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Studies with naphthenic acids in the bush bean, phaseolus vulgaris L. Severson, John George

Abstract

The overall objective of these experiments was to augment our understanding of how naphthenic acids stimulate metabolism and growth of bean plants. Three separate studies were carried out with bush bean plants (Phaseolus vulgaris L. cultivar Top Crop) to determine: 1) the effect of potassium naphthenates (KNap) on the uptake, distribution, and incorporation of phosphorus-32, 2) the metabolism of the individual naphthenic acid, potassium cyclohexanecarboxylate (KCHC), in leaves and roots, and 3) the effect of KNap and KCHC on the uptake and metabolism of glucose by excised root tips. 1) Fourteen-day-old plants growing in a phosphate-free (-P) or a complete (+P) nutrient solution were sprayed to drip with a 0.5% solution of KNap. Twenty-four hours after spraying, the roots of both control and treated plants were exposed for 2 hours to a nutrient solution containing ³²P. Following the exposure to ³²P, the plants were returned to their original nutrient solutions. Control and treated plants were withdrawn 4, 8, 12, and 24 hours after exposure to ³²P, and were separated into leaf blades, stems, and roots. Acid soluble, acid insoluble, and total ³²P activity, or total phosphorus were determined at each sampling time. KNap treatment increased by 7 to 9% the intake of ³²P by plants grown in the -P or +P nutrient solution. The increases, however, lacked statistical significance at the 0.05 level. The rate at which ³²P was translocated out of the roots of plants grown in the -P nutrient only was increased significantly by treatment, in spite of the fact that at the 24 hour sampling time 84% of the total ³²P label remained in root tissues. At the same sampling time 32% of the total ³²P label was found in the roots of plants grown in the +P nutrient. While KNap treatment significantly increased ³²P activity in stems of -P grown plants over the sampling period, activity in stems of control and treated plants grown in the +P nutrient was similar. Naphthenate treatment increased the rate of incorporation of ³²P into both the acid soluble (sugar phosphates, nucleotides, phospholipids) and acid insoluble (nucleic acids, phosphoproteins) fractions of leaves of plants grown in the +P nutrient solution. Acid soluble ³²P activity declined in all root tissues over the sampling period as acid soluble ³²P-containing compounds, primarily orthophosphate, were translocated acropetally. The percentage acid insoluble ³²P activity in the roots of KNap-treated plants was significantly greater than that found in the roots of control plants at the 24 hour sampling time. Naphthenate treatment did not affect the amount of total P (³¹P + ³²P) in the two P fractions of the three plant organs. The augmented incorporation of ³²P into the acid soluble and acid insoluble fractions is further evidence of the KNap-stimulated P metabolism reported by other workers. 2) KCHC-7-¹⁴C administered to leaf disks in the light or to roots of intact seedlings in the dark was rapidly converted to a mixture of two conjugated metabolites: the glucose ester and the aspartic acid amide. The root-feeding experiment indicated that following their synthesis in root tissues both conjugates were translocated acropetally. The results of amino acid analyses of the acid hydrolysates of several unidentified metabolites strongly suggest that KCHC-7-¹⁴C was also conjugated with a low molecular weight polypeptide. 3) Three sets of root tips cut from 7-day-old seedlings were incubated in a medium containing ¹⁴C glucose for 3 hours. Two of the three sets were pretreated in a solution of KCHC or KNap for 6 hours. Each naphthenate treatment significantly increased ¹⁴C activity in the ethanol-soluble (amino acids, glucose, etc.), ethanol-insoluble (polysaccharides, protein, etc.), and respired CO₂ fractions. The individual naphthenic acid, KCHC, had the greater effect on the uptake and metabolism of labelled glucose. Results also indicated that not only were the uptake of glucose and CO₂ production increased significantly by each treatment, but also amino acids containing the glucose carbon passed more quickly through soluble amino acid pools in root tissues, and were more rapidly fixed into protein. In light of the finding that naphthenate conjugates and not the free acid were detected in the tissue, it may be that the conjugates were associated in a causal way with the stimulated uptake and metabolism of labelled glucose.

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