Breeding and Genetics

Transcription

1 Breeding and Genetics THE PHOTOPERIODIC CONTROL OF FLOWERING IN SACCHARUM M. H. R. Julien Mauritius Sugar Industry Research Institute RCduit, Mauritius ABSTRACT Flowering in 2 clones of Saccharum spontaneum and 1 clone of S. robustum was found to be controlled by photoperiod. In the 2 S. spontaneum clones the earliest stages of development-induction and initiation of inflorescence axis primordium-required intermediate days of about 12 hr, 30 min, while the subsequent stage-initiation of inflorescence branch primordia-had a short day response, with a critical photoperiod 13 hr. The final stagesinitiation of spikelet primordia and growth-were quantitative short day responses with optimum photoperiods of 11 hr and 9 hr respectively. In the S. robustum clone all stages were short day responses with gradually shortening critical photoperiods from induction to growth. At all stages of development in the 3 clones the presence of young leaves was necessary for the perception of the photoperiodic stimulus. The effects of inductive short or intermediate day cycles could be annulled by night-break treatments; it was shown that red and green were the effective wavebands. It is clear that normal floral development in these Saccharum clones requires a precise sequence of photoperiods. The relevance of this for the manipulation of date of flowering in breeding material is discussed. INTRODUCTION The aim of the present work was to obtain additional information on the mechanism controlling flowering in Saccharum spontaneum and S. robustum. It was hoped that such knowiedge would lead to the rationalization of the use of treatments for synchronizing flowering in different species and thus allow interspecific hybridization. Previous work on the physiology of flowering has been reviewed in detail by Coleman (1969) and by Davies and Vlitos (1970). It has been established that the following factors may influence flowering in sugarcane: photoperiod, temperature, soil moisture status, the condition of ripeness to flower and the presence of specific leaves on the plant. Most of the knowledge was obtained from work conducted on a few varieties, particularly H ; iurthermore, since,, as in most cases, the time of onset of initiation was not determined critically, it is difficult to assess at what development stage the treatments were actually applied. Preliminary experiments conducted in Mauritius had shown that photoperiod and night-break treatments had inhibitory effects when applied several weeks after the onset of initiation (Julien, 1968). Consequently, in the present work, the different stages of development were identified prior to experimental work, and the effects of photoperiod, night-break and defoliation treatments

2 324 BREEDING AND GENETICS applied at these stages of development were studied in S. spontaneunz var 51 N(; 2 and Mandalay and also in S. robusturn var Mol EXPERIMENTAL PROCEDURE The experimental methods adopted to study normal floral development have been described in detail by the author (Julien, 1970b), hence only the main points will be outlined in the present article. Random samples of 6-8 stalks were taken at intervals of 3-6 days from control plots under natural conditions. The apices were dissected out and embedded in wax for sectioning when they were less than 'b'.3 cm in length. The stage of development and the length of each apex were recorded. Stalk-to-stalk variation was small during the early stages of development but greater during the last stages (elongation) (Table 1). Table I. Stages of development and mean length of inflorescence in var Mandalay. Mean length of inflorescence Coef. of Date Stage of development -- (mm) Trar (%) Feb 10th Vegetati~e Feb 13th Feb 19th Feb 24th Pel-, 27th March 2nd March 9th March 13th Onset of initiation of inflorescence axis Initiation of infloresce~lce axis 0.35 t Onset of initiation of inflorescence branch primordia Initiatio~l of inflorescence branch primordia Initiation of spikelet primordia (early) March 19th Initiation of spikelet primordia March 2211d Initiation of spikelet primordia (late) March 28th Ohset of elongatio~l of inflorescence March 31st Elongation of inflorescence April 7th "growth" April 13th April 21st Onset of lag period April 27th Lag period t The pattern of development of the inflorescence was generally similar in the 3 var. The following stages were recognized and are illustrated in Fig. 1

