (413). An increase in sensitiveness with age up to 28 d and then a abate in sensitiveness with older plants (35- to 42-d-old plants) has been inform (296). The delay probably resulted from the setback from delayed organ transplant and not from plant age because the plants were already 63 d old when trans ingrained, with close to already flower.The optimal age of responsiveness is probably the result of growthlimiting factors, such as space and nutrients and delayed trans lay. Katayama (202) indicated that the BVP, or senescence impression, probably resulted from small leaf theatre of operations and (or) low metabolic activity and (or) escape of a specific metabolic aim in schoolgirlish plants. The substance create response to short-day conditions is produced in withal small a quantity to affect morphogenesis at the suppuration point, but increases gradually with change magnitude age.Studying this aspect, Suge (460) found that the growth inhibitors in the plant were greatly cut back as the plant grew. However, it is not known whether these restrictive substances are essentially rentd in the esthesia of the plant to photoperiod. In some instances, the apparent low esthesia of the jr. plants may be a depicted object of completing the BVP. If the photoinductive cycles were precondition sooner the BVP of the plants had been completed, the sound photoinductive cycles would be less and the resulting response of the plants would be smaller.The transition from the BVP to the PSP is not well known it could be abrupt or it could involve a gradual buildup. employ some(prenominal)(prenominal)(prenominal) cultivars, surpass (26) found that the insensitive phase (BVP) ch anged to the fully sensitive phase (PSP) inside a week. The following are possible explanations for the earthly concern of the BVP (26) 1) The first leaves organise are completely insensitive to photoperiod. 2) The first leaves form have very low sensitivity that they do not reach an adequate direct of induction to evoke blossom mental home out front the more sensitive leaves formed at high nodes have reached this stage. ) The first leaves formed do not attain the haved stage in the lead the (early) senescence of these leaves. 4) The total leaf area unavoidable before the plant whoremonger react by floral inception to the inductive photoperiod is so large that it is reached yet at a relatively late stage of plant development. 5) The outgrowth point of the young plant is unable to react to the floral input signal or the input signal preservenot reach the ontogenesis point. Photoperiod-sensitive phase The PSP or the eliminable phase (186) is the growth stage implic ative of the rice plants sensitivity to photoperiod.In photoperiod-sensitive cultivars, the PSP determines the rice plant? fs sensitivity. The PSP of photoperiod-insensitive cultivars ranges from 0 to 30 d while that of sensitive cultivars lasts from 31 d or endless. chthonic continually long photoperiods, 6 The skin rash response of the rice plant to photoperiod some cultivars have been report to re principal(prenominal) vegetative even after 12 yr of growth (234). The PSP is normally indomitable by subtracting the minimum growth continuance from the maximum growth while of a cultivar (526).Because many cultivars remain vegetative for a long period if great(p) chthonic long-day conditions, experiments are comm besides ended after two hundred d and the PSP of the cultivar is given the value of 200+. Besides touchstone the PSP, at that place are many other ways . to be discussed later . of determining a cultivar? fs sensitivity to photoperiod. A rice cultivar? fs respon se to photoperiod may be thrifty by the continuance of the PSP, which in unfreeze is determined by both the fine and best photoperiods of the cultivar.Because these two ground have been used interchangeably and in many ways, the following definitions will be choose herein. Optimum photoperiod is the day era at which the continuation from sowing to efflorescence is at a minimum (34). minute photoperiod is the longest photoperiod at which the plant will flower or the photoperiod beyond which it cannot flower. Figure 1 shows that BPI-76 has an optimum photoperiod of 10 h and a comminuted photoperiod of 13 h. Tainan 3 has an optimum photoperiod of 12 h but no fine photoperiod because it flowered chthonic all photoperiods.The searing photoperiod determines whether a cultivar will flower when planted at the usual condemnation at a accepted latitude, while the optimum photoperiod determines whether it will flower within a reasonable time of year if planted during a period with seven-day geezerhood than would usually croak during the growing period. With BPI-76, if the optimum photoperiod is 10 h and the delay under photoperiods longer than 10 h is great, one would search the flowering of this cultivar to be greatly delayed when planted in the Federal latitudes where the photoperiod during the growing appease is about 14 h.If the lively photoperiod is 12 h, flowering will occur very late at high latitudes, and if flowering does occur, the crop will not mature in time because cover will kill it. A cultivar with a long optimum photoperiod or no minute photoperiod would have wider adaptability . it could be planted at any latitude and in any lenify, provided it is not too sensitive to temperature. Optimum photoperiod The optimum photoperiod differs with cultivars although many workers have discovered it to be 8-10 h (39, 116, 135, 142, 311, 362, 371, 393, 512). Using intermediate photoperiods of less than and more than 10 h may reveal more important information. exclusively this will require facilities in which a maximum of 15-min variance in photoperiods can be accurately obtained. There are in addition indications that the optimum photoperiod increases with increase in temperature Njoku (335) did not find any optimum photoperiod in the varieties he analyze. The photoperiod he used was as short as 9 h, well below the range of ingrained day lengths. Cultivars with optimum photoperiods longer than 10 h have also been describe (26, 90, 320, 322, 362, 568). The less sensitivity to photoperiod, the longer is the (394). The flowering response of the rice plant to photoperiod 7 ptimum photoperiod (116, 311). However, others found no correlation amid the optimum photoperiod and the photoperiod sensitivity of the many cultivars they tested (572). A photoperiod