Breeding and Genetics

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1 Breeding and Genetics I FLOWERING OF SUGARCANE WITH REFERENCE TO INDUCTION AND INHIBITION E. D. Paliatseas Louisiana Agricultural Experiment Station Baton Rouge, Louisiana ABSTRACT The minimum,time required for flower initiation was studied in 9 hybrid varieties of sugarcane under Louisiana conditions. A minimum of inductive days were required for initiation of easy flowering varieties. Sixty to 70 days were required for reluctant flowering varieties. Comparable periods for flower initiation were+obtained by the leaf count method as well as by direct examination. The duration of the other 3 stages of flowering was calculated. lntercalation of minimal treatments totaling 45 days with 5 long days totally inhibited flower initiation. Intercalation of minimal treatments with 5 short days diminished but did not totally cancel flower initiation. Intercalation of minimal or longer photoperiod treatments with 5 low temperature nights had no effect on flower initiation. Long days and low night temperatures adversely affected flower emergence. INTRODUCTION The flowering of sugarcane has been given attention since 1888, when it was discovered that sugarcane produced sexual seed. Photoperiodism has been known since 1920 when Garner and Allard (12) made this outstanding discovery. However, there was no important work on photoperiodic induction of flowering in sugarcane until 1938 when Allard (1) and Sartoris (14) induced a variety of Saccharum spontaneum (28 NG 292) to flower. In 1945 Dutt et al. (11) and in 1950 Lea and Lin (13) reported artificial induction in some types of S. spontaneum. Work on photoperiodic induction of commercial varieties of sugarcane was unsuccessful until 1954, when Chilton and Moreland (3) reported the photoperiodic induction of 7 easy flowering varieties by gradually decreasing day length. According to Burr et al. (2), up to 1957 attempts to induce flowering or to change the time of flowering in reluctant flowering varieties was completely unsuccessful. The lack of progress at first was largely because sugarcane breeders depended on natural flowering and made little effort to explore the physiology of flowering in the plant. Progress has been made on artificial or photoperiodic induction of sugarcane since the middle fifties. A better understanding of the physiology of flowering has since been achieved, much more is expected, and the day is not too remote when the mechanism of flowering of the plant will be established in detail. This work was reviewed by Burr et al. (2) in 1957, and general reviews of the subject have been given in the Proceedings of the 12th and 13th ISSCT Congresses (8,9,10). Research most pertinent to that reported in this paper was by Clements

2 - E. D. PALIATSEAS 355 E et al. (9) and Coleman (8). Clements et al. distinguished 5 stages in flowering oi: sugarcane. One nyctoperiod of 11 hr 39 min was used througl~out the whole flowering process. The 5 stages were: initiation, bolting, emergence, full extension of the inflorescence and flower opening. In the initiation stage, 2-3 new leaves were formed, then the flower primordia. The duration of this stage was days under favorable temperatures ( F) and days under lower night temperatures ( F). This was the only temperature dependent stage. It required 100 days from the beginning of the experiment until flower emergence of easy flowering varieties and weeks for reluctant flowering varieties. About 155 days were required for flower emergence of easy flowering varieties under conditions of lower night temperature ( F). Coleman (8) concluded that 12 hr 35 min was the optimum photoperiod for flower initiation of the varieties studied. A deviation of + 15 min inhibited Rower initiation (4). He concluded that the 12.5 hr light period must all be high intensity light. When part of the light period was low intensity light, many more days (60-90 as compared to 20 in nature) were required for induction (5). The photosynthetic activity associated with a high intensity of light was a requirement for the production of a maximum amount of stimulus. He concluded that the inability of the plant to flower in the spring was probably due to the lack of a 12.5-hr high intensity light period at that time of year. In studies of light interruption during part of the 20 inductive nights in Hawaii, various percentages of flowering occurred. Fifteen inductive nights were necessary for maximum flowering, and no flowering occurred with less than 10 inductive nights (7). The light interruption work and the intercalation of non-inductive nights (long days) during the 20 inductive nights of Hawaii showed the accumulative nature of stimulus production. With less than the minimum 15 inductive nights (7), less flowering stimulus was produced, which resulted in a primordium partly developed or reverted to the vegetative condition. CoIeman (6) concluded that 65 F and below during the induction period inhibited flowering. The effect of low night temperature on flowering was similar to the effect of non-inductive nights due to light interruptions (6). Some of the practical problems that interest sugarcane breeders raise certain questions. Should flowering of sugarcane be treated entirely as 1 phenomenon or divided into stages? Is each stage controlled by the same photoperiod or different photoperiods? Is inhibition of flowering due to unfavorable photoperiod and unfavorable temperature similar in their effect on flowering or different? The purpose of this paper is to try to answer some of these questions for sugarcane breeders. MATERIALS AND METHODS One set of experiments was conducted to determine the minimum time required for flower initiation and the other stages of flowering. The material for this study was 9 CP and L varieties. Six were easy or medium flowering (CP , CP 52-1, CP 52-68, CP 58-48, CP 59-22, and L 62-96), and 3 were reluctant flowering varieties (CP , CP , and I, 64-67). The varieties were grown in 10-gal galvanized cans until 5-6 exposed internodes were formed. The stalks were then marcotted by wrapping 2 joints of each in a polyethylene bag filled with a wet mixture of soil and peat moss. After roots formed, the

