THE USE OF ETHEPHON FOR PREVENTION OF FLOWERING IN SUGARCANE IN SUDAN

Plant Physiology THE USE OF ETHEPHON FOR PREVENTION OF FLOWERING IN SUGARCANE IN SUDAN G. Hardy, H. Dove, Africa Sugar Consultants Limited, Mauritius M. Awad, Kenana Sugar Company Limited, Sudan ABSTRACT In experiments carried out at Kenana Sugar Company, Sudan, ethephon [(2-chloroethyl) phosphonic acid] applied at 840 g a.i.lha at one to three weeks before flower initiation provided excellent control of flowering in sugarcane varieties Co527 and Co6806, aged five to seven months at initiation. Increased yields due to flowering control attained 30 tons canelha (+ 33%) and 4.1 tons sugarlha (+ 36%) when variety Co527 was treated at five months. The extent of response to treatment varied with the date of application of ethephon and the age of cane at initiation. Responses were mostly due to increased growth and reduced pithiness. Quality of treated cane was improved, with lower fibre and sometimes, higher sugar content. INTRODUCTION Important environmental factors influencing flowering in sugarcane include day length, moisture and temperature. Day length is possibly the most important factor in determining when initiation takes place, while the other factors, moisture and temperature, affect the intensity of flowering (Moore8). The initiation of flowering for most commercial varieties takes place within fairly narrow parameters of day length, namely 12h30 and 12h35 (Brett, Harding3 and Brett4). It has been shown in Hawaii that high moisture and high temperature during the pre-initiation stage result in a high intensity of flowering (Anon.'). Flower initiation will not occur in sugarcane unless the cane plant has completed its tillering phase and is physiologically able to receive the photoperiodic stimulus, a stage which is normally reached approximately three to four months after harvest and four to five months after planting. Flowering could be beneficial during the early part of the harvest season, when there is a relatively short interval between flower initiation and harvest, by enhancing maturity and thus improving the sucrose content of the crop. However, adverse effects on yield can be expected when the crop is harvested from four to five months or more after flower initiation. Most of the adverse effect is due to a loss of cane yield because of excessive development of pithy tissue within the stalks. In certain varieties, poor cane quality has also been associated with high intensity of flowering. Keywords: Ethephon, sugarcane flowering, flowering control, cane fibre, cane quality. 305 i i (/

306 PLANT PHYSIOLOGY Milling performance could also be adversely affected by an increased fibre content of flowered stalks. The control of flowering in sugarcane using growth regulating chemicals has been practised for many years. Experiments carried out in various parts of the world from about 1965 to 1975 showed positive effects using diquat, which became the preferred chemical for flower control until recently (5,2,6,12,7). However, diquat has the disadvantage of causing severe scorching of the crop foliage, which can lead to adverse effects on sugar production. The inhibition of flowering with ethephon was observed in about 1976 (Rostron"). When applied at the correct time before flower initiation, ethephon inhibits the process of initiation without detrimental effects on the growth of the crop. Good control of flowering was obtained in trials carried out in Hawaii (Osgood9) and trials in Brazil and in Malawi were also conclusive. Ethephon is now used commeri' cially in these three\countries for the inhibition of flowering. A veiy severe/ffowering problem exists at Kenana Sugar Company Ltd and in other parts of the Sudan sugar industry, where the main variety grown, Co527, flowers intensively. Kenana Sugar Company has an area of 35 280 ha under cane. It is located at latitude 13'05' N and at an altitude of 410 m above sea level. The dry season (November to April) is characterised by a mean diurnal temperature range of 17 'C to 36 "C. The wet season (May to October) is characterised by a mean diurnal temperature range of 22 "C to 34 "C and an average rainfall of 325 mm. The harvest season is from November to April. The main flower initiation period occurs between 13th and 20th August. Because of the relationship of time of flower initiation with the harvest season, approximately 55% of the cane grown on the estate is prone to adverse effects from flowering, as illustrated in Figure 1. lnltiation Matur~ty Yield perlod benefit decline J J A S O N D J F M A M 1 - - - - - - - - - - - - - - - - - - - Harvest FIGURE 1. Possible effects of flowering on sugar production at Kenana, Sudan. This paper reports the results of six field experiments conducted at Kenana in 1984-85 in an investigation of the effect of ethephon on flowering and on yield. MATERIALS AND METHODS Five replicated small-plot trials and one large-scale trial were conducted to assess the efficacy of ethephon in controlling flowering and to quantify the benefits derived

