Gibberellic Acid Activity in Sugarcane as a Function of the Number and Frequency of Applications

Abstract

A treatment-efficiency study was conducted in relation to growth and sugar activity of applied gibberellic acid (GA) in sugarcane. The work was performed at the greenhouse and laboratory level with plants grown in sand culture. All plants received an equal absolute amount of GA during varying periods of time. Three GA treatments were initiated at 10 weeks: 1, A single application of 0.01 percent; 2, increments of 0.0033 percent each given at 0, 10, and 20 days; and 3, increments of 0.0033 percent each given at 0, 35, and 70 days. Treatments were designed to test the theory that maximum growth must be obtained as a short-term cumulative effect, and that long-delayed increments are partially wasted against a growth reversion which follows GA growth stimulation. A second objective was to prove that GA-induced growth is accompanied by an increased sugar-synthesizing and sugar-accumulating capability, and to determine which of the frequency-concentration differentials was most effective for sugar production. Samples were taken at 2-week intervals over a period of 4 months for growth and sugar analyses. The following results were obtained: 1. Growth data for total green weights, internode elongation, internode thickening, and millable-stalk weights revealed positive effects for all GA treatments as compared to controls. 2. Maximum green weight, internode length, and stalk weight was achieved with three GA increments given over a short-time interval (0, 10, and 20 days). Widely-spaced increments produced no appreciable growth increases over those of a single application. 3. Internode elongation reached a maximum average of 8.6 inches with short-term increments, while a single GA application produced a maximum of 7.4 inches. With increments the positive GA effect extended over a timegrowth period of seven internodes, while the single-dose effect extended over four internodes. None of the treatments affected the total internode number. 4. All GA-treated plants experienced growth reversion toward subnormal levels as indicated by internode-length data. While the slope of growth decline was constant, the initiation of decline was much delayed by use of short-term GA increments. For long term-increments the 35- and 70-day applications encountered strong growth reversion already in progress. The latter increments succeeded in checking decline temporarily, while the decline itself prevented each increment from achieving its stimulatory potential. Net growth increases from delayed applications were not appreciably greater than those from a single application. 5. Reversion of internode elongation was accompanied by increased lateral growth, i.e., internodal thickening. Lateral growth was greatest for the single GA treatment. However, only with short-term increments did lateral growth compensate for losses in length. On the basis of millable-stalk weight, the most important growth criterion, the closely-spaced increments were most successful in countering growth reversion. 6. Lengthwise splitting of internodes was a common problem among plants given all GA in a single dose. This damage was seldom noted when GA was applied in increments.. Contrary to an earlier assumption, the splitting did not appear to be a function of rapid growth rate, since by far the most rapid growth was achieved with closely-spaced increments. It is concluded that weakened stalk structure results from a temporarily unbalanced growth physiology, which, in turn, is sensitive to absolute GA level at the moment of tissue penetration. 7. Brix, polarization, and direct sucrose analyses of milled juice from whole stalks showed that GA had either accomplished modest sugar increases per unit of storage tissue, or had maintained levels comparable to control plants during the 4 months of study. Coupled with the additional growth produced by GA, all GA-treated plants yielded significantly more sugar than did controls. For total sucrose production the short-term increments were superior to other GA treatments, largely on the basis of superior growth. The frequency of GA increments had less effect on sucrose than on growth. 8. Analysis of juice from individual internodes showed that those internodes already laid down at the time of initial GA treatment suffered severe sucrose losses. After about 4 weeks GA-treated cane began accumulating sucrose above control levels, even though high growth activity was progressing simultaneously. Thus the utilization of stored sucrose by the GAstimulated plants was an interim measure, one apparently designed to "hold the line" while the new sugar-forming potential could be geared to growth requirements. 9. It was concluded that GA stimulation of sugar synthesis is of sufficient magnitude to satisfy added growth needs, and to send additional sucrose into storage as well. 10. Based on earlier findings of GA-increased levels of leaf fructose and glucose, and on the lack of inversion products in GA-treated stalks, it was proposed that sugar requirements for GA-stimulated growth are satisfied by a portion of the freshly-formed sucrose diverted toward the meristematic area. 11. Paper chromatography of juice from individual internodes revealed an apparent maturation effect of GA on the stalk segments. Reducingsugar levels were notably lessened in GA-treated internodes, as compared to controls, while sucrose was generally increased above the fourth internode. The single GA application was more effective in this respect than either of the increment treatments. 12. The importance of exploring a broad range of physiological-biochemical GA potentials, at the greenhouse and laboratory level, is briefly discussed.