3 356 BREEDING AND GENETICS stalks were severed and planted in stovepipe tubes filled with a mixture of soil, peat moss and river sand. On May 1, 1969 the tubes with the plants were placed on carts that could be rolled in and out of photoperiod houses, according to schedule. An inductive photoperiod of 12 hr 30 min was given for 45, 55, 65, 70 and 150 days. At the end of the photoperiod treatment, 2 plants of each variety were removed from the carts and placed both under non-inductive long days (13-14 hr) and non-inductive short days (11 hr 40 min). Flower initiation was examined in plants under long days by cutting the growing point logitudinally days after removal from the carts. Under short days, flower initiation was assumed to have occurred in plants that showed flower elongation 30 days after removal from the carts, and was assumed not to have occurred in those that did not elongate flowers. The rest of the plants continued to receive the inductive treatment until flowering (about 150 days).. The duration of the 4 stages of flowering-initiation (stage I), reorganization and early development of the flower (stage 2), full elongation (stage 3), and emergence (stage 4) -was estimated in this group of plants using the leal count method developed by Clements (9). The duration of the 1st stage was estimated by counting the leaves produced from the beginning of the treatment to the emergence of the last leaf (flag). Nine leaves were present in the spindle at the time of initiation and these were subtracted from the total number of new leaves produced. The time required for the emergence of a single leaf was calculated and multiplied by the number of new leaves, to give the approximate minimum time in days required for initiation. To determine the duration of the 2nd stage, the duration of stage 1 was subtracted from the number of days from the beginning of the treatment to the beginning of elongation. To determine the duration of the 3rd stage, elongation, the duration of stage 1 and stage 2 was subtracted from the total number of days to emergence. To determine the duration of stage 4, which is the full upward thrust of the inflorescence, the number of days to emergence was subtracted from the number of days to full extension of the inflorescence. A 2nd set of experiments was conducted to study the effect on flower initiation of intercalation of non-inductive long and short days and noninductive low night temperatures. Three easy flowering varieties, CP , CP 52-68, and CP , were used in experiments on intercalation of long and short non-inductive days and low temperature nights. The plants were grown as previously described. On June 1, 1970, photoperiodic inductive treatments were started totaling 40, 45, 50, 55 and 60 days. These treatments were interrupted (intercalated) by 5 long or short non-inductive days, and 5 nights of low temperature. The photoperiod for the inductive treatment was 12 hr 20 min. The photoperiod for the non-inductive long and short days was 14 hr to 14 hr 7 min and 11 hr 40 min, respectively. A few days after treatment the plants were cut and examined for flower initiation. Low night temperatures of F were established in an insulated air-conditioned cold room. A 3rd group of experiments was conducted on the effect of long and short days and low night temperatures on flower emergence. Ten CP and L varieties, growing as previously described, were used in this experiment. Ten stalks of each variety were selected that showed definite signs of flowering after a 90-day photoperiod-inductive treatment. Half of the plants were removed from the inductive treatment and placed under 40 non-inductive long days and