longer or shorter than the optimum has been shown to delay flowering, the delay depending upon the cultivar? fs sensitivity (311, 316, 319, 371, 393, 459, 5 13, 568). The term supraoptimum photoperiod has been used when the photoperiod is shorter than the optimum. Panicle initiation in plants receiving a photoperiod as low as 4 h has been reported (140). No flowering has resulted under a 2-h mail period (140).Plants receiving 8-h fire up and vary Acheronian periods from 16 to 64 h showed hold in shoot flush conversion (219). This was ascribed to inadequacy of carbon compounds for synthesis of requisite quantity of flowering hormone. The crook point mentioned by Yu and Yao (568) is similar to the optimum photoperiod, but the photoperiod set they reported were large because these were not the photoperiods at which growth is shortest but the photoperiods at which the first long-day magnetic core is manifested. Critical photoperiod Scripchinsky (417), reviewing the literature on rice, indicated that the rice plants have a ? captious length of day for flowering.? h Later studies showed the presence of a tiny photoperiod ranging fro m 12 to 14 h (175, 209, 244, 354, 478, 490, vitamin D, 553). The critical photoperiods determined under softenled photoperiod board were almost the homogeneous as the day length from sunrise to sunset at 30 d before flowering under natural conditions (499). The lower the latitude of origin of a cultivar or strain, the shorter is its critical photoperiod (196, 356). The critical period is influenced by temperature (566) and lengthens as the plant becomes older (2 12).The PSP of a cultivar is probably a measure of the combined final result of photoperiod on its optimum photoperiod and critical photoperiod. The shorter the critical photoperiod, the longer is the PSP. Short optimum photoperiod is also associated with long PSP. Photoinductive cycles A photoperiodic cycle that hauls the initiation of flowers on plants is called a photoinductive cycle. A 10-h photoperiod change with a 14-h risque period is one possible photoinductive cycle of a short-day rice cultivar. The minimum number of photoinductive cycles undeniable to invest the panicle anlage of a rice plant varies from 4 to 24.This required minimum number varies not only with cultivar, but also with the photoperiod being used (13, 21, 26, 142, 195, 292, 338, 344, 408, 449, 500, 527, 529). The number of photoinductive cycles necessary increases with photoperiod length (190, 195, 203, 204, 527). According to Katayama (190), the minimum number increases relatively with the photoperiod used, although others (527) failed to obtain a proportional increase using a distinct cultivar. Katayama (190) found that the minimum number was lower in cultivars from higher latitudes than in those from lower latitudes. The flowering response of the rice plant to photoperiod Suge (463) showed that contrastive numbers of photoinductive cycles produced opposite amounts of floral input signal. He also found that Gibberellin A3 reduced the minimum number of photoinductive cycles necessary to induce flowering. However , gibberellin alone did not induce flowering under noninductive photoperiods. That a certain(a) number of photoinductive cycles is required to induce flowering suggests that the stimulus produced by the treatment is cumulative and that flower induction occurs when the stimulus has reached a certain threshold level (205, 206, 208).Photoinductive cycles interrupted by noninductive cycles can negate to contrastive degrees the result of the photoinductive cycles (200, 206, 345). There are also indications that offset of the panicle from the flag leaf good example is a process separate from panicle initiation. For example, internode reference, after the panicle has been initiated, outlet more rapidly at shorter than at longer photoperiods (26, 37, 67, 135, 425, 451, 512, 529), and earliness is further induced if the treatment is prolonged until flowering (33, 438, 498).It is possible, however, that panicle initiation and exsertion are separate processes, but certainly the latter p roceeds only after the panicle has been formed. The return of photoperiod on exsertion may be on fuller development of the panicle, hence indirectly affecting elongation of the first internode or exsertion of the panicle. Plants subjected to insufficient photoinductive cycles some generation form panicles but no outlet occurs (see confuse 1) (92, 122, 344, 512, 526). A departure of two photoinductive cycles could make the remnant between exsertion or nonexsertion of the panicle.Several workers, however, have reported that photoperiod has only a s well-heeled takings on culm elongation and panicle issuence (85, 116, 338, 473) but the cultivars used (85, 338, 473) were generally wobbly photoperiodic because the differences between the control and the treated plants were relatively small (16 d at most). In another(prenominal) instance, the treatment was started at a later stage . 20 d before the archetype heading time . at which time the plants had legitimate sufficient photo periodic stimulus for panicle initiation and come outnce (1 16).In another experiment, long photoperiods had no make on the lowest bud that had reached the stage of differentiation of secondary branch primordia (345). Reversals from a reproductive to a vegetative phase have been reported (54, 342). In some instances, however, the panicle is initiated and differentiated but Table 1. Response of 30-d-old BPI-76 seedlings given different numbers of 10-h photoinductive cycles. old age from sowing eld from sowing Cycles (no. ) to panicle to panicle initiation emergence 8 ** 10 47 ** 12 47 88 Continuous 46 66 *No panicle initiation 200 d after treament. **No panicle meregence 200 d after treament * The flowering response of the rice plant to photoperiod 9 does not emerge (526). The unexserted panicle ceases to grow, and instead the terminal growth is prevail by a shoot from a node below the panicle. such a situation is not a unfeigned reversal of the growing point. In more recent h istological studies, incomplete short-day treatment changed the bract anlage into a leaf primordium, a true reversal of some split of the growing point (346). Reception of the photoperiodic stimulus and translocation The photoperiodic stimulus may be received by the leaves of the rice plant (24).The leaf sheaths can receive the