Chemical Ethephon Ethephon Ethephon Rate 840 g a.i. ha-' 840 g a.i. ha-' 840 g a.i. ha-' Application Ground Ground Aerial Treatment Applied o n 1 25 Jul, 1984 18 Aug, 1984 4 Aug, 1984 2 28 Jul, 1984 22 Aug, 1984 10 Aug, 1984 3 1 Aug, 1984 26 Aug, 1984 Control 4 5 Aug, 1984 30 Aug, 1984 5 9 Aug, 1984 Control 6 Control Control Experiments 1 to 5 Ethephon was applied at 840 g a.i./ha using a knapsack sprayer fitted with an overhead boom carrying 3 TK 1.5 flood-jet nozzles. These covered an effective swath of two cane lines at a row spacing of 1.67 m and delivered 113 L/ha. Treatments in Experiments 1-4 consisted of five dates of application of ethephon, at intervals of three or four days over the period July 25 to August 9, 1984 (shortly before flower initiation) and an untreated control. In Experiment 5, there were four dates of application, at intervals of four days, covering the period approximately

308 PLANT PHYSIOLOGY at and shortly after flower initiation. The design in all these experiments was a 6 x 6 latin square. Nett plot size was four cane lines, 8 m long (53.4 m2). Experiments 1,2 and 3 were harvested at 11.3 months, 11.8 months and 12.5 months respectively. Experiments 4 and 5 were harvested at ten months. All these trials were harvested by hand. A tractor-mounted mechanical grab fitted with a scale was used for weighing the yields of individual plots. Cane mass, stalk population and the percentage of flowered stalks were determined for individual plots. Twenty representative stalks per plot were taken for analysis of cane quality. Experiment 6 In this large-scale experiment there were two dates of application, August 4 and 10, and an untreated control. Ethephon was applied at 840 g a.i./ha using a fixed-wing aircraft delivering 30 L/ha. The effective treated area for each of the sprayed plots was 600 m x 25 m (1.5 ha). There were two replications per treatment, with an untreated control area of 600 m x 50 m (3.0 ha) on both sides of each treated area. The trial was harvested at 12.3 months. Three 'data' plots, each consisting of four cane lines, 8 m long (53.4 m2), were harvested within each of the two replications of the sprayed plots. Six plots were also harvested from the untreated control areas. Cane mass, stalk population, percentage of flowered stalks and sucrose content were determined as in Experiments 1 to 5. Data were analysed as a nested (hierarchical) design of two replications. RESULTS AND DISCUSSIONS General effect of ethephon on growth Within a few days following the application of ethephon, the cane foliage turned to a yellow-orange colour, which seemed to be more pronounced on the lower leaves. This effect was short-lived and disappeared within two weeks, after which the production of green leaves was restored. A reduction in length of the leaves emerging after application was noticeable, with an average of six to eight leaves being affected, after which growth was back to normal. A premature senescence of the lower leaves was noticable and this was slightly more pronounced in variety Co6806. Ethephon also affected the development of the internodes on both varieties. There was a reduction in length of one internode only, after which the length of subsequent internodes exceeded the length of internodes below the affected one. A few stalks produced side-shoots from nodes in the area of the stalk around the shortened internode. The side-shoots grew to only 2 to 10 cm in length before dying off (Figure 2). Similar effects of ethephon on growth have been reported previously (Rostronl0). Experiment 1 Results from Experiment 1 are shown in Table 111. There was a marked reduction in the intensity of flowering with a mean of 14% fbr the first four dates of application, compared to 93% in the unsprayed plots. The increased incidence of flowering obtained by the last application, 52%, might indicate that the period of flower initiation was well defined and that this application was too late to be highly effective.

