( 1984) pp.lt-20 ICKEI"L L.G. A Review of Plant Growth Regulators in the Sugar Cane Industry.

Transcription

1 s ( 1984) pp.lt-20 ICKEI"L L.G. A Review of Plant Growth Regulators in the Sugar Cane Industry.

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3 A Review of Plant Growth Regulators in the Sugar Cane Industry L.G. Nickell Plant growth regulators (PGRs) have been used in the sugar cane industry for over two decades to increase the production of sucrose in sugar cane. The first commercial success was in the prevention of flowering. followed by the application of gibberellic acid (GAl) for the increase of stalk elongatio-n which utlimately resulted in increased sugar production. Currently. the greatest interest centers around the use of chemical compounds for the control of maturation. the so-called chemical 'ripeners'. In addition. research has been successful in affecting both germination of the vegetative 'seed pieces' used for propagation and on the tillering of young plants of sugar cane. Because of the importance of and the success with ripening control. these latter two uses have not been pursued rigorously during the past few years. Consequently. they are not at the commercial stage at this time. It is recognized that the sugar industry throughout the world and especially in Hawaii. is using chemicals of the PGR type at almost every stage of the development of the crop. The author presents herein the use of PGRs sequentially. as they would be used in the development of the crop. i.e., from planting through to harvest. l.g. NickeD is Vice President of Research and Development with Veliscol Chemical Corpora- tion. Chicago. Illinois. Permission to excerpt certain passages has been granted by CRC Germination. Because of the problems involved in emergence of sugar cane after planting (including such factors as depth of planting, angle of the bud, adverse weather conditions, particularly excess moisture and cold, and fungal or bacterial infection), it has always been the aim of sugar cane planters to try to obtain a 'good stand'. To obtain a good stand. germination and rapid early growth is required, i.e., emergence of the first green leaf must be adequate. In an empirical search through many compounds looking for those which might stimulate the rate of young sugar cane plant. it was found that the amino acid. arginine. caused a considerable stimulation of germination. This aminio acid was also shown to cause an increase in the growth rate of the plant at later stages. resulting in increased production of both cane and sugar. Additional studies showed that the effect is evident at the cellular level. i.e., cells of sugar cane grown in suspension culture respond rather dramatically to arginine. Subsequently. considerable work has been done with arginine in relation to its effects on sugar cane growth and metabolism. The dramatic results obtained with arginine suggest that. if these findings could be exploited commercially, they could easily lead to one of the greatest increased in sugar production yet achieved by the application of chemicals. However, in order to achieve this much more whole-plant physiology and field work must be carried out. Tillering. A tiller is a branch (stalk) arising from the base of the plant or from the axil of a lower leaf, and is a general characteristic of grasses. The manner of tillering provides a means for dividing grasses into two groups: tufted grasses and sod formers. In sod formers, there is intense underground branching which permeates the soil. These underground branches combined with the root system form a coherent mat which is found in lawns and in permanent pastures. In tufted grasses, which include sugar cane, the underground branching is limited and is followed by the formation of a number of erect stalks, which makes individual germination or early growth of th~ Press. Inc.. Boca Raton. Florida. MARCH 1984 &~~ - ~ plants (in clumps) easily distinguishable. Of the many variables involved in the production of sugar from the cane plant, probably the most significantly related factor is the number of stalks per unit area of land at harvest. To obtain the maximum number of stalks support,able by an area of cane, it is necessary to induce branching at an early stage. A search for compounds that would induce tillering in sugar cane was started several years ago and several compounds, such as ethephon, have been found to be active. Recent results from Jamaica confirm the Hawaiian results in response to ethephon. Two of the three cultivars studied tilkr~d freely in response to ethephon treatment; the third did not tiller, regardless of treatment. Results from Brazil show no effect of ethephon in the plant crop but a significant increase in the number of millable stalks in the ratoon. In the Philippines, both ethephon at high concentrations and Bualta at low concentrations significantly stimulated tillering while in Taiwan, ethephon and chlormequat were effective. To date, very little has been done to carry these studies beyond the screening stage. Stalk Elongation. The control of plant and organ size can be of great importance in agriculture. If maximum weight, length, or diameter affects final yield, then an increase in size is desirable. On the other hand, if it can be of commercial benefit, it may be important to be able to reduce the overall size of the plant. The elongated 'foolish seedling' effect in rice, caused by infection with the fungus Gibberella [ujikuroa, has been known for many decades. It was not until 1938, however, that a metabolite of this fungus was isolated and shown to be the causative agent of the disease. The isolation, crystallation, and structural determination of this material in

