Use of Silicon to Preserve Sucrose in Sugarcane Desiccated with Paraquat and Diquat

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Sugarcane studies have shown that the desiccants Paraquat and Diquat cause severe sucrose losses as a result of interrupted photosynthesis and abnormal behavior of sugar-metabolizing enzymes. It was theorized that an appropriate agent, selective for the sucrose-limiting enzymes, could modify the sugar-destroying features of otherwise useful chemicals. Experiments are reported herein in which silicon (Si), an in vitro inhibitor of cane enzymes, was applied to sugarcane foliage prior to treatment with Paraquat and Diquat. There were three objectives: 1, To incorporate sufficient Si into living tissues to control hydrolytic and oxidative enzymes; 2, to help retain photosynthetic activity; and 3, to maintain higher sucrose levels as a consequence of lessened sugar metabolism and continued photosynthesis. Plants of the variety P.R. 980 were grown in sand culture and treated with the following percent solutions of Paraquat and Diquat: 0, 0.0002, 0.0006, 0.0018, 0.0054, and 0.0162. For each experiment half of the plants were pretreated with 500 p.p.m. of Si (Na2SiO3-9H2O) as a foliar spray at 8, 4, and 1 day prior to desiccant application. The following results were obtained: 1. The 0.0018 to 0.0162 percent levels severely damaged cane foliage, desiccated leaves and sheaths, and caused major sucrose losses in plants not pretreated with Si. 2. Si-pretreated plants retained near-normal foliage and near-normal sucrose levels for approximately two Paraquat increments above those of non-pretreated plants. For example, damage typical of the 0.0018 percent level, without Si, did not appear until the 0.0162 percent level was reached when Si was present. 3. Paraquat desiccation, as reflected by sheath-moisture values, was ultimately constant regardless of Si treatment. 4. Paper chromatography revealed ribose in leaf extracts of Paraquat-and Diquat-treated plants. Si pretreatment retarded the appearance of ribose below the 0.0162 percent level of Paraquat, and at all Diquat levels. This indicated a Si "protection" of photosynthetic reactions, possibly those of phosphoribose isomerase. 5. The leaf enzymes phosphatase, ATP-ase, and peroxidase were suppressed by increasing desiccant concentrations. Si pretreatments acted to preserve near normal enzyme levels regardless of desiccant action. Leaf amylase, and invertase from immature storage tissue, were greatly stimulated by Paraquat. In these instances Si served to suppress the excessive enzyme activity. 6. Si did not seem to act as an inhibitor in living tissues, as it is known to do in the test tube. Rather, a series of Si-enzyme complexes appeared to form and to retard activity shifts in either direction. 7. A third experiment involving 0.001 percent Paraquat, and a single Si pretreatment given 1 day prior to Paraquat, generally verified the sucrose, enzyme, and visible foliar effects noted earlier with more severe desiccant and Si treatments. The results support the thesis that undesirable effects of a chemical on plants can be selectively modified by control of critical enzymes. The value of this concept in increasing the scope and productivity of agricultural chemicals is noted.