Enzymatic Factors in the Sucrose Decline of Sugarcane Desiccated With Paraquat, Diquat, and Dinitro-butylphenol

Abstract

Enzyme behavior and sugar changes were studied in sugarcane treated with the desiccants Paraquat, Diquat and Dinitro-butylphenol (DNBP). The chemicals were applied as foliar sprays to immature cane of the variety P.R. 980 or P.R. 1059 grown in sand culture. There were three objectives: 1, To confirm abnormal enzyme and sugar responses during sugarcane desiccation; 2, to explore enzyme sensitivity within the dimensions of chemical concentration and time; and 3, to determine whether a common pattern of enzyme changes is triggered by the three distinct chemicals. Paraquat was applied once to P.R. 1059 at levels ranging from 0.0008 to 0.5 percent. All Paraquat treatments quickly wilted foliage. There was little relationship between leaf desiccation and the moisture content of sheaths and stalks, the latter remaining near normal. Leaf sucrose declined sharply to confirm a similar Paraquat effect recorded earlier in P.R. 980. The appearance, of ribose in leaf extracts indicates that Paraquat impaired photosynthesis at the level of 0.004 percent and higher. Since desiccation occurred below this level it is suggested that photosynthesis can proceed simultaneously with a limited amount of desiccant action. Evidence was found of Paraquat-induced interconversion of sucrose with raffinose and unidentified sugars. Leaf amylase activity was increased by 0.5 percent Paraquat and all Paraquat levels increased the enzyme in immature storage tissue. Amylase action is believed to account for sucrose decline in spite of a severe inhibition of invertase. Leaf peroxidase was greatly suppressed by as little as 0.0008 percent Paraquat, and tyrosinase by higher levels, indicating a sensitivity of cane oxidases to the desiccant. Diquat, 0.5 percent, was applied to 6-months old P.R. 980 and tissue samples were taken at intervals during the ensuing 144-hour period. Injury symptoms were identical to those of Paraquat. Again, the early desiccation of leaves was not reflected in sheath moisture values. Leaf sucrose was lowered markedly by Diquat within 24 hours but immature storage tissue and stalk juice did not reveal sucrose decline until 96 hours. Ribose was detected by chromatography of leaf extracts. Invertase was severely inhibited. Amylase and peroxidase was stimulated by Diquat in immature storage tissue. Leaf phosphatase and ATP-ase were clearly retarded by Diquat. Their suppression was accompanied by major and persistent increases in leaf organic phosphorus. Like Diquat, a single 1.0 percent level of DNBP was studied over a prolonged period of time. The chemical was lethal within 8 hours and symptoms are described. Desiccation was generally inferior to that of Paraquat or Diquat. DNBP significantly lowered leaf sucrose within 8 hours. The sucrose decline continued throughout the study and was accompanied by increased reducing sugar content. Chromatography revealed accumulations of fructose and glucose, but no ribose. Invertase was lowered to an insignificant level within 8 hours after DNBP application. Amylase activity was greatly increased in both leaf and immature storage tissues, apparently denying storage tissues an opportunity to accumulate sucrose. Peroxidase was abnormally high in leaf and immature storage tissues in response to DNBP. Phosphatase and ATP-ase was not affected by DNBP; nevertheless there were definite accumulations of organic phosphorus in DNBP-treated leaves. Importance of the desiccant-enzyme-sugar relationships to the sugar industry is discussed. It is proposed that these invertase-suppressing chemicals have potential as ripening agents. They are, however, presently too non-specific in their enzyme relationships for full potential to be realized either as desiccants or ripening agents. Amylase and oxidases apparently must be controlled to accumulate or even retain sucrose.