4 I E. D. PALIATSEAS 357 i the other half were placed under 40 non-inductive short days. The effect on flower emergence was determined. Plants of another set of 10 varieties were placed under different night temperature after a 60-day inductive treatment. Half of the plants of those showing definite signs of flowering were placed outside under prevailing temperatures at Baton Rouge, Louisiana, in October and November Outside minimum temperatures for this period averaged 40.6 F. The other half of the plants were kept in the greenhouse, where minimum temperature was maintained at F. The effect on flower emergence was determined. RESULTS Minimum Time Required for Flower Initiation and the Other Stages of Flowering. The results on flower initiation after varying the number of inductive days (12 hr 30 min) followed by 15 non-inductive long days are shown in Table 1. The 6 easy flowering varieties and the 3 reluctant flowering varieties Table 1. Determination of the minimum time required for flower initiation in sugarcane after varying numbers of inductive clays (12 hr 30 min), followed by 15 long days. Length of inductive treatments (days) h Varieties No. plants treated. 2 No. plants initiated. had no visible floral initials after an inductive period of 45 days. The 6 easy flowering varieties had visible floral initials after an inductive treatment of 55 days; however, there were no visible floral initials in the 3 reluctant flowering varieties. After 60 or 70 inductive days, initiation was complete in the easy flowering varieties and partial in the reluctant flowering varieties. More of the reluctant flowering varieties initiated after 70 days than after 60 days. The same varieties treated with the same lengths of inductive photoperiod behaved differently when treatments were followed by 30 non-inductive short days (11 hr 40 min) (Table 2). Six easy flowering varieties initiated and elongated following only 45 days of inductive photoperiod. However, the reluctant flowering varieties required 60 or 70 days of inductive photoperiod to initiate and elongate flowers. The minimum time required for flower initiation of the 9 varieties was

5 358 BREEDING AND GENETICS Table 2. Determination of the minimum time required for flower initiation in sugarcane after varying the number of inductive days (12 hr 30 min), followed by 30 non-inductive days (11 hr 40 min). Varieties No. of plants with signs of eloilgation after following inductive treatme~ltsl 1 Two plants were given the inductive treatment in all instances. also estimated by the leaf count method (Table 3). According to this method the 6 easy flowering varieties were estimated to have initiated in days. The minimum time required for flower initiation is in agreement with that shown by the 6 varieties when given a 45-day inductive treatment followed by 15 long days. The calculated minimum time required for flower initiation of the 3 reluctant flowering clones was days, which was again in good agreement with the time (60-70 days) found in the previous 2 experiments. Knowing the approximate minimum time required for initiation it was possible to calculate the approximate duration of the other 3 stages (Table 4). From stage 1 to stage 2, i.e., from flower initiation to the beginning of elongation, required an average of 32 days for the 9 varieties. The duration of this stage varied from days or the 9 varieties. The duration of stage 3, elongation, was an average of 61 days and varied from days for the 9 varieties. Stage 4, emergence, required an average of 14 days for the 9 varieties and varied from days. The total time from the beginning of the experiment to emergence (stages 1, 2 and 3) varied from days for the 9 varieties, with an average of 141 days. Eflect of Intercalation of Non-inductive Long and Short Days and Non-inductive Low Night Temperatures The effect on flower initiation of intercalating inductive photoperiods of 40, 45, 50, 55, and 60 days with 5 non-inductive long days is shown in Table 5. No flower initiation occurred for the 40-day treatment whether intercalated or continuous. When the 45-day inductive treatment was intercalated with 5 long days ( ), some initiation occurred with continuous treatment (CP and CP 52-68) but no initiation occurred in the intercalated treatment. Initiation was complete for all varieties after the continuous 50-day inductive photoperiod. However, initiation was diminished or completely inhibited as in the case of CP after intercalation with 5 non-inductive long days ( and ). Initiation was complete in all 3 varieties after 55 and 60 days of continuous inductive photoperiod. In intercalated treatments

6 Table 3. Determination of the minimum time required for flower initiation in 9 varieties of sugarcane by the leaf count method after treatment for 150 days with a 12 hr 30 min inductive photoperiod. F c s M % Av No. plants No. No. No. number emerged or Av no. No. new No.new daysfl- daysfrom plants leaves in flag leaves leaves formed leaves formed leaf May 1 - Varieties treated May 1 Sept 30 Sept 30 May 1-Sept 30 May 1-initiation emergence initiation

