Effect of Acetic, Propionic, and Butyric Acids on Young Rice Seedlings' Growth1

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Abstract
Anaerobic bacteria in flooded soils degrade various types of organic matter, including rice straw and green manure crops, with the production of low carbon, monobasic aliphatic acids. The major fermentation products are formic, acetic, propionic, butyric, and lactic acids. Organic acids produced in flooded soils may adversely affect the growth of rice seedlings.Rice seedlings, Oryza sativa L. ‘Earlirose’, were grown in sterile nutrient solution containing 1, 5, and 10 mN concentrations of acetic, propionic, and butyric acid adjusted to variables of pH 3.0, 5.0, and 7.0. The seedlings were grown in treated nutrient solutions for 7 days before harvesting for plant growth measurements.Root elongation, shoot height, and weight were retarded by all organic acids, the severity increasing with concentration and decreasing pH. Root elongation was affected most. Residual seed reserves were greater with higher acid concentrations, corresponding to decreased growth of the plant root and shoot suggesting that the translocation of stored seed reserves was inhibited. The magnitude of organic acid effects was in the order of butyric > propionic > acetic. At 1 mN concentration, all organic acids evaluated, reduced root length by 40 to 50%, but did not inhibit root initiation. At 5 mN concentration, root elongation was severely inhibited and most root tips showed severe root injury. At 10 mN concentration, both elongation and initiation of roots were completely inhibited. An increase in pH from 3 to 7 decreased the inhibiting effects of all organic acids at all concentrations, indicating that the concentration of undissociated acid was more critical than total acid concentration in inhibiting seedling growth. Anaerobic fermentation products, particularly organic acids, are potential problems in rice stand establishment in flooded soils. A proper time interval for their decomposition prior to planting rice must be observed to avoid seedling damage.

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