Enzyme Studies of Sucrose Decline in Sugarcane Desiccated with Paraquat

Abstract

Two experiments are reported in which sugar and enzyme transformations were measured in immature sugarcane treated with Paraquat (1,1'-dimethyl- 4,4'-bipyridilium salt). The chemical is a powerful desiccant whose effects on sugar yield have been inconsistent. There were two objectives: 1, To determine whether sugar changes could be correlated with abnormal behavior of hydrolytic or oxidative enzymes; and 2, to verify the persistence of sugar and enzyme changes in two varieties under conditions of high nitrate (NO₃) and gibberellic acid (GA) growth stimulation. All plants were grown by sand culture in the greenhouse. For experiment 1, growth, sugar, and enzyme data were taken periodically following a single foliar treatment of 0.5-percent Paraquat on 16-week old plants of the variety P.R. 980. The second experiment employed variable NO₃ (1.0 and 50 meq./liter) and GA (0 and 0.01 percent solutions) to establish extreme growth regimes prior to Paraquat treatment. Plants were harvested once 7 days after desiccant application. The following results were obtained: 1. All Paraquat-treated plants were visibly wilted within 8 hours after treatment. Leaf and spindle tissues were severely wilted within 36 hours and foliar discoloration had begun. At 72 hours the spindle, leaf, and sheath tissues were desiccated and a greyish-yellow color was predominant. 2. Severe loss of leaf sucrose was detected within 24 hours. Brix and polarization values were low at 72 hours and had declined still further at 168 hours. 3. An excessive rise in leaf amylase activity accompanied the leaf sucrose decline. This supports an earlier theory that active amylase stimulates a drain of glucose-1-phosphate reserves into polysaccharide synthesis. 4. Paper chromatography revealed the appearance of ribose in desiccated leaves between 24 and 48 hours after treatment. This is believed to signify a blockage of photosynthesis during the conversion of triose phosphate to pentose phosphate. It is suggested that action of a phosphoribose isomerase is blocked, and subsequent accumulations of ribose-5-phosphate are hydrolyzed by ribose-5-phosphatase. Raffinose, a trisaccharide bearing a sucrose linkage, and several sugars of lesser mobility increased in Paraquat-treated leaves. Increased activity of a transferase employing sucrose as a hexose acceptor is suspected. 5. Invertase in immature storage tissue was suppressed by Paraquat. This possibly accounts for initial increases of Brix values and of sucrose in immature storage tissues. 6. Immature storage tissue enzymes were generally less affected by Paraquat than leaf enzymes. All leaf enzymes, peroxidase in particular, eventually declined in response to Paraquat. 7. Growth, sugar, and enzyme changes induced by Paraquat generally persisted regardless of variety, NO₃, or GA variables. 8. GA-treated plants of both varieties were more severely desiccated than controls. GA increased internode length and weight of stalks, but did not permit greater stalk desiccation. 9. Three conclusions were drawn as guidelines for future desiccant research: a. Foliar desiccation appears to defeat its own purpose if photosynthesis is curtailed too early. b. A biochemically acceptable desiccant must be specific and selective in its action against enzymes. c. The objective of cane desiccation should be a wilted top, no longer growing, but one still green and photosynthetically active.