4 1938, led to the discovery of a new class of hormones, the gibberellios. In most plants, the outstanding effect of the gibberellins is to elongate the primary stalk. This effect occurs in the young tissues growth centers and is caused by an increase in cell length, an increase in the rate of cell division, or a combination of both, depending on the specific types of plant treated. Although the gibberellins can induce flowering in many plant species, a significant commercial use has been in stimulating the growth of sugar cane to increase the length of the primary stalk. During the 1950s when experimental amounts of gibberellic acid and related gibberellins became avajlable, they were applied to sugar cane, and stalk elongation was demonstrated. Because of the high cost of crystalline gibberellic acid (GA J ) and lack of availability in quantity, more than a decade passed before such materials were evaluated to determine whether or not they would increase crop production on a commercial scale. During.the early studies of this class of plant hormones, more and more potential uses became apparent. Consequently, fermentation companies produced larger amounts of gibberellins, and eventually unrefined fermentation liquids became available for evaluation, at greatly reduced costs. These products were shown to increase cane tonnage per unit area, particularly when applied during the cold or winter months wlrerrgrowth is slowed aown considerably. Gibberellic acid has been evaluated on sugar cane since the mid-1950s and evaluated under commercial field conditions since the mid Since the yield increases are relatively small compared to the total crop yield, they are difficult to measure in the field. As a result, there has been a trend toward evaluating the effects on the basis of individual stalks or on those portions of the stalk showing the greatest responses. Taking into consideration the high variability among plots under most sugar cane cultural conditions, it has been calculated that at least sixty plots would be necessary to give an 85 per cent probability of measuring a 5 per cent yield difference at the 5 per cent level of significance. Recently such a yield trial was carried out in Hawaii on the sugar cane cultivar HS9-377S. In this experiment, half of 120 plots, each ISO m I, were sprayed with two application of 70 gallons per hectare of gibberellic acid. The results showed that GAl significantly increased the yield of sugar cane by 3.7 per cent. Sugar yields were increased by 2.8 per cent and there was no effect of GA J on the quality of the cane or juice. 18 Flowering. Control of flowering in one of the most important practical aspects of horticulture and agriculture. With many horticultural crops, the key to financial success is the capability to induce flowering and more importantly, to induce it on command in order to meet certain major market and holiday dates. Conversely, the ability to prevent flowering is extremely important in agricultural crops when flowering causes a decreased economic benefit. To improve yields, it is commercially worthwhile to prevent flowering in some crops, such as sugar cane. The flowering process in sugar cane is extremely sensitive to the environment. This is true of flower initiation, emergence and pollen fertility. The optimum photoperiod appears to be about 12.5 hours and most commercial varieties respond to this throughout the world. The 12.S hour day occurs about 2 September in Hawaii. and night interruption experiments ndicate this is the approximate date on which initiation begins in that state. For most commercial varieties studied so far, initiation is usually complete by 20 September. The highly reproducible short period of floral initiation makes possible the suppression of flowering in young cane on a commercial scale. Experiments in Hawaii have established that flower inhibition by night lights, chemical sprays, or the withdrawal of water can be expected to increase yields by J 0-20 per cent under conditions of heavy flowering. After it had been determined that night interruption from 1-20 September would inhibit flowering (commonly referred to as tasselling or arrowing in the sugar cane industry) for the varieties worked with in Hawaii at the time. field experiments were carried out to determine what measurable effect flowering might have on the yield of sucrose. The 1949 results of a replicated field trial of ten paired plots using a current commercial variety supported the belief, long held by sugar growers, that flowering reduces sugar yields. In this field trial, t!je average gain from suppression of flowering was IS per cent the equivalent of a sucrose yield of 3.2!Dnnes per hectare. In the years immediately following these st udies, the factors affecting flowering were extensively studies as were methods for preventing its occurrence. Effective ways found to prevent flowering included: night interruption with light, lower temperature, leaf and spindle trimming, withdrawal of water, and application of chemicals. Because temperature cannot be controlled in the field and