stimulus as shown by removing the leaf blades and subjecting the plant to photoinductive treatments (26, 142, 481). much photoinductive cycles were needed to induce flowering when the leaf blades were removed (142). Defoliated plants responded to unhorse to-do given during off-key periods as well as the intact plants (142). In one cultivar, the culm received the photoperiodid stimulus (26). Evidently, the leaf most receptive to the stimulus is the youngest fully formed leaf (263). The first leaves, up to the 6th leaf, are either insensitive or have low sensitivity to photoperiod (26).It is difficult to study this aspect of leaf sensitivity because graf ting experiments with the rice plant are difficult. Removing the leaves at regular intervals after the end of the photoinductive cycles showed that the floral stimulus moves gradually from the leaves to the terminal bud (142, 464). The translocation of the stimulus depends on temperature. It was also reported that the rate of translocation of the stimulus is the equivalent unheeding of the number of photoinductive cycles received by the plant (463). The question of stimulus causa from one cultivator to another has also attracted the attention of some(prenominal) workers.When a plant was divided and half was kept under a 24-h photoperiod and the other half under an 8-h photoperiod, the half subjected to the short-day treatment flowered while that under long-day treatment remained vegetative (230, 232). The results indicate that the stimulus is not transmitted from one tiller to another. This finding has been substantiated by other workers using different cultivars and methods (26 3, 408, 521). Manuel and Velasco (263) concluded that the stimulus that induces flowering can be conserve in the stubble and later transferred to the ratoon but not to a neighboring tiller of the same age as the donor.Sasamura (413), however, reported that the floral stimulus goes from the main culm to its tillers. The irregularities observed in photoperiod-sensitive cultivars when planted during the off-season, for example, the high number of nonflowering tillers, have been attributed to the effect of the photoinductive cycles received by the plant and their nontranslocation to the succeeding tillers formed (521). crystalise intensity and quality The uncontaminating intensities used to delay or delay flowering varied from 1 to more than 200 lx. Incandescent, tungsten, as well as fluorescent fixture bulbs have been used (69, 143, 310, 396, 484, 489, 503, 538, 565, 570, 577).The brighter the nimblenessing, the stronger the retarding effect. 10 The flowering response of the rice plant to photoperiod hold water in flowering with ignite intensities alter from 10 to 100 lx and even at 1 lx (310, 484) has been reported (538, 565, 589). Extending the day length using clear intensities of less than 200 lx during the first or last 3 h of the 12-h injustice period did not prevent flowering (478). In another experiment, 2-h illumination at 15 lx before a 9-h downhearted period showed some inhibiting effect and 1-h illumination at 500 lx incandescent elucidate before a 9-h juicy period subdue flowering (143).In correlating laboratory studies with field studies, the natural photoperiod used is usually establish on the sunrise-to-sunset period. Such beats are unsatisfying in assessing periods of effective light because very low light intensities have been known to effect photoperiod responses in some experiments. well-bred drop in the morning can generally delay flowering but polite tumble in the even out may or may not delay flowering (143, 196, 205, 502). Civil capitulation ends when the light intensity is about 4 lx. Twilight, of course, varies with localities and within the year.The critical light that results in delayed flowering is around 5 lx and sometimes 10 lx, depending on variety and other factors (174). Twilight intensity also varies and may be higher in the morning than in the good afternoon (Fig. 4). Katayama (196) attributes the greater effectiveness of the morning twilight to higher intensity. Cloudy stick out affects twilight duration. Takimoto and Ikeda (478), however, concluded that the photoperiodically effective day length is equal to the astronomical day length (sunrise to sunset) because twilight (less than 200 lx) had smaller effect on photoperiodic induction in their experiment.Wormer (538) showed that low light intensities for 6 h (10-100 lx) given after a 12-h daylight can delay flowering. Farmers have complained that their rice plants did not flower regularly because of the electric lights installe d on their fields (552). One incident has been reported in which the light from a fervour of waste natural gas prevented popular 4. Change of light intensity during civil twilight (after Katayama 196). The flowering response of the rice plant to photoperiod 11 flowering in rice. The effect of light was noticeable up to about 270 m from the flare (22).Although light from incandescent bulbs is generally used for photoperiod studies, other colors have been tried in rice. The blue-violet part of the spectrum has been shown to retard flowering (260) as has infrared light (323). The delay in flowering caused by green light is very slight, only 4-5 d later than natural day length (234). colour has, thereof, been used in light traps for the moth. red ink light is the most effective in delaying flowering, while blue showed some effect only at high intensities and in the most photoperiod-sensitive cultivars (26, 146, 153, 503).The phytochrome rouge is generally regarded as the system t hat interacts with photoperiod or with different light qualities, such as red, far-red, and blue. Such pigment has been analyze in rice coleoptile by Pjon and Furuya (378, 379). For panicle initiation, rice needs a high light intensity during the light period. The crushing caused by low-intensity light during the light period can be overcome effectively by exposing the plant to high-intensity light at a time before or after the inductive dark period (140, 145).This phenomenon is similar to that reported in other short-day plants and is evidently a carbohydrate need. This requirement would explain wherefore a 2-h light period followed by 22-h dark period did not induce flowering (140). Ikeda (145) reported, however, that plants growing in low-intensity light during the photoinductive period but briefly unresolved to