Effect of Water Regime on the Sucrose-Enzyme Relationships of Sugarcane Desiccated with Paraquat

Abstract

Immature sugarcane was given variable-water regimes in sand culture and subsequently treated with a powerful desiccant, the bipyridylium herbicide Paraquat, applied as a 0.05-percent aqueous foliar spray. There were two objectives: (a) To evaluate the effects of water regime on sucroseenzyme relationships in desiccating sugarcane; and (b) to determine whether controlled water regimes could effectively modify Paraquat activity in sugarcane. Tissue samples were harvested for moisture, sugar and enzyme analyses at 1,3 and 9 days after Paraquat application. Low water supply (1 liter per day) reduced total fresh weights and stalk weights, and increased sucrose content of immature storage tissue. Paraquat significantly lowered total fresh weights, stalk weights, sheath moisture and leaf sucrose by the 9-day harvest. Desiccant action was generally more rapid within the low-water regime. High- and intermediate-water regimes tended to modify Paraquat activity at 1 or 3 days, but its ultimate effects were comparable regardless of water regime. No evidence was found to support the theory that desiccating cane accumulates water as a function of continued water absorption when transpiration has ceased. Acid invertase was suppressed by Paraquat, an effect consistent with earlier findings. The suppression was most severe in the low-water regime. Low-water supply significantly lowered invertase level but the response was not consistent at all harvests. Acid phosphatase and ATP-ase were severely repressed by Paraquat in leaves but not in immature storage tissue. An explanation was offered in terms of distinct chloroplast and mitochondrial enzymes rather than localized Paraquat action. For both enzymes the desiccant repression was significantly more severe in the high-water regime at 1 or 3 days, but water regime showed no effect at 9 days. Paraquat significantly increased ß-amylase in leaves (consistent with earlier studies), particularly within the high-water regime. In immature stem tissue ß-amylase was repressed by high water in Paraquat-free plants. Paraquat eliminated the water effect. Peroxidase was increased in storage tissue by Paraquat. This response was statistically significant only under conditions of low-water supply. It is concluded that variable water regimes can modify the rate of initial Paraquat activity in sugarcane; however, the ultimate effects of Paraquat will not be changed under conditions of thorough chemical application. Under field conditions of marginal chemical penetration, the plant's moisture status might play a more decisive role in determining the desiccant's effectiveness.