Relationships of Light to Enzyme and Sucrose Transformations in Sugarcane Treated with Paraquat

Abstract

Light relationships to enzyme and sugar transformations were studied in sugarcane treated with Paraquat. A bipyridylium herbicide, Paraquat has shown promise as a preharvest-desiccant and flower-controlling agent but causes undesirable sugar and growth restrictions. The present studies were conducted as part of a program having chemical control of sucrose as an ultimate purpose. There were three objectives: 1, To clarify the dependence of Paraquat on light for its desiccative action; 2, to clarify the roles of light in abnormal enzyme behavior induced by Paraquat; and 3, to define the role of light in sucrose losses triggered by Paraquat. Two experiments were conducted with immature sugarcane of the variety P.R. 980. All plants were grown by sand culture in the greenhouse under strict water and nutritional control. For the first experiment plants were treated with 0 or 0.05 percent Paraquat solutions and subjected to continuous darkness or maintained under natural illumination. During the second experiment one group of plants was darkened for 72 hours followed by 72 hours of natural illumination, and a second group was maintained for 144 hours with natural light exposure. During both studies leaf and immature storage tissues were frozen at appropriate intervals for enzyme and sugar analyses. Enzymes assayed included acid phosphatase, ATP-ase, amylase, invertase, peroxidase and polyphenol oxidase. Changes in sucrose, fructose, glucose and ribose were studied by paper chromatography. The following results were obtained: 1. Darkened cane did not experience typical Paraquat destruction of green tops as did those kept under normal light. Leaf sucrose declined markedly among all darkened plants. Nearly identical sucrose losses were recorded in Paraquat-treated plants receiving normal illumination. 2. Regarding sucrose synthesis, it was concluded that Paraquat treatment of sugarcane has an effect equivalent to a continuous night of several weeks or more duration. 3. During the first experiment ATP-ase was greatly suppressed by Paraquat under normal light conditions. The herbicide's effect was blocked by darkness. The involvement of Paraquat in the adenylic acid system and its consequences is discussed. Invertase was suppressed both by darkness and by Paraquat. 4. During experiment 2, undarkened plants experienced severe foliar drying within 72 hours after Paraquat treatment. At this time the darkened plants still appeared green and normal. Removal to sunlight initiated typical foliar destruction by the herbicide. 5. Plants not treated with Paraquat lost sucrose and reducing sugars in darkness, but the sugars reappeared when plants were reintroduced to light. The 0.05 percent Paraquat level caused typically severe sucrose decline. Sucrose did not recover under natural illumination nor in darkened plants upon reexposure to light. 6. During both experiments, Paraquat-treated plants accumulated ribose in their leaves, indicating a blockage of photosynthesis at the site of phosphoribose isomerase action, i.e., in the conversion of ribose-5-phosphate to ribulose-5-phosphate. 7. In spite of continuing ribose production, reexposure of darkened Paraquat-treated plants to sunlight led to recovery of fructose and glucose production. It is suggested that an alternate method of ribulose-5- phosphate formation, from xylulose-5-phosphate, and catalyzed by ribulose phosphate-xylulose phosphate isomerase, was activated during darkness. This presupposes that a dark reaction occurs in which foliar mechanisms are sensitized to danger before the herbicide assumes phytotoxic capability. 8. Failure of leaf sucrose to recover after reexposure to sunlight suggests that Paraquat is also acting against sucrose synthetase or related catalysts of sucrose biosynthesis. 9. Darkened, herbicide-free cane accumulated excessive amounts of sucrose in leaf and immature storage tissues within 6 hours after reillumination. The possibility of effecting more efficient sucrose production by stress of darkness, as opposed to limitations of sugar transport, are discussed. 10. The leaf enzymes ATP-ase, acid phosphatase, amylase, peroxidase, and polyphenol oxidase, plus invertase from immature storage tissue, were all significantly altered by light or Paraquat treatments during experiment 2. ATP-ase: The enzyme was severely suppressed by Paraquat in light, but not in darkness, and was rapidly suppressed upon reexposure to light. Evidence is presented of a non-toxic dark reaction which disposes of steps normally rate-limiting in daylight. Similar effects were noted upon acid phosphatase. Amylase: Amylase was stimulated by Paraquat in light which verified earlier findings. Amylase is considered a major factor in the herbicide's stimulation of metabolic processes. Darkness eliminated the stimulatory effect. A Paraquat free-radical produced after darkness may not be identical to that formed upon immediate exposure to light. Peroxidase: This enzyme declined sharply in iUuminated plants given Paraquat. Darkened plants strongly resisted the Paraquat suppression, although some loss of activity still occurred. It is suggested that traces of peroxide were formed and "alerted" peroxide-destroying mechanisms. Such an alarm system would be distinct from the darkened Paraquat-ATPase relationship in which the plant apparently made no effort to save an endangered enzyme. Polyphenol oxidase: Moderate Paraquat inhibition took place in light; none occurred in darkness or following darkness. Inhibition was delayed for about 3 days. An indirect Paraquat effect such as increasing moisture stress is suspected. Invertase: The enzyme's activity pattern was vastly altered by light and Paraquat. Darkness and Paraquat suppressed invertase with identical decline curves. Reillumination led to full invertase recovery plus significant overcompensation in zero-Paraquat plants. No recovery occurred in cane given Paraquat. A light-sensitive mechanism of invertase synthesis is proposed, with darkness and Paraquat giving reversible and non-reversible inhibition, respectively. 11. Sugarcane growth, sugar, and enzyme data concur with the supposition that a free radical of Paraquat is produced by photoreduction. In addition, prior to reduction and reoxidative transformations, there appears to be several types of herbicide-enzyme reactions occurring independently of light. These include: 1, Disposal of secondary but rate-limiting steps which speed an ultimate enzyme inhibition in light; and 2, sensitizing reactions in which critical enzymes are forewarned of phytotoxic compounds and essentially defensive or protective mechanisms are alerted.