Effects of Environmental Conditions and the Coadministration of Growth Retardants on the Response of Sugarcane to Foliar Treatment with Gibberellin

Poor yields of sucrose from sugarcane are often associated with low sucrose:cane ratios, particularly when climatic conditions tend to favor stem elongation rather than sucrose accumulation. Plant growth regulators may be used to modify environmental effects on the growth of sugarcane and thus alter the balance between utilization and storage of sucrose. The objectives of this study were to investigate the influence of temperature, soil moisture, and the coadministration of plant growth retardants on the response of sugarcane to treatment with gibberellic acid. Single foliar applications of GA (gibberellic acid) commercial sugarcane varieties (interspecific hybrids of Saccharum) stimulated stem elongation for from 2 to 6 weeks, but this response was not reflected in higher cane yields unless the plants were harvested within 6 weeks of treatment. A second application of GA at 4 weeks prolonged the growth response. In the field low temperatures were associated with the more prolonged growth responses, which were of smaller amplitude. Moisture stress had no apparent effect on GA response. No correlations were evident between environmental conditions or application rates and the duration of growth responses in greenhouse trials, but the amplitude increased with application rate within the range of 25 to 200 ppm. Three to five immature or semimatttre internodes were affected. In field trials under warm conditions a period of growth retardation followed the initial GA-induced growth stimulation. This did not occur in greenhouse trials. Stem-sugar concentrations fell during the growthstimulation phase, but recovered as growth rates reduced. Attempts were made to accentuate this recovery, and so to increase the accumulation of sugars, by coadministration of growth retardants with GA. Dalapon had a delayed effect on cane growth, slightly curtailing the duration of the GA response, and suppressing growth thereafter. The phase of growth suppression was accompanied by increased stem sugar concentration in greenhouse-, but not in field-trials. Sodium silicate (at up to 200 ppm) had no significant effect on growth or sugar accumulation and did not modify the response to GA. Treatment with azauracil (at up to 400 ppm) strongly inhibited cane growth and increased stem-sugar concentrations, but showed no significant interaction with GA.