because leaf trimming and light interruption on a commercial scale were not operationally feasible, emphasis was placed on water withdrawal and application of chemicals. Withdrawal of water was possible only on irrigated plantations and, because such a practice had other operational problems, using chemicals eventually became standard practice in Hawaii. The first potentially useful commercial chemical was maleic hydrazide, which gave about 60 per cent control at best. Rapid developments led to the establishment of first monuron, and then diu ron as the most popular chemicals. Properly applied, 4.5 kilograms per hectare of either chemical gave virtually complete control of flowering in the heavy-flowering varieties used in Hawaii during the 19SQs and Continued testing for lictive chemicals, to prevent flowering, led to the discovery that diquat applied from the air at rates as low as 140 gallons per hectare of the cation form was as active as monuron at 4.S kilograms per hectare, making diquat one of the most active compounds yet evaluated for this substantia!. Tests included both the heavyflowering varieties in use at that time, and the ligher-flowering varieties that were emerging commercially in the late Positive effects with chemical control of flowering, using diquate, have been obtained in Guyana, Mexico, the Philippines and Taiwan, as well as in Hawaii. Ripening. Compounds that affect crop metabolism, particularly those that regulate crop maturity, are especially likely to have a dramatic impact SUGAR Y AZUCA~

5 on agriculture in the years ahead. Several such compounds are already used commercially and their success is largely responsible for the increased interest in PGRs in agriculture. Most of the compounds used on economic crops have a direct or indirect effect on final yield, on quality, or on both. Ripening is considered one of the most important aspects of sugar cane production, from both a research and an operational point of view. To say that the phenomenon of cane ripening is extremely complex would be, at best, a gross understatement. Studies on the use of PGRs for ripening have appeared in the literature spasmodically since The first material reported to be effective was 2,4-0, which was followed by studies with maleic hydrazide, triiodobenzoic acid, dalapon, CMU, DCMU, EDTA, Trysben, Pesco 1850 (a mixture of MCPA and Trysben), as well as a number of enzyme inhibitors and metabolic inhibitors. No large-scale program was launched, however, until basic studies on the effects of defoliation on translocation in sugar cane had furnished a solid basis for such a program. The screening test used is relatively simple, consisting of adding the test material by pipette, or by needle and syringe, into the whorl of leaves at the top of the sugar cane stalk, which is field-grown and almost at the stage of normal maturation. At a specified time or times (4,5 and/or 8 weeks) after application of the test material, 5-10 stalks are harvested, analyzed, and compared with an untreated group of stalks. The effectiveness of a test compound as a ripener is based on its ability to increase the quality of the treated stalks in two major parameters for sugar production (juice purity and sugar as a percentage of field cane weieht). Although attempts were made for several decades to control the ripening of sugar cane by the use of chemicals, no concerted effort was made until the start of a research program in Hawaii in the early 1960s. This effort was soon joined by investigators in Australia and Trinidad. The initial success resulted in extensive field testing throughout the sugar cane world. Originally, very few commercial companies were involved; chemicals used were primarly those available from chemical supply houses together with the few materials synthesized by research organizations in the sugar cane industry. The initial success led a number of companies to become interested in supplying chemicals for evaluation, and the end result is a surprising number of chemicals that increase the sucrose content of sugar cane at harvest. Some compounds were never developed beyond the initial screening stages; others are two new to have MARCH.1984 been reported other than through an initial publication or an issued patent. The first material seriol1sly considered as a potential ripener, for increasing sucrose yields of sugar cane, was the dimethylamine salt of 2,3,6-tri-chlorobenzoic acid. Because of a number of technical, environmental, and legal problems, this material did not prove successful commercially. Nevertheless, it served as a standard for comparison in screeing tests aimed at finding better sugar cane ripeners. It continued to be the standard for comparison until the registration of the first ripener for sugar cane in the United States namely, N,N-bis(phosphonomethyl) glycine, known generically as glyphosine and marketed by Monsanto as the product Polaris. Polaris has been evaluated over a period of several years on close to 40,000 hectares in Hawaii and other sugar-producing areas and has given substantial yield gains of about per cent, which results in an increase of 2.5 tonnes per hectare or more sugar when applied to certain varieties grown on the rainy coasts of the Island of Hawaii. More recent work has shown that varieties previously thought to be non-responsive to this ripener have been found to respond positively when surfactants are