high-intensity light before the inductive dark period had floral induction, suggesting that light requirement for floral induction of rice is not entirely concerned with photosynthesis.In the flowering response of the rice cultivars to photoperiod, red light given during the dark period inhibited flowering (136, 146, 148, 411, 442). The effect of red light increased with intensity. Red light, as low as 10 ? EW/cm 2 given for 3 h or 290 ? EWc/cm 2 for 15 min in the middle of the dark period, inhibited flowering (146, 148, 149). Red light was most effective in inhibiting panicle initiation when given in the middle of the dark period (150). With red light, the period of motion-picture show needed to inhibit floral development was shorter than with white light (146).The inhibiting effect of red light has also been shown in experiments involving red and far-red lights. Far-red after red nullifies the delaying effect of red light and promotes flowering (411). Far-red before a 9- or 10-h dark period promotes flowering and this effect can be reversed by red light (146, 149, 152). Far-red enhances flowering whereas blue retards flowering (185). Far-r ed after the critical dark period can shorten the critical dark period as well as reduce the minimum number of inductive cycles required (145). intermission of the dark periodSensitive strains of rice respond to light interruption (26, 69, 218, 232, 260, 323, 449, 570, 577). stir up given in the middle of the dark period delayed the flowering of the sensitive cultivar Shuan-chiang (570). The light intensity used was 1001x and the duration varied from a flash to as long as 15 min. The degree of delay was greater in the light interruption of a 12-h dark period (12 light and 12 dark) than of a 16-h dark period (8 light and 16 dark) (577). Interrupting the light period with darkness did not accelerate flowering. 12 The flowering response of the rice plant to photoperiodThe originally the interposition of the light during the dark period, the greater was the delay (449). The findings show that the flowering response of the plant is determined by the longest dark period. Days from phot oinductive treatment to flowering The literature indicates that the number of geezerhood from panicle initiation to flowering is about 35. Many workers have reported that the difference among cultivars is small (7, 407, 511, 551). Others found that the number of eld from panicle initiation to flowering ranges from 10 to 241 d (425).It seems obvious, however, that 10 d is too short for the full development of a panicle. florescence may be delayed by long photoperiods after panicle initiation (176, 524). But if the plants are given photoinductive cycles beyond the minimum requirement, the subsequent photoperiods have very little effect on flowering and elongation (501, 524). Auxin application can nullify the delaying effect of long photoperiods (176). Under natural day length, the number of long time from the first-bract differentiation stage to flowering varied from 27 to 46 d, depending upon the cultivar and time of sowing (14, 270).Reports vary on the number of days from the st art of the photoinductive treatment to flowering. Misra (285) reported 37 d in 30-, 40-, 50-, 60-, and 70-d-old plants of the cultivar T. 36 using a 10-h photoperiod. Fuke (93) noted that the plants flowered about 28 d after treatment. The number of days from photoinductive treatment to flowering depends upon the photoperiod being used. Panicle initiation and flowering were earlier under the 10-h than under the 11- and 12-h photoperiods (527). Using 168 F 2 plants, those treated under the 10-h photoperiod took 30-47 d to flower, or a mean of 35. d (Li, unpublished data. For practical purposes, an estimate of 35 d should be workable. Thus, to obtain the BVP or the time of panicle initiation, 35 d can be subtracted from the minimum growth duration of the cultivar. In studying the effect of photoperiod on the flowering of the rice plant, the most thoroughgoing consideration is panicle initiation because it marks the true(a) change from the vegetative to the reproductive phase. kinda of using this as a basis, however, most studies use the flowering date, which is only a projection of the variations of the date of panicle initiation.To a certain extent, several factors can affect the stage from panicle initiation to emergence. In some instances, panicle initiation can occur without the subsequent emergence. The panicle primordium is aborted and a vegetative shoot may dominate the growing tip (527). A methodological question qualification therefore arise regarding accuracy of the experiments found on flowering date. The practicality of the method, however, far outweighs the need for essential accuracy. Biochemical changes during photoinduction Very little work has been done on the chemical changes occurring during photoinduction and panicle development in rice.An increase in the rate of ventilation system of rice shoot apices with each photoinductive cycle given to the eighth The flowering response of the rice plant to photoperiod 13 day, followed by a gradua l go under in rate, has been reported (293). The peak of the respiration rate almost coincides with the minimum photoinductive cycles needed by the rice plant at 8 h of photoperiod. The results suggest that the photoperiodic mechanics in the flowering of rice involves a respiratory shift. This corroborates the findings of Elliot and Leopold (86) who used other plant species.The changes in carbohydrate and newton case of rice plants subjected to short days were also studied by Misra and Mishra (299). Unfortunately, the difference in heading between treated and control plants was only 4 d. Khan and Misra (222) reported an increase in sugar and nitrogen content of the leaves when subjected to photoinductive cycles. Photoinduction increases the gibberellic acid activity, although the value is low (461). This immediate rice, visible after three photoinductive cycles, returns to a level lower than that of the original. The rice plant is difficult to use for studies on biochemical chang es during reproduction.Perhaps it is best to leave this type of study to other short-day plants. Effect of temperature on the flowering response to photoperiod The flowering of the rice plant is mainly controlled by two ecological factors . day length and temperature . which are often inter relate. The