G. HARDY, H. DOVE AND M. AWAD 309 / FIGURE 2. Effect of ethephon on growth of Co 527 a: shortening of internode b: side-shooting average gain of 18.1 tons/ha (17%) for the first four application dates, even though the trial was harvested somewhat ear1y:at 11 1.3 months. There was a slight but not significant reduction in sucrose content in the cane in the treated plots; from 12.1 to 11.4% ERS (estimated recoverable sucrose). This may have been due to the relatively short drying off period of three weeks, which was probably to the disadvantage of the treated plots. The benefit of higher cane yield was somewhat depreciated by the reduced ERS 070 cane and although there

310 PLANT PHYSIOLOGY TABLE Ill. Effect of ethephon on Co 527,7 months old at initiation (Experiment 1) Date of Cane ERS ERS Fibre Flowering application tonslha % cane tonslha % cane % stalks Jul 25 126,9** 11,82 Jul 28 126,9* * 10,99 Aug 1 124,0** 11,16 Aug d 125,7** 11,76 Aug 9 120,5* * 11,49 Control 107, f1 12,08 ERS = estimated recoverable sucrose. * Significant at 5% level of probability. ** Significant at l0/0 level of probability. was a mean increase of 1.4 t ERS/ha (11%) for the first four spraying dates, differences between the treatments in terys of sugar were not significant. There was a highly significant reduction in the fibre content of the cane for the first four dates of application, from 18.6% to a mean of 16.4%, with a trend of increasing fibre content from the first to the last date of application. Experiment 2 This trial site was accidentally burned and an accurate assessment of the intensity of flowering could not be made. However, from spot checks on a few plots and from the yield results, it is obvious that a high degree of control of flowering was achieved by most of the treatments. All treatments resulted in significantly higher yields of cane compared with the unsprayed plots, with an average increase of 17.6 TABLE IV. Effect of ethephon on Co 527,6 months old at initiation I(Experiment 2). Date of Cane ERS ERS Fibre application tonslha % cane tonslha % cane Jul 25 121,9** 13,77* 16,79** 17,92 Jul 28 117,4** 12,92 15,17** 18,30 Aug 1 120,2* * 13,05 15,69** 17,51 Aug 5 113,6** 12,74 14,49 17,69 Aug 9 113,6* 12,85 14,60* 18,74 Control 100,7 12,87 12,96 18,80 ERS = estimated recoverable sucrose. * Significant at 5% level of probability. ** Significant at 1 % level of probability.

G. HARDY, H. DOVE AND M. AWAD 311 I (35%). tons/ha (17%) for the first four applications. Reduced benefit from the applications made nearer the time of initiation was again evident (Table IV). The treatment with ethephon had no general effect on cane quality in this experiment, but there was a significant increase in ERS % cane from treatment on the first date of application. Treatment on the first four dates of application resulted in highly significant increases in yield of sugar, with a mean increase of 2.6 tons sugar/ha (20%). The greatest increase in yield of sugar, 3.8 tons/ha (30%) was obtained from the first treatment. Experiment 3 In Experiment 3, in which the cane was approximately five months old at flower initiation, there was almost complete control of flowering in all cases, and there was no trend of reduced control of flowering when ethephon was applied near initiation, as was observed with Experiments l and 2 (Table V). It appears that younger cane is more sensitive to treatment with ethephon. Treatment with ethephon on all five dates provided highly significant increases in cane yield, averaging 29.7 tons/ha Cane yields were similar in all the treated plots, reflecting the uniform control of flowering. TABLE V. Effect of ethephon on Co 527,5 months old at initiation (Experiment 3). Date of Application Cane tonslha ERS % cane E RS tonslha Fibre % cane Flowering % stalks Jul 25 Jul 28 Aug 1 Aug 5 Aug 9 Control 17,43 16,38 17,69 16,81 17,55 18,87 ERS = estimated recoverable sucrose. * Significant at 5% level of probability. ** Significant at 1 % level of probability. There was a significant increase in the sucrose content of cane in the plots treated on the last date of application and in general, sucrose content was increased slightly. All treatments resulted in highly significant increases in yield of recoverable sugar; such increases were greatest for the last two dates of treatment and the mean increase for all treatments was 3.9 tons/ha (39%). As in Experiments 1 and 2, the treatments showed a tendency fok reduced fibre content. Experiments 4 and 5 As the yields of the control plots in Experiments 4 and 5 on variety Co6806 were similar, the data from these two trials were combined and analysed as one experiment. The results are summarised in Table VI.