3 A= Apex LF= Leaf prirnordiurn VEGETATIVE BP: Branch prirnordiurn I i INITIATION OF INFLORESCENCE INITIATION OF SPIKELET PRI - BRANCH PRlMORDlA ( IBP) MORDIA ( ISP) Fig. 1. Illustrations of L.S. of apices of var Mandalay at different stages of development. I I and 2 for var Mandaly: 1) induction-14-0 days prior to onset of initiation of / inflorescence axis, recognized by histological changes in the zonations of the apex (Julien, 1969) ; 2) after the onset of initiation of the inflorescence axis, the young primordium increased in size; subsequently inflorescence branch primordia were initiated spirally on it. The period between the onset of initiation of the inflorescence axis and the onset of initiation of inflorescence branch primordia on the axis will be defined as the initiation of inflorescence axis stage (IAP) (Fig. 1) ; 3) the stage initiation of inflorescence branch primordia (IBP) started from onset of initiation of the first branch primordium and ended when all inflorescence branch primordia had been initiated on the axis (Fig. 1) ; 4) the spikelet primordia were then initiated on the inflorescence branch primordia, defined as the initiation of spikelet primordia (ISP) (Fig. 1) ; 5) the ISP stage was followed by the elongation of the inflorescence (growth) (Fig. 2) ; 6) a lag phase (Fig. 2) ;

4 326 BREEDING AND GENETICS 501 t I DIFFERENTIATION I ELONGATION I L AO I Y I INFLORESCENCE T l NTERNODE FE 0 MARCH APRI L M AY Fig. 2. Length of the iilfloresceilce aid 1st floral i~lterilode from initiation to anthesis ill var Mandalay. and 7) elongation of the first floral internode lead to the emergence of the inflorescence (Y) (Fig. 2). The actual dares of onset of initiation and of other devel- STAGES OF DEVELOPMENT DIFFERENTIATION I I I 42to28 28to 14 lnductionl IAP I IBP I ISP Elongation I Day.+ I Days+ I I = Growth I I I I I I I Janl4 Jan.28 FeblO Feb.24 March 10 March24 Apr.6 Apr.20 I - [ Treatment period Natural daylength + = Prior to onset of initiation Fig. 3. Illustration of the method of experimentation for the photoperiodic experiment on var Mandalay.

5 M. H. R. JULIEN 327 The following general method was adopted to study the effects of various treatments applied at different stages of development. The experimental period started several weeks prior to onset of initiation (in control plots under natural conditions) and ended when the elongation phase was terminated. This period was divided into periods of days, which were planned so as to correspond to different stages of development. These are illustrated for var Mandalay 42 TO 28 DAYS PRIOR INITIATION OF INFLORESCENCE INITIATIONOFSPIKELET PRIMOFDIA OF INITIATION " " 0..., INFLORESCENCE PRI MORDIA GROWTH OF INFLORESCENCE Q '(IND) DAYLENGTH HR. DAYLENGTH HR. DAYLENGTH HR. Fig. 4. Effect of daylength treatments applied at 7 stages of development on time to emergence in var Mandalay. 0 z I NO ---. I AP - -. I BP... ISPEARLY.-- I SP LATE --- GROWTH,-- EMERGENC Fig. 5. Effect of daylength treatments applied at 7 stages of development on arc sin vyo emergence, in var 51 NG 2.

6 DAYLENGTH Fig. 6. Effect of daylength treatments applied at 5 stages of development on length of inflorescence (cm) sampled 38 days after onset of initiation in var 51 NG 2. HR Fig. 3. Samples taken from control plots at intervals of 4-6 days confirmed that the treatment periods corresponded to their respective stages of development. Natural daylengths ranged from 13 hr to 11 hr from the beginning to the end of the experimental period. In the photoperiod experiments, the following photoperiods were used: 7, 9, 11, 13, 15 and 17 hr. For photoperiods shorter than natural daylength (7, 9, 11 hr), the pots were placed on a cart and pushed in and out of dark rooms at specific times; while for photoperiods longer than natural daylengths the pots were illuminated with low intensity light (20 fc) obtained by using loow incandescent bulbs. The shortest photoperiod (7 hr) reduced the incoming energy by 25%; no other treatment changed this by more than 8%. The night-break treatments were applied by illuminating the plants between 10 pm. and 2 am. every night for a day period, wllicl~ corresponded. to a particular developmental stage. The types of lamps and filters used to give the night-breaks, as well as the intensity of light and wave-length transmitted have been described earlier (Julien, 1970a). Two defoliation treatments were used in the defoliation experiments: either only the spindle and 1st 2 leaves or the lower 3rd, 4th, 5th and 6th leaves were left on the plant. The youngest fully expanded leaf was designated leaf 1, as previously defined by the author (Julien, 1969). Each experiment included undefoliated plots, which were considered as a control. RESULTS The results obtained with the 2 S. spontaneurn clones, in the photoperiodic experiments, were generally similar and are summarized in Fig. 4, 5, and 6. The most striking features of these results are: 1) the plants were shown to be photoperiodically sensitive prior to initiation, and this period was defined