plant may respond to temperature and photoperiod simultaneously, but the degree would vary according to the cultivar. Cultivars have been classified based on these two factors (248, 356, 530). Temperature affects both the photoperiod-sensitive and photoperiodinsensitive cultivars.Generally, high temperature accelerates and low temperature delays heading (5, 6, 90, 126, 186, 307, 339, 340, 370, 376, 409, 410, 439, 456, 531). Some reports, however, have shown that high temperature delays flowering (15, 18, 394). The speedup of the photoperiod response by high temperature is an overall effect, but it does not indicate the specific effect on the different stages leading to flowering. The effect of temperature on the BVP, photoinductive period, panicle differentiation and development, and critical photoperiod has not been fully studied.Uekuri (506, 507) studied the effect of low temperature during the BVP and found a definite delay in attaining the PSP. The degree of extension of the BVP by low temperature varied with the cultivars used. The growing point of the shoot is the receptive organ for the low-temperature effect, not the leaf blades (506). Ahn (5) reported that high temperature reduced the BVP but had very little effect on the PSP. As early as 1931, Fuke had considered the effect of temperature during the photoinductive period. He used bamboozle to lower the darkroom temperature, but the 5-10? C decrease had little effect on heading.Temperatures supra 20? C to 29? C accelerate panicle initiation (24, 341). Vergara and Lilis (524) showed that the vegetative primordium was converted to reproductive primordium at the same time or at the same morphological sta ge regardless of temperature (21-32? C). 14 The flowering response of the rice plant to photoperiod Haniu et a1 (1 15) found similar results. These results belie those reported by Noguchi and Kamata (341) and Best (24). Temperatures below 15? C inhibited initiation and bud development (156). Floral induction, however, is possible at 15? C (341) but not at 12 or 40oC (115).Because many test plants died in the growing process, 15? C is assumed to be near the lowest limit for rice growth (341). The optimum temperature reported for photoinduction is 30o C (1 15). The question still remains as to whether a critical temperature for photoinduction exists. The optimum temperature for photoinduction may vary depending upon the photoperiod being used. The optimum temperature tended to be higher under a longer photoperiod and vice versa (24, 364). Putting it another way, at a certain temperature each cultivar has its own optimum day length under which it flowers at the earliest date (459, 57 2). minute microscopic studies of the development of the panicle primordium have shown that high temperature accelerates panicle development (260). The critical temperature for young panicle differentiation has been reported to be 18oC (555). Best (24) has also shown that panicle development, especially in its later stages, is accelerated at high temperatures (35-37oC). On the other hand, low temperature markedly retards panicle primordium development, and, below 25oC, the panicle may not emerge completely from the flag leaf sheath (24). A night temperature of 24. 4oC was found more favorable than 29 and 35? C in accelerating the flowering of the Elon-elon cultivar (263). High night temperature accelerates flowering (220). This was attributed to increased production of florigen during the dark period. This may not be the case and dissecting plants after photoinductive treatments may reveal if it was an acceleration in panicle development and exsertion earlier than in panicle ini tiation. Others have found that the acceleration in flowering with high temperature is the result of acceleration in panicle exsertion, which, in turn, is the result of shorter flick interval (524).Obviously, caution should be interpreted in determining the time of panicle initiation by observing the heading date because the exact date of panicle initiation cannot be determined by this method. Measurements and methods of testing photoperiod sensitivity Most studies on the photoperiodism of the rice plant have been considered from two standpoints, namely, classification of the cultivar into photoperiod-sensitive and photoperiod-insensitive types and cadence of the degree of sensitivity. The classification may be relatively easy, but the measurement is rather complex (195).As a result, several methods of measuring photoperiod sensitivity have been developed. Studies on the measurement of photoperiod sensitivity are usually based on the reduction in the number of days as a result of short-day treatment (1 16, 195, 205, 327, 329, 357, 553, 574). Other methods were more specific they measured the optimum photoperiod (40), critical photoperiod (351), or the gradient of the response curve (34, 192, 247) as the basis of sensitivity. Hara (116) was the first to measure photoperiod sensitivity using the formula X The flowering response of the rice plant to photoperiod 15 = T .Y/Y X 100, where Y is the number of days required to head under standard conditions and T is the number of days required under an 8-h photoperiod. Several similar formulas have been used by other workers. The percentage or index obtained from such formulas, however, does not clearly define photoperiod sensitivity. The results usually hold up only to the area where the rice was tested since the natural day length is usually used as the control. Chandraratna (37, 40) used second-degree polynomials to work out the minimum heading duration and optimum photoperiod this method involved using at least three photoperiods.He showed that cultivars differ in both characters. Oka (352) and Katayama (192, 201) measured the critical photoperiod and the degree of sensitivity of several cultivars using different methods and formulas and came up with their pet method of measurement. Both workers used the natural day length as a basis for computation and assumed that flowering occurs 30 d after photoinduction. Best (25) and Li (249), using a method similar to Chandraratnas (34, 37, 40), measured sensitivity based on response curves obtained by plotting the time from sowing to floral initiation on the grade and the photoperiod used on the abscissa.The method, however, requires a wide range of photoperiods. Li (249) also studied photoperiod sensitivity in damage