TABLE VI. Effect of ethephon on Co 6806,5,5 m6nths old at initiation (Experiments 4 and 5). Date of Cane ERS ERS Fibre Flowering application tonslha % cane tonslha % cane % stalks Jul 25 Jul 28 Aug 1 Aug 5 Aug 9 Aug 18 Aug 22 Aug 26 Aug 30 Control ERS = estimated recoverable sucrose. * Significant at 5% level of probability. ** Significant at 1 % level of probability. Excellent control of flowering was obtained by all the ethephon treatments throughout the period up to harvest. From the responses in cane yield that were obtained, it appears that the treatments between July 25 and August 9 in Experiment 4 were applied before flowtl ~~~itiation and that the later treatments in Experiment 5 were probably applied during and after flower initiation (Figure 3). If this is so, the good control of flowering in Experiment 5 must have been due to the failure of initiated flowers to develop, rather than inhibition of flower initiation.? Initiation - c cb Jul Aug 0 Application dates FIGURE 3. Effect of ethephon applied before and after flower initiation on flowering and yield of Co 6806. It was observed that flower emergence for untreated Co6806 occurred approximately four to six weeks later than that of Co527. Also, the observable effects of

G. HARDY, H. DOVE AND M. AWAD 313 ethephon treatment - shortening of internodes and development of side-shoots - were generally more pronounced on Co6806 than on Co527. These suggest that the "normal" level of ethylene within Co6806 at initiation is higher than that of Co527 and it is possible that a lower rate of application of ethephon could be used to prevent flower initiation in Co6806. Despite the experiments having been harvested two months earlier than normal, a highly significant increase in cane yield was recorded, averaging 16.5 tons/ha (16%) for the first five dates of application. Even greater benefit could have been expected if the trials had been harvested at 12 months. As in the case of Experiment 1, the drying off period was very short because of the early harvest, consequently, the sucrose content was significantly reduced by the treatments applied on the first five spraying dates in Experiment 4. In Experiment 5, the treatments applied from August 18-30 had only a slight effect on sucrose content. The increases in cane yield benefit were depreciated by the reduction in sucrose content and, although not statistically significant, a mean increase of 1.2 tons/ha (9%) was obtained for the first five dates of application. It seems likely that, had the experiments been harvested at 12 months after a normal drying off period, much greater increases in yield of sucrose would have been obtained. The effect of ethephon on fibre content can best be shown by the data in Experiments 1 through 4. These experiments have common treatments, permitting an analysis of variance of fibre data by using experiments as replications. The average fibre content (18.15%) of the untreated controls was significantly higher (5% level) than that in each of the five treatments where ethephon was applied. The mean fibre content of the ethephon'treated cane in these experiments was 16.90%. Experiment 6 The results of Experiment 6, in which ethephon was applied aerially, are shown in Table VII. Excellent control of flowering was achieved with the treatment applied on August 4, but the treatment applied six days later was somewhat less successful, possibly because of inaccurate application, causing severe streaking in the treated plots. 1 TABLE VII. Effect of ethephon applied aerially on Co 527, 5,5 months old at initiation (Experiment 6). Date of Cane ERS ERS Fibre Flowering application tonslha O/O cane tonslha % cane % stalks Aug 8 131,7** 13,62 17,94* 16,32 3 Aug 10 123,8* * 14,06 17,41* 17,33 32 Control 97,6 13,73 13,40 18,ll 92 ERS = estimated recoverable sucrose. * Significant at 5% level of probability. ** Significant at 1% level of probability.

