AbstractVariable 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.
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