7 M. H. R. JULIEN 329 as the induction period (Fig. 4, 5, 6) ; 2) the response to photoperiod at this stage and the following IAP stage was of the obligate intermediate day type with an optimum photoperiod of 12 hr, 30 min to 13 hr (Fig. 4, 5, 6) ; 3) the subsequent IBP stage showed a typical short day type of response with a critical photoperiod of 13 hr. Photoperiods shorter than 13 hr (7, 9, 11 hr) were not inhibitory at this stage in contrast to their marked inhibitory effects when applied at the 2 earlier stages of development, induction and IAP; 4) the subsequent ISP stage and early stages of elongation of the inflorescence were quantitative short day responses, with optimum photoperiods of 11 to 9 hr (Fig. 6) ; and 5) the latter part of elongation of the inflorescence was generally insensitive to photoperiod (Fig. 5). Thus, all stages of development from inhction to the early part of elongation of the inflorescence are sensitive to photoperiods. The pattern is for precise intermediate days during the early stages (induction and IAP) and short days in the later stages. The IBP stage was the most sensitive and required photoperiods of less than 13 hr. The successive subsequent stages were progressively less sensitive and their optimum photoperiods were progressively shorter. Unfavourable photoperiods at stages subsequent to initiation commonly resulted in reversion of the apex to the vegetative condition. Such reverted apices were most frequently found when treatments were given during the IAP and IBP stages. At later stages (ISP and early elongation) unfavourable photoperiods usually arrested or delayed inflorescence development but reversion was rare. The pattern of response in the S. robustum clone was a series of short day responses with gradually shorter critical photoperiods (Fig. 7). In contrast with /;;\ 4 s rc-* DAYS* ---a IBP GROWTH EARLY t 4. *...a I N D c-. I SP EARLY 35 r-r IAP GROWTH LATE 25 W I- UI Z 5 P DAYLENGTH HR + PRIOR TO INITIATION Fig. 7. Effect of daylength treatments applied at 9 stages of development 011 time to emergence in var Mol the S. spontaneum clones, precise intermediate days were not required at the early stages and the ISP stage was the most sensitive. The effects of inductive intermediate or short day cycles could be nullified

8 330 BREEDING AND GENETICS by certain of these night-break treatments. The intermediate day effects on the induction stage in S. spontaneum could be reversed by night-breaks of red light (Fig. 8) but not of incandescent light (4000 fc min) (Fig. 9). Fig. 8. Effect of night-breaks with,light of different spectral composition on time to emergence in var 51 NG 2. The favourable effects of inductive short day cycles at later stages, particularly at the IBP stage in S. spontaneum and at: the ISP stage in S, robustum, could be nullified by night-breaks with incandescent, red or green light. The presence of the young leaves (spindle + 1, + 2) was necessary for the promotive effects of photoperiod at all stages from induction to the early part of elongation of the inflorescence (Fig. 10 and 11). The removal of the young leaves had the maximum effect at the stages of development which were most sensitive to photoperiod and night-break treatments, i.e., the IBP and ISP