314 PLANT PHYSIOLOGY Highly significant increases in yield of cane, averaging 30.2 tons/ha (31%) for the two treatments, were again obtained. The first treatment gave the bigger response and this was associated with the greater degree of control of flowering. There were no significant effects of the treatment on sucrose content and the highly significant increases in yield of sugar reflected the increases in yield of cane. The mean increase in sugar yield from both treatments was 4.3 tons/ha (32%). Treatments with ethephon again showed a trend for reducted fibre content and this effect was greater with the first application, associated with the better control of flowering. CONCLUSIONS This series of experiments confirms the adverse effects of flowering when harvesting is delayed unduly after flowering. The experiments also show that ethephon can be used effectively to control flowering without any adverse effect on the growth of the cane, under the conditions prevailing at Kenana Sugar Company, in Sudan. The period of flower initiation for varieties Co527 and Co6806 at Kenana has been fairly well defined as occurring between August 13 and 20. Responses to treatment with ethephon, both in terms of cane and sugar yields, were greatest with younger cane at initiation. This is to be expected because of a greater degree of control of flowering when young cane is treated but mainly because of the longer period for effective growth between treatment and harvest (Figure 4). 5 mths 6 rnths 7 mths Age of cane at initiation FIGURE 4. Effect of ethephon applied to Co 527 at various ages at initiation on cane and sugar yields.

Jul Application dates FIGURE 5. Effect of ethephon applied at different dates on cane yields of Co 527. A significant relationship between fibre % cane and flowering intensity was evident from the experiments. Control of flowering therefore has a positive benefit in reducing the fibre content of cane, which is an added advantage in milling and processing. REFERENCES 1. Anon. Prediction of sugarcane flowering. Internal Communication USDA, ARS, Western Region Sugarcane Research Laboratory, 4 pp. 2. Benedicto, F. (1967). Victorias Milling Co. Exp. Stn. Bull., March-April, p.3. 3. Brett, P.G.C. and Harding, R.L. (1974). Artificial induction of flowering in Natal. ISSCT Proc. 15:55-66. 4. Brett, P.G.C., Harding, R.L. and Paxton, J.G. (1975). Time and intensity of flowering as influenced by certain temperature and photoperiod treatments. SASTA Proc. 49:202-205. Aug

316 PLANT PHYSIOLOGY 5. Evans, H. and Bates, J.F.'(1966). Proc. Br. West Indies Sugar Technol. 6. Humbelt, R.P., Lima, M., Goveas, J. (1968). Proc. 13th Int. Soc. Sugar Cane Technol. Taiwan, 462-467. 7. Moore, P.H. (1974). Flowering control with diquat. Hawaiian Planters' Record 58(23): 323-329. 8. Moore, P.H. (1983). Personal Communication. 9. Osgood, R.V., Moore, P.H., Carr, J.B. (1983). Comparison of diquat and ethephon for reve en ti on of flower initiation in sugarcane (Saccharurn spp. Hybrids). Proc. Plant Growth Regulator Soc. of America, loth Annual Meeting, 266-269. 10. Rostron, H. (1977). Prolonged chemical ripening of sugarcane following multiple applications of Ethrel. ISSCT Proc. 16, 1743-1753. 11. (1977). A review of chemical ripening of sugarcane with eth~el