Interrelationships of Gibberellic Acid and Nitrate in Sugar Production and Enzyme Activity of Sugarcane

Abstract

Variable nitrate (NO₃) and gibberellic acid (GA) were applied to sugarcane in order to clarify NO₃-GA interrelationships which affect growth, sugar production, and enzyme activity. Since both low NO₃ and foliar GA were known to increase sucrose synthesis as separate entities, their combination posed the following questions: 1, To what degree can NO₃ be withheld before growth decline offsets sucrose gains? 2, To what extent will increasing NO₃ supply offset the beneficial effects of GA application? 3, Is there a combination of high NO₃ and GA which will permit both increased tonnage and increased sucrose synthesis as simultaneous effects? Healthy, 12-week-old plants grown in sand culture were treated for 5 weeks. Three levels each of NO₃ (0, 5, and 30 meq./liter) and GA (0, 0.01-, and 0.10-percent solutions of foliar spray) were given in a 3 X 3 factorial combination. The following results were obtained: 1. Both NO₃ and GA caused moderate growth increases as evidenced by fresh weights. A combination of high NO₃ and medium GA gave the maximum yields recorded. Medium GA stimulated internode elongation while retaining stockiness of the cane. High GA caused excessive elongation plus weakening and occasional lodging of the plants. 2. High GA eliminated all NO₃-induced growth increases. Withholding NO₃ did not seriously curtail growth so long as GA was applied. Plants given GA without NO₃ grew comparably to those receiving high NO₃ without GA. 3. Withholding NO₃ caused major sucrose increases in both leaf and immature storage tissues. Raising NO₃ increased synthesis of total ketoses but caused striking decline of sucrose. GA caused significant sucrose increases in storage tissue when NO₃ supply was low. However, GA-induced sucrose increases could not offset sucrose losses due to high NO₃. 4. The most favorable treatment for growth and sugar production was a combination of low NO₃ and medium GA. This induced moderate growth plus major sucrose increases. 5. Leaf phosphatase and ATP-ase were greatly stimulated by NO₃. High GA alleviated the NO₃ effects. 6. A strong NO₃ X GA interaction affected both hydrolytic and oxidative enzymes in immature storage tissue. Phosphatase, ATP-ase, peroxidase and polyphenol oxidase were all involved. 7. NO₃ greatly increased protein content of immature storage tissue, but GA had little effect. GA was able to stimulate both growth and sugar formation without the major protein changes characteristic of NO₃ treatments. Practical usage of the NO₃ and GA data are discussed. It is felt that GA treatment should be delayed after heavy nitrogen fertilization, and will not prove fully effective if given simultaneously with normal fertilizer programs.