AbstractVariable combinations of gibberellic acid (GA) and silicon (Si) were applied to the foliage of 12-week-old sugarcane grown in sand culture. GA levels were equivalent to 0, 0.01-, and 0.10-percent solutions of the pure acid, and Si levels were 0, 100, 1,000, and 10,000 p.p.m. of elemental Si. A 3 X 4 factorial design was employed with four replicates. There were four objectives: 1, To reconfirm the ability of both GA and Si to stimulate sucrose formation; 2, to determine whether sufficient Si could be absorbed by the leaves to accomplish its sucrose-promoting effect; 3, to determine whether GA and Si can accomplish these functions when combined in a single application; and 4, to evaluate growth and enzyme responses to combined GA plus Si. A single harvest of leaf and immature storage tissue was taken for analysis 5 weeks after treatment. The following results were obtained: 1. Fresh-weight data showed that 10,000 p.p.m. of Si severely retarded growth. GA increased internode elongation and moderately increased fresh weight at the medium level. Si toxicity was partly alleviated by medium GA. 2. Sucrose content of leaf and immature storage tissues was increased by both GA and Si as main effects. Maximum sucrose was achieved with a combination of 0.01-percent GA plus 100 p.p.m. of Si. 3. Sucrose increases were partially a result of stimulated total ketose production. Leaf fructose was generally lowered by high GA and Si treatments. 4. Enzyme behavior verified earlier observations. Phosphatase, ATP-ase, peroxidase, and polyphenol oxidase were all retarded by high Si. However, sucrose increases appear to have been due almost entirely to invertase suppression by Si and GA. 5. Results confirm that both GA and Si can increase sucrose production and storage as independent entities, and when combined they can further increase sucrose to levels unattainable by either factor acting alone. Relatively small amounts of the two constituents are needed and these can be combined readily within a single foliar application.
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