PLANT ASSOCIATED N_2 FIXATION (C_2H_2-REDUCTION) BY FIVE RICE VARIETIES, AND RELATIONSHIP WITH PLANT GROWTH CHARACTERS AS AFFECTED BY STRAW INCORPORATION

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Five rice (Oryza sativa L.) varieties with different growth durations were grown in a flooded field with and without straw amendment in the dry and wet seasons of 1984. Plant growth components, plant-associated acetylene-reducing activity (ARA), bacterial enumeration, and soil ARA were measured throughout the rice growth cycle. Plant-associated ARA significantly differed among the varieties with and without straw incorporation in both the dry and wet seasons. Among the different growth stages, it was at the heading stage that i) differences in ARA among rice varieties were most evident, ii) coefficient of variation (CV) of rice plant-associated ARA among plots was lowest (13-17% in the dry season), and iii) partial correlations between ARA and dry weight of root (shoot wt. constant ) and shoot (root wt. constant) were significant. ARA of different varieties at heading was also positively correlated with total dry matter yield at maturity. Long-duration varieties had higher ARA per plant at heading than short-duration varieties, mainly because of differences in plant biomass. Amendment with straw increased soil ARA, plant ARA, and N_2-fixing bacterial population more in the dry season than in the wet season. It effect on plant and soil ARA appeared only during the earlier stages of rice growth. The possibility of developing selection criteria for screening rice varieties for their capacity to stimulate N_2 fixation is discussed.