added to the formulation. Similarly, it has been found to be effective on irrigated areas when surfactants are added. Glyphosine treatment results in a reduced rate of terminal cane growth, but how this relates to its mode of action has not yet been established. Until late 1980, Polaris was the only compound registered for this use in the USA. In the Autumn of 1980, phosphonomethyl glycine was also register~d as a ripener for sugar cane. This compound, known generically as_ glyphosate, is marketed by Monsanto as the product Polado; it is the sodium salt of the active ingredient of the herbicide Round-Up. As a ripener, glyphosate is almost an order of magnitude more active than glyphosine. Glyphosate formulations improve the sucrose content over a wide range of climatic conditions, are less cultivar specific, and the ripening response they induce in sugar cane is more consistent and rapid than that obtained with glyphosine. Three other chemicals: Ripenthol, chlormequat, and mefluidide have been registered under experimental labels in the United States for field evaluation as commercial ripeners. Ripenthol, the monoamine salt of Endothall, was one of the first materials found to have significant activity on sugar cane in Hawaii. Numerous relatives of this compound were tested in the early screening stages, and it was found that, although the acid itself had very low activity, amine salts were more active than di-substituted arnines. Ripentho! (also know as Hydrothol) has considerable phytotoxic activity and, because of this, care must be taken in its application, especially to avoid drift when applied by air. Chlormequat (2-chloroethyl-trimethylammonium chloride), also know as Cycocel, is among the most widely used PGRs in the world on crops other than cane. It has been evaluated on more than 400 hectares of sugar cane in Hawaii, but preliminary results suggest that its activity may be too low to be commercially successful. Mefluidide, also known as Embark, is being tested at the present time under an experimental label in.hawaii, the Philippines and certain other countries. The ethylene-producing compound ethephon is used commercially in sugar cane in South Africa and Rhodesia. The effectiveness of ethephon as a sugar cane ripener has not been comparable to that of glyphosine in some areas of the world, although its effects on growth result in an increase in yield. These effects are currently being evaluated in reserch programs in the sugar industry. The number, and diverse chemical nature, of compounds found to be active as sugar cane ripeners suggests that there are several modes of action to enhance the ripening of sugar cane. There are also varietal differences, in addition to differences due to (a) the fertilizer status (particularly nitrogen), (b) the age of the crop and its condition, (c) the climate (both during the growth of the crop and prior to harvest), (d) the physiological state of the cane, and (e) the purity of the juice in the young growing tops. These variables, and probably many others, suggest that there is scope for a number of sugar cane ripeners. Additional variables to be considered are (a) phytotoxicity of the ripeners, (b) the cost effectiveness of the compound under consideration, and (c) the effects on the processing of sugar cane. The effect on growth of subsequent ratoons is an extremely important consideration for registration as well as long-term use of any ripener. Recent comparative studies with four active sugar cane ripeners: glyphosine, glyphosate, ethephon, and mefluidide, have demonstrated two models for increasing sucrose per stalk in sugar cane. Glyphosine and glyphosate increase sucrose per stalk by increasing the partitioning of dry matter toward sucrose storage and away from fiber production. Both mefluidide and, to a much larger extent, ethephon increased the production of sucrose per stalk by increasing the total amount of dry matter produced, with a greater portion

6 incorporated as fiber and a lesser amount as sucrose. Probably because glyphosine was the first registered sugar cane ripener, the use of this compound increased dramatically, at least in the sugar areas of the U.S. From its registration in 1972 and experimental use on a few hundred hectares, it reached over 25,000 hectares in Hawaii by A similar situation occurred in Florida, increasing from 72 hectares in 1972 to more than 18,000 hectares in Because of the greater activity and considerably lower costs with glyphosate, it is expected that glyphosine will be replaced very quickly. Ethephon is said to be much more effective than Polaris in southern Africa, whereas the reverse is true in Hawaii and other places where two have been compared. Glyphosate has been found to be reasonably active in the sugar cane-growing regions of southern Africa it is expected that it will be an effective competitor for ethephon. Interest seems to be declining for three materials that received early consideration: chlormequat, disugran and Ripenthol. In several major sugar growing areas, mefluidide appears to be losing poplarity, but it is being more seriously evaluated in tropcial countries such as the Philippines. The finan<;ial return to the grower is substantial through the use of ripeners; increased sugar yield produced by such compounds can be as much as 20 per cem, depending on the variety of sugar cane treated as well as on prevailing weather and soil