AbstractImmature sugarcane was subjected to variable water and phosphorus (P) supply and then treated with foliar gibberellic acid (GA). All plants were grown in sand culture and received initial water and P treatments at 88 days of age. Water regimes of inadequate, adequate, and abundant supply were established with 1, 2, and 4 liters of water per day, respectively. Variable P included 0, 6, and 30 meq./liter. Foliar GA was given as 0-, 0.01-, and 0.10-percent solutions. There were three objectives: 1 To determine the effectiveness of GA as a growth stimulant and regulator of sugar-enzyme relationships under conditions of water and P stress; 2, to explore physiological limits within which GA-enzyme relationships persist; and 3, to explore the enzyme basis of water and P performance under extreme conditions of GA-stimulated growth. The following results were recorded: 1. Both water and GA had greatly increased stalk weight and intemode length 5 weeks after GA treatment. 2. Water supply strongly affected GA-growth responses. Water-deficient plants were proportionately more stimulated by GA than water-rich plants. However, maximum growth required both GA and abundant water. 3. GA appeared to increase the efficiency of water utilization, regardless of the amount of water supplied. 4. Variable water supply severely transformed the behavior patterns of ATP-ase, amylase, invertase and polyphenol oxidase. 5. GA treatment of low-water plants appeared to increase the severity of water shortage. Hydrolytic enzymes were severely retarded by GA when water supply was low, but not when adequate or abundant water was available. On the basis of growth and enzyme data it was proposed that GA caused an internal redeployment of water so that the net quantity available for enzymatic functions was reduced. It was also proposed that GA might decrease the internal water supply while increasing growth, in contrast to the commercial practice of externally withholding water which decreases growth. 6. Low P was inadequate for maximum growth, but severe P deficiency was not achieved. GA was proportionally more effective in promoting fresh weights and internode elongation when P supply was low. 7. GA moderately increased leaf P content when P supply was low. The increase was primarily organic P (PO) and this was attributed to GA suppression of phosphatase and ATP-ase. The significance of GA alteration of PO is discussed. 8. Evidence was found of a GA-induced PO decline mediated by increased amylase activity. 9. Leaf peroxidase was extremely sensitive to P supply, and to GA in P-hungry plants. The enzyme was excessively active in low-P X low-GA plants. 10. It is shown that cane growth and enzymology is far more sensitive to P than field experiments have indicated. The importance of PO, phosphatases and phosphorylase, as contrasted to total P content, is stressed.
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