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Physiological and biochemical aspects of growth and yield stimulation of bean (Phaseolus vulgaris L.) plants by 2, 4-D-mineral sprays Rathore, Vikram S.

Abstract

Recent reports have emphasized that foliar applications of stimulatory concentrations of 2,4-dichlorophenoxyacetic acid (2,4-D) induce greater growth and productivity of several crop plants. This stimulatory action of the hormone has been found to be augmented by simultaneous application of such micronutrients as iron, manganese, zinc, copper, and boron (Wort, 1964). However, no systematic attempt has so far been made to investigate physiological and biochemical changes induced in a relatively short period following these treatments. A correlation of such changes with final improvements in growth and yield may provide a better understanding of the mechanism of action of the hormone. In addition, the biochemical nature of the augmentory role played by micronutrients on 2,4-D action is yet undefined. It was therefore felt desirable to investigate these aspects. Bean plants (Phaseolus vulgaris L. Var. Top Crop) were grown in plastic cans under controlled environment in a growth room. When the plants were 2 weeks old, the following treatments were applied as foliar sprays: (1) 2,4-D (1 ppm), (2) micronutrient solution (Fe, Mn, Zn, Cu, B; 5x10⁻⁴ M) and (3) 2,4-D-mineral solution, with the same components as (2) plus 1 ppm 2,4-D. Measurements of juvenile growth, chemical composition of the plants, respiration and photosynthetic rates and activities of some of the key enzymes of carbohydrate and nitrogen metabolism were made 5, 10, and 15 days after the treatment, to determine the course and basis of stimulation. Yield of pods and seeds and the vitamin C content of pods were also determined. The results revealed that: (1) maximum stimulation resulted from the use of hormone-mineral spray, (2) treatments involving 2,4-D resulted in progressive increase in plant height, leaf number, and leaf area; fresh and dry weights of root, stem and leaves; chlorophyll content and total sugar in leaves, (3) sucrose and reducing sugars were lower in stems and roots of treated plants; (4) moisture content was not affected significantly by any treatment; (5) quantitative chromatographic determination of the free, ethanol-soluble amino acids revealed a reduction in amino acid content in 2,4-D-treated plants, particularly in leaves, and an increase in plants to which micro-nutrient spray had been applied. The amino acid content of plants treated with 2,4-D plus minerals was intermediate between those resulting from the other two treatments; (6) measurement made with intact plants using an infrared CO2 analyzer revealed increases in rates of respiration and photosynthesis of the aerial portions 10 and 15 days after treatment; (7) significant increases in the activity of phosphorylase, phosphoglyceryl kinase, succinic dehydrogenase, catalase, nitrate reductase, and transaminase in all organs were also apparent at 10 and 15 days after treatments involving 2,4-D; (8) treatments involving 2,4-D also resulted in significant increases (P=0.05) in number and fresh weight of green pods, in seed number, and in total seed weight. Weight of the individual seed was not significantly altered; (9) the green pods from treated plants had higher vitamin C content at harvest time, but the moisture content was not altered by treatment. The green pods of plants which had received 2,4-D alone or 2,4-D plus minerals lost less ascorbic acid and moisture during 4 days storage at room temperature. The stimulation of enzyme activities is explained on the basis of hypothesis proposed by Wort (1964) that 2,4-D may participate in the formation of substrate-enzyme-regulator complex. Minerals may affect the activity of formation of this complex. In proposing a physiological and biochemical basis for the stimulation in growth and yield under 2,4-D action, the following points are emphasized: (1) the stimulated rate of photosynthesis produced a larger amount of photosynthate which could be utilized in the biosynthesis of all cell constituents or serve as substrate for respiration; (2) the stimulated rate of respiration and activity of such enzymes as phosphoglyceryl kinase and succinic dehydrogenase resulted in an increased supply of available energy, as ATP and reduced nucleotides, for biosynthesis, and in larger amounts of keto acids which provide the carbon skeletons of amino acids; (3) the greater activity of nitrate reductase and transaminase resulted in an augmented supply of amino acids responsible for the enhanced synthesis of protein evident in greater growth and productivity.

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