conditions. In fact, chemical control of maturation in sugar cane is now such a well-established practice, that many research organizations in the sugar industry are shifting part of their efforts to investigation of other stages in the development of the sugar cane crop, for additional potential uses and times of chemical treatment. In particular, ethephon may have potential for economic use, not unlike that found for gibberellic acid. Desiccation. Preharvest burning or 'detrashing' is a common practice, in sugar cane culture, designed to lower the percentage of extraneous material shipped to the factory as well as to facilitate harvest procedures. When leaves accompany stalks during the milling operations, the recovery of sucrose is decreased. In addition to (a) the extra costs of harvesting and transportation due to trash, (b) the extra fiber contributing to the milling operation without sugar, and (c) the impurities in the trash which contribute to low juice purity, the presence of trash leads to greater insect infestation, larger rat 20 populations, and reduced effectiveness of pre-emergence herbicide treatments. The importance of eliminating trash before milling operations is widely recognized, but is seldom accomplished to the satisfaction of either growers or factory superintendents. Buring has been the traditional means of eliminating trash. However, burns are seldom uniform, seldom completely effective and, under humid conditions, are often ineffective. One method of accomplishing trash removal would be chemical defoliation. Such compounds are commonly used on other crops and often the process is facilitated by the sensitive abscission layer found at the base of the petiole. No such structure exists in sugar cane. As in most grasses, leaf fall is usually prompted by mechanical disturbance after the death or partial decomposition of the leaf sheath which clings tenaciously to the stalk. A more practical means of leaf removal in sugar cane would be by desiccation of the attached leaf and its removal by fire. Systematic searches were carried out during the 1940s and 1950s, with a number of compounds, and several decades of testing have shown that the herbicides diquat and paraquat are the most effective chemicals for desiccation. In recent years, these two compounds have become almost synonymous with the term 'desiccant' in sugar cane-growing regions. The advantageous properties of these two herbicides are (a) low mammalian toxicity, (b) solubility in water, (c) rapid absorption, and (d) inactivation upon contact with the soil. Currently, after more than two decades of evaluation of these quaternary ammonium herbicides, paraquat has emerged as the preferable compound. Unfortunately, the use of paraquat has failed to produce consistent effects as a desiccant, with results varying from no effect on sucrose production and yield to slight negative effects. However, as higher tonnage varieties are developed and with the persistent losses in both harvesting and milling, there has been renewed interest, particularly in areas using mechanical harvesters, in pre-harvest desiccation with paraquat. Conclusions. Traditionally, the world wide sugar industry has supported a multi-pronged investigation of the activity of chemicals on most of the stages of cane development, from germination through to ripening and harvest. Although the use of PGRs is still in its infancy, success to date with ripening alone, in terms of yield increases greater than 10 per cent substantiates the belief that the regulation of crop growth and metabolism may result in one of the most important quantitative gains yet achieved in agriculture. The monumental task of producing raw materials to supply the world's food and to supplement its energy requirements may depend to a large degree on achievements of this magnitude in a wide range of crops. Hawaii, with its high costs of operation and high yields of sugar cane, nonseasonal environmental conditions, and the necessity to harvest the year-round, can afford high-priced chemicals. In other cane-producing countries this may not be the case. For example, Australian investigators were among the first to study the use of chemical ripeners. In fact, during the early I 960s, there was a cooperative program between Hawaiian and Australian workers on this subject. This was not pursued when it was realized that Australian conditions, with cool and dry weather at harvest, were for the most part conducive to excellent natural ripening. In other cane-growing areas in the world, however, as more knowledge is gained about the relationship of a given chemical to the process which it affects, and as sugar cane agronomy improves and its economy becomes more favorable, the use of chemical ripeners wiii undoubtedly become more widespread; essentially the same can be said for the use of gibberellins and chemicals for flower control and other uses. Historically, agricultural research has been primarily concerned with improvement of total crop yield by the removal of limitations to optimal production. Now that many of these limitations can be overcome with herbicides, pesticides, fertilizers, irrigation and improved management practices, the stage is set for further yield increases by the use of sophisticated techniques of physiological manipulation of the plant and its metabolism. SUGARyAZUCAR

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