7 360 BREEDING AND GENETICS Table 4. Determination of the duratiou of the 4 stages1 of flowering in 9 varieties of sugarcane. Duration of Duration of Duration of Duration of Total no. stage 1 stage 2 stage 3 stage 4 days to Varieties (days) (days) (days) (days) emergence CP CP 52-1 CP CP CP L CP CP L Average 1 Stage 1 = initiation; stage 2 = reorganization and early development of flower Stage 3 = full elongation; stage 4 = emergence. Table 5. The effect on flower initiation of intercalating inductive photoperiod treatments with 1 or 2 periods of 5 long days in 3 sugarcane varieties. CP CP CP Photoperiod treatment (days) No. plants No. initiated No. plants No. initiated No. plants No. initiated Continuous Continuous Continuous Continuous Continuous Continuous ( and ) initiation was somewhat diminished but no variety was completely inhibited from flower initiation. The effects on flower initiation of intercalating the same lengths of inductive treatments by 5 non-inductive short days are shown in Table 6. No floral initiation occurred for the 40-day treatment whether intercalated or continuous. There was complete flower initiation in the continuous 45-day treatment, but initiation was diminished for all 3 varieties in the intercalated treatment ( ). However, intercalation with short days did not result in the complete cancelling of initiation as was the case when the 45-day inductive treatment was intercalated with 5 long days. Initiation was complete in all 3 varieties after 50, 55, and 60 days of continuous treatment. Intercalation of these treatments by 5 short days resulted in only slight inhibition of flower initiation. The effect on flower initiation of intercalating 40- to 60-day inductive

8 E. D. PALIATSEAS 36 1 Table 6. The effect on flower initiation of intercalating inductive photoperiod treatments with 1 or 2 periods of 5 non-inductive short days in 3 sugarcane varieties. CP CP CP Continuous Continuous '1 Continuous Continuous 50 Continuous Continuous 60 2 photoperiod treatments with 5 low temperature nights (55-60 F) is shown in Table 7. Again, no initiation occurred for the 40-day continuous treatment; Table 7. The effect on flower initiation of intercalating inductive photoperiod treatments with 5 low temperature nights in 3 varieties of sugarcane. CP CP CP Photoperiod treatment (days) No. plants No. initiated No. plants No. initiated No. plants No. initiated Continuous Continuous however, some initiation occurred in the intercalated 40-day treatment ( ). It should be remembered that in this treatment the canes actually received 45 photoperiodic inductive days since the 5 low temperature nights were photoperiodically inductive. Apparently, the 45-day inductive photoperiod was sufficient to induce some initiation. Intercalation of or 60-day inductive photoperiods with 5 nights of low temperature did not inhibit or increase flower initiation. All these treatments contained a minimum number of days (45 or more) with optimum photoperiod and temperature so the 5 low temperature nights had no effect.

9 362 BREEDING AND GENETICS The Effect of Long and Short Days and Low Night Temperature on Flower Emergence. The effects of 40 long and 40 short days on flower emergence in 10 sugarcane varieties which had initiated and partially elongated after a 90-day inductive photoperiod treatment are shown in Table 8. The percentage of Table 8. Effect of 40 noa-inductive long days or short days, following flower initiation, on flower emergence in 10 varieties of sugarcane. No. plants Flower emergence (yo) No. plants with flowering Varieties treated stages 1 and 2 40 long days 40 short days flower emergence was far superior when initiation was followed by 40 short days than 40 long days. Every variety responded positively to the short day treatment. The effect of low temperature on emergence is shown in Table 9. Eight Table 9. Effect of low night temperature, following flower initiation, on flower emergence in 10 varieties of sugarcane. - Flower emergence (yo) No. plants No. plants showing signs outside greenhouse Varieties treated of flowering 49.6 F (75-78 CP CP CP CP CP CP NCo L L L of 10 varieties completely failed to emerge under the prevailing outside temperatures (50 F). All varieties showed emergence, ranging from 2'0-75%, under the higher temperatures in the greenhouse. DISCUSSION Forty-five days was selected as the minimum duration of inductive photo-