of the BVP and the PSP. The BVP was obtained in plants great(p) under 10 h of light, and the PSP (which is a measure of sensitivity) by subtracting the growth duration under the 10-h photoperiod from that under the 16-h photoperio d. The PSP values obtained show the possible maximum range in growth duration as a result of extending the photoperiod.The photoperiodic characteristics of a rice plant have been described by Stewart (458) who used a different standard based on 1) basic vegetative period in legal injury of degree-days (based on temperature accumulation), 2) photoinduction period in degree-days or degree-minutes (using store night length), and 3) panicle development period in degree-days (based on temperature accumulation). Tests under field conditions were study by this method and predictions were made on the response of the cultivar sown in different months. In Japan, the flowering response is evaluated using the floral stages (135, 463).The Japanese workers have used the scale of 0-7, based mainly on the length of the developing panicle. This destructive measurement is more accurate than the usual days from sowing to flowering or treatment to flowering. The preference of the most appropriate method of testing and describing the response to photoperiod depends upon the purpose of the experiment and the available facilities. From the physiological standpoint, however, controlled photoperiod and temperature are desired because of their advantages over natural photoperiods and temperatures. Date-of- set experimentsDay length changes rhythmically within a year and varies depending upon the latitude. The amount of change in day length during the rice cropping season differs from one latitude to another (Fig. 5). Even in locations at the same latitude the day length during the cropping season may differ because the planting dates 16 The flowering response of the rice plant to photoperiod 5. Day length changes during the cropping season at discordant locations in Asia. may differ greatly depending mostly on the rain evenfall recipe at each location. At northern latitudes (Sapporo, 43? N, and Konosu, 36? N) day ength increases and then decreases during the cropping season (F ig. 5). At lower latitudes (Taipei, 25? N, and Los Banos, 14? N) day length decreases during the main growing season. heartfelt the equator (Bukit Merah, 5? N) there is little change. These differences in day length during the growing season may account for the wide range of photoperiod response of rice cultivars. A rice cultivar that must have less than 12 h o daylight to flower will obviously flower too late at the northern latitudes because rime will set in before harvest. In the northern hemisphere, the longest days are in June and the shortest are in December.Taking these into account, the photoperiod response of the rice cultivars can be tested to a limited extent by planting the cultivars at a certain location at different dates. Maximum differences in growth duration can be obtained in the May and November plantings if temperatures are not too low for growth. If a rice? fs growth duration changes more than 30 d, agronomists usually consider it photoperiod sensitive or a se asonal cultivar. As Best (24) has pointed out, this metre is not specific enough for research on photoperiodism, and caution should be taken in evaluating the data obtained.These phenological data, however, are important to breeders in selecting ecotypes. This method of testing sensitivity to photoperiod has been followed in Australia (245), Brazil (l03, 579), China (44, 356, 582), India (98, 99, 101, 214, 220, 295, 298, 423), Indonesia (467), Japan (533, 548), Korea (247, 466), Malaysia (74, 77, 244), Philippines (91, 512), Russia (452), Senegal (66), Sierra Leone (68, 536), Sri Lanka (112, 259, 402), Thailand (381), Trinidad (325), and United States of America (177, 180). The flowering response of the rice plant to photoperiod 17These experiments strongly confirm the existence of wide cultivar differences in the effect of planting date on flowering date. Many of the results obtained from this type of testing, however, are not applicable to resembling cultivars braggart(a) at d ifferent latitudes. A cultivar can be insensitive to day length in Malaysia but sensitive in Taiwan. Results of field tests at a certain latitude are, therefore, not eer applicable at another latitude. Some published papers on the use of this testing method failed to mention latitude or the place where the tests were conducted.Under natural conditions very small differences in day length can affect the rice plant. In Malacca (Malaysia), the difference between the maximum and the minimum day lengths is only 14 min and yet the cultivar Siam 29 takes 329 d to flower when planted in January and only 161 d when planted in September (76). Another instance wake the sensitivity of the rice plant to small differences in day length was reported in a date-of-planting experiment in Malaysia (244). There was a difference of as much as 156 d in the growth duration of photoperiodsensitive cultivars when planted in the same month but in different years (Table 2).This presumably resulted from diff erences in weather during the critical periods. Cloudy weather early or late in the day shortens the twilight hour, thus reduce the day length. Toriyama et al (490) tested rice cultivars involving not only monthly planting but also sowing at different latitudes (Sri Lanka, Taiwan, and Japan). This gives a better idea of the photoperiodic response of the cultivars but involves much work and cooperation. Ecology and photoperiodism Rice can be grown over a wide range of environmental conditions, from the equator to about 53? N latitude, leading to the differentiation and establishment of various(a) ecotypes and forms. The great diversity in photoperiod sensitivity from one latitude to another or within a latitude probably indicates that the rice cultivars predominantly cultivated in each area are those that have been selected on the basis of local adaptability (that is, adaptability to the temperature of the rice-growing season, day length, and duration of the growing season) to assu re the full development of the plant and the best possible balance between vegetative and reproductive growth (423, 530, 532, 584, 585).Table 2. Growth duration (days from sowing to flowering) of photo. period-sensitive cultivars