9 M. H. R. JULIEN X I AP ISP -' GROWTH - IND 50- Y > LSD PQ05 LSD P W U z w M e 0 a w \,' \ Y "* " b- 4 il lirim 4 il ~ b l i a 4 11 ts n li LIGHT INTENSITY FOOT CANDLRS Fig. 9. Effect of intensity of light during night-breaks applied at different stages of development on time to emergence in var 51 NG 2. stages in S. spontaneum and S. robustum, respectively (Fig. 4, 7, 8, 10 and 11). Reversion to the vegetative condition commonly occurred when the young leaves were removed during the IAP and IBP stages in S. spontanez~m, and this effect was similar to that observed when unfavourable photoperiods or night-break treatments were given at these stages. DISCUSSION The complexity of the mechanism controlling flowering in Saccharum as compared to that of other plants, e.g., Xanthium (Salisbury, 1969) is emphasized. Different photoperiodic requirements for the different stages of development have been observed in many other species, e.g., temperate grasses (Calder, 1966) ; Chrysanthemum (Cathey, 1969), but the pattern in S. spontaneum of a precise intermediate day requirement followed by a series of short day requirements with different optimum photoperiods is unusual, although a similar pattern has been observed in some strains of Oryza satiua (Chandratna, 1954). These photoperiodic requirements at most stages of development suggest a continual requirement for flowering stimulus, which appears to be produced by the young expanding leaves. Although night-breaks with red and far red light have similar effects to those in other short day plants (Evans, 1969), the effect of green light seems to be peculiar and might possibly suggest a different mechanism for the control of the dark reactions, particularly as attempts to reverse the effects of red light by far red have been unsuccessfu1 (Coleman, 1969). This work suggests several methods for the manipulation of time of flowering in breeding material. Thus, long days (13 hr) or night-break treatments applied at stages after initiation will lead to delay of emergence, while shorter

10 332 BREEDING AND GENETICS 'i photo periods (9 and 11 hr) may advance flowering. Furthermore, in clones which fail to flower because the inflorescence primordium does not develop under naturh conditions, gradually shorter photoperiods (9 hr-11 hr) during the later stages of development (ISP and elongation) could be tried as an inducing method. ACKNOWLEDGEMENTS I wish to thank Mr. R. Antoine, Director, Sugar Industry Research Institute, Mauritius, for his support and encouragement, and Dr. P. M. Cartwright, Department of Agricultural Botany, University of Reading, for her advice and criticisms. +. PRIOR TO ONSET OF INITIATION Pig. 10. Effect of 2 defoliation treatments applied at 7 stages of development on tim? lo cmcrgence in var 51 NG 2.

11 M. H. R. JU1,IEN 333 S PINDLE +I + 2 J >- a LOWER LEAVES L J + PRIOR TO ON ONSET OF INITIATION Fig. 11. Effect of defoliation treatments applied at 10 stages of development on time to emergence in var Mol REFERENCES Calder, D. M Inflorescence induction and initiation in the Gramineae. In the Growth of Cereals and Grasses. Ed. F. L. Milthorpe and J. D. Ivins. Butterworths Scientific Publications, London. p Cathey, H. M Chrysanthemum morifolium. In the Induction of Flowering. Ed. L. T. Evans. The McMillan Co. Ltd., Melbourne. p Chandratna, M. F Photoperiod response in rice (Oryza sativa) I. Effects on inflorescence initiation and emergence. New Phytol.; 53:397. Coleman, R. E Physiology of flowering in sugarcane. Proc. ISSCT, 13: Davies, W. N. L., and A. J. Vlitos Some aspects of flowering in sugarcane and its relationship to sucrose metabolism. In Cellular and Molecular Aspects of Floral Induction. Ed. G. Berneir. Longman Group Ltd., London. p Evans, L. T The Induction of Flowering. Ed. L. T. Evans. The McMillan Co. Ltd., Melbourne. Julien, R Investigations on the physiology of flowering. Rep. Maurit. Sugar Ind. Res. Inst., 15: Julien, R Ibid., 16: Julien, R. 1970a. Ibid., 17: Julien, R. 1970b. The photoperiodic control of flowering in sugarcane. Ph.D. Thesis. University of Reading. Salisbury, F. B Xnnthium strzrmarium L. In the Induction of Flowering. Ed. L. T. Evans. The McMillan Co. Ltd., Melbourne. p