10 E. D. PALIATSEAS period for these studies. Preliminary experiments showed that a 40-day induction treatment was insufficient to bring about flower initiation under these experimental conditions in Louisiana. Insufficient induction (40 days or less) resulted in either continued vegetative growth or bunch top, a condition common when insufficient induction is given to sugarcane. The data from the experiments reported here indicated that the minimum time required for floral initiation was between 45 and 55 days for easy flowering varieties and between 60 and 70 days for reluctant flowering ones. The minimum time was determined not only by the number of days of continuous induction treatment but also conditions that followed. The 45-day minimum for easy flowering varieties and the 60-day minmum for the reluctant flowering varieties was lengthened if treatment was followecl by long days but was insufficient if treatment was followed by short days. The data show that there is a different effect on the time required for initiation if the minimum inductive period is intercalated with long days, short days or low temperature nights. Intercalated minimum treatments (45 days) by long days resulted in complete inhibition of flowering. Intercalation of longer inductive treatments by long days diminished but did not completely inhibit initiation. Apparently some stimulus was annulled or negated by the intercalated long days. There was insufficient stimulus to induce initiation by the plants in the minimum 45-day induction treatment after intercalation. The amount of stimulus was still sufficient in some of the plants after intercalated longer inductive treatments. However, flower initiation occurred after a 45-day photoperiod intercalated with 5 short days. Apparently no stimulus was annulled by the intercalated short days. Apparently low night temperatures intercalated with a minimum inductive period did not annul or negate any flower stimulus. Initiation occurred as readily in the intercalated 45-day treatment as in the 45-day continuous treatment. Furthermore the results suggest that stimulus was produced during the cold nights since some initiation occurred in a 40-day intercalated treatment. The data on the effect of long and short days on flower emergence indicated that long days not only adversely affected flower initiation but also adversely affected floral development (elongation and emergence). Flowering was far superior if a 90-day inductive period was followed by 408 short days than by 40 long days. Unless inductive day lengths are applied until tassel emergence, the breeder must be concerned about photoperiodic conditions that prevail in the area when inductive treatments are concluded. Exposure of treated canes to longer natural day lengths should be avoided. Exposure of treated canes to shorter (sub-inductive) natural day lengths like those prevailing in subtropical areas in October and April (for Northern and Southern hemispheres) should be preferred. Furthermore, exposure of photoperiodically treated canes to low minimum temperatures (below 60 F) should be avoided. REFERENCES 1. Allard, H. A Behavior of some plants which are unable Lo flower or flower less readily when the days are either too long or too short. J. Agr. Res., 57: Burr, C. O., C. E. Hartt, B. T. Tanimoto, H. P. Kortschak, D. Takahashi, P. M. Ashton, and R. E. Coleman The sugarcane plant. Ann. Rev. Plant Physiol., 8:

11 364 BREEDING AND GENETICS 3. Chilton, S. J. P., and C. F. Moreland Experiments on the flowering of sugarcane. Sugar Bull., 32: Coleman, R. E Factors involved in the flowering of sugarcane (Saccharum spp). Proc. ISSCT, 10: Coleman, R. E Control of flowering and the use of pollen storage as techniques in a sugarcane breeding program. Proc. ISSCT, 11: Coleman, R. E Effects of temperature on flowering of sugaicane. Intern. Sugar J., 65: Coleman, R. E Effect of intercalaled non-inductive nights on floral initiation in sugarcane. Phyton., 22: Coleman, R. E Physiology of flowering in sugarcane. Proc. ISSCT, 13: Clements, H. F., and M. Awada Experimenls on the artificial induction 01 flowering in sugarcane. Proc. ISSCT, Daniels, J Improving sugarcane breeding methods to increase yield. Proc. ISSCT, 13: Dutt, N. L., and N. D. Yusuf Control of cane arrowing. Current Sci., 14: Garner, W. W., and H. A. +Allard Effect of the relative length of day and night and other factors of the environment on growth and reproduction in plants. J. Agr. Res., 31: Lee, S., and IC. A. Lin Studies of the photoperiodic effects on sugarcane. Proc. ISSCT, 7:33-43.