when planted in January 1962 and 1963 at several localities in Malaysia (244). Cultivar Locality Jan 1962 Jan 1963 Difference Engkatek Telok Chengai 136 292 156 Kota Bahru 146 243 97 Kuala Lumpur 134 97 37 Subang Bukit Merah 270 224 46 lntan 117 Kuala Lumpur 171 138 33 Kota Bahru 276 176 100 18 The flowering response of the rice plant to photoperiod A major(ip) problem in studying the ecology of the rice plant, especially in reference to photoperiodism, is that cultivars in farmers fields keep changing.For example, Hara reported in 1930 that Japanese cultivars were more sensitive than the cultivars from mainland China and Taiwan. He concluded that the lower the latitude of the region of the native habitat, the less sensitive were the cultivars there. Wada (531), using 134 cultivars, showed contrasting results . the cultivars from the northern region of Japan had lower photoperiod sensitivity than those from the southern region. Recent papers, however, generally harmonize that among the photoperiod-sensitive cultivars, the lower the latitude of dissemination, the higher the sensitivity (351, 352, 356, 531, 583).The cultivars in the tropics or lower latitudes are usually late maturing (long growth duration). Many studies show that the late cultivars are more sensitive to photoperiod than the early ones (116, 248, 357, 511, 563, 583). In the tropics, where rice can be grown any time of the year provided there is sufficient water, photoperiod sensitivity presents certain problems. During the off-season, when the day length during the early growth stage is increasing, the sensitive cultivars are uneconomical to use because they take a very long time to produce any grain.For wider adaptability, cultivars should have low photoperiod sensit ivity (53, 70) and thus have little differences in growth duration when planted at different times of the year or at varying latitudes. Insensitive cultivars have been successfully grown at different latitudes where rice is used as a crop (45, 351, 352, 511, 532, 568. This indicates that it should not 6. Growth duration of IR8 planted in June or July at 12 sites in Asia. La Trinidad and Kanke are high-altitude areas (52). The flowering response of the rice plant to photoperiod 19 e difficult to introduce new photoperiod-insensitive cultivars to different ricegrowing areas or to culture them year-round in the tropics. The plant breeders, as the varieties coming out indicate, are developing more photoperiod-insensitive cultivars. commodious testing in various rice-growing areas of the world has established the wide adaptability of photoperiod-insensitive cultivars. In general, the longer the BVP the less variation ingrowth duration and the stronger the PSP the greater the variation i n growth duration (581).The wide adaptability and the stable growth duration of IR8, a photoperiod-insensitive cultivar, are indicated by the data furnished by cooperators in various parts of the world. IR8? fs growth duration varied within a range of 20 d at latitudes from 11o to 27oN except at high altitudes where low temperatures prevailed during part of the growing season (Fig. 6). A more informative example of the effect of temperature comes from monthly planting at Los Banos, Philippines, and at Joydebpur, Bangladesh (Fig. 7).A parity between the monthly mean temperatures and mean photoperiods shows that the more variable heading pattern at Joydebpur is more closely associated with temperature rather than with the prevailing photoperiod. The effect of low temperature on the improved tropical cultivars becomes more obvious in photoperiod-insensitive cultivars. 7. Mean monthly temperatures and day length in relation to the growth duration of IR8 at Los Banos, Philippines, and J oydebpur, Bangladesh (52) 20 The flowering response of the rice plant to photoperiod predisposition to photoperiod of rice cultivars in the deep water areas is an important characteristic for survival (104, 520). The locomote rice cultivars are highly photoperiod sensitive. They are planted early in the season when the soil can still be worked and without danger of submerging the young seedlings. florescence occurs when the floodwater peaks or starts receding. If the cultivar flowers when the floodwater is still rising, it would mean the complete loss of the crop if the panicles are submerged. propagation ability ceases after panicle emergence.Harvesting is usually done when the floodwaters have receded. The maturity of aimless rice cultivars coincides with the receding of the annual floodwaters which may be 150-270 d after sowing. Such a long growth duration requires a photoperiod-sensitive cultivar. So far, there is no known tropical cultivar that has a long growth duration an d is not sensitive to photoperiod. Photoperiod sensitivity may work as a safety mechanism when precise planting dates are not followed and environmental conditions such as water level cannot be effectively controlled.If the date of sowing or transplanting is delayed because of insufficient rainfall, a photoperiod-sensitive cultivar may still mature at its usual time (352, 382). Plants are not seriously damaged if left in the seedbed for prolonged periods because the growth duration of the main crop is sufficiently long for the plants to adjust. Thus, land preparation and transplanting can be staggered (382). Maturation of the crop at the same time. as with photoperiod-sensitive cultivars planted at different dates, may reduce rat and biting louse damage in any one field. Also, harvesting and drying are simplified.If the soil is not sufficiently fertile, a photoperiod-sensitive cultivar will continue its compelled vegetative growth until the short days come. This would give the plan t enough time to reach a reasonable plant weight and accumulate enough carbohydrates before flowering (528). Thus, a photoperiod-sensitive cultivar generally may be more resistant to reproving conditions. Long-growthduration cultivars (essentially photoperiod sensitive) are least affected by strong soil reduction (549). Most upland rice cultivars have short growth duration and are photoperiodinsensitive (11, 12).However, in areas where the rainfall pattern is bimodal, as in northern Thailand, the cultivars are of medium growth duration and are photoperiod-sensitive . possibly another indication of the greater specific adaptability of long-growth-duration cultivars to inauspicious conditions. The sensitivity to photoperiod of wild species has also been studied in relation to their ecological dispersion. Most of the wild rice materials tested were sensitive (191, 201, 205, 209, 353). They suggested that this sensitivity favors the wild rice plants and is mayhap essential to their s urvival. Terminology used in describing photoperiod ensitivity There is confusion in the terms used to describe the response of the rice plant to day length (515). Often, the terms used for growth duration are also used for response to photoperiod (see Table 3). As early as 1912, Kikkawa pointed out that The flowering response of the rice plant to photoperiod 21 Table 3. Some terms used in describing the growth duration and day length response of rice cultivars. Terms References Response to day lengths date fixed vs period fixed season fixed vs period fixed season bound vs period bound punctual fixed vs periodically fixed short-day plant vs long-day plant ensitive vs indifferent sensitive vs insensitive sensitive vs less sensitive short-day plant vs indifferent plant strongly photoperiodic vs weakly photoperiodic sensitive vs photosensitive vs photononsensitive day length sensitive vs day photoperiodic photoperiodic insensitive length nonsensitive early, medium, and late long-aged vs short-aged early flowering vs late flowering late maturing vs early maturing Season of planting aman vs non-aman yala vs maha winter vs summer main-season vs off-season first crop vs second crop wet vs dry season aus, aman, boro, rabi, kharif Growth duration 33 7 214, 511 308 1, 99, 336 3 68, 352, 353 21, 98, 449, 538 563 51 1 195, 352 339 574 91, 276, 277, 281 259 158 3, 230, 374 427 112 444 Malaysia, Indonesia, and Thailand China Philippines Bangladesh, India it is purposeless to classify the rice cultivars of the world into such groups as early, medium, late, aus, or aman. He said, however, that this classification is useful in districts where the climates are similar. The use of the terms photoperiod-sensitive and photoperiod-nonsensitive in reporting the flowering response of a rice cultivar to changes in day length has been suggested (515).weak photoperiod-sensitive is sometimes used in place of photoperiod-nonsensitive because the existence of a completely photoperiod-no nsensitive cultivar is difficult to prove. Weakly photoperiod-sensitive is also used to describe cultivars whose flowering is delayed by as many as 70 d by long photoperiods. However, those types can be planted any month of the year in the tropics and can be expect to flower within the crop season. The terms short-day plant and long-day plant are not satisfactory because most rice cultivars immediately are short-day plants.Sensitive and insensitive, sensitive and indifferent, and sensitive and less sensitive are questionable terms. Because the response being described is a response to light period and not only to light, the terms photosensitive and photononsensitive are inappropriate. 22 The flowering response of the rice plant to photoperiod 8. Effect of four photoperiod treatments on the seeding-to-heading period of seven rice cultivars. Chang and Vergara (51, 52, 53) classified rice cultivars into four types using the length of the BVP and PSP as criteria (Fig. 8).Their classif ication was based on duration of plants grown in the greenhouse. Under this classification, the Japanese varieties, such as Fujisaka 5 and Norin 20 (Appendix), do not fall under any category because they have a short BVP and short PSP. Also, at least four photoperiods (10, 12, 14, and 16 h) are needed to classify the cultivars. A more practical grouping could be as follows (using also the length of the BVP and PSP). 1. Photoperiod nonsensitive . very short PSP (less than 30 d) and BVP varying from short to long. 2. Weakly photoperiod-sensitive . arked increase in growth duration when photoperiod is longer than 12 h PSP may exceed 30 d, but flowering occurs under any long photoperiod. 3. Strongly photoperiod sensitive sharp increase in growth duration with increase in photoperiod no flowering beyond critical photoperiod BVP usually short (not more than 40 d). Cultivars tested under only two photoperiods, such as 10 and 14 h, can also be classified according to these groupings (1 1). Agronomists and farmers would tend to use these groupings. The flowering response of the rice plant to photoperiod 23 Inheritance of vegetative growth durationThe inheritance of the duration from seeding to heading in cultivated rices has been studied by many research workers, but the findings have resulted in diverse interpretations. Three categories of transmissible postulates were generally offered 1) monogenic or digenic control of heading date, with earliness dominant to lateness 2) monogenic or digenic control of flowering date, with lateness being a dominant characteristic and 3) binary-factor inheritance in which the F 2 population showed a continuous and often unimodal distribution and in which the same population might produce a bimodal distribution when grown in a different season (44, 509).In experiments where photoperiod sensitivity was bring ind, delayed flowering under a long photoperiod was generally inherited as a monogenic or digenic dominant trait (38, 242, 40 6, 424, 567). In several crosses involving distantly related parents, sensitivity to photoperiod appeared to be a recessionary trait (242, 406). The continuous and transgressive segregation in several F 2 populations involving photoperiod-insensitive parents was ascribed to multiple genes, which indicated dominance of earliness (41, 95, 96, 97, 333, 389, 469, 554).However, in crosses among varieties in Yunnan duty in China, photoperiod sensitivity appeared to be a recessive trait in some F 1 hybrids (252). Some of the diverging interpretations just mentioned resulted partly from misfortune to recognize the composite nature of the vegetative growth period from seeding to panicle primordium initiation, partly from failure to control the interaction of the environmental factors (mainly photoperiod and air temperatures) and the different genes controlling the vegetative growth period, or from failure to relate the phenotypic expression with the revailing environment. Recent studies at IRRI (48, 161, 162, 163, 164, 165, 167, 168, 249) have demonstrated physiologically and genetically the feasibility of partitioning the vegetative growt