The effect of cool weather conditions on nitrogen (N) and phosphorus (P) accumulation by soybean based on dry matter production was examined in both pot and field experiments. Pot experiment: Soybean plants, Glycine max L. Merr. cv. Tachiyutaka, were grown with or without rhizobium (Bradyrhizobium japonicum A1017) inoculation in a mixture of sterile soil, sand, and perlite under glasshouse conditions. After the growth of the plants for 46 d, half of the pots were transferred to a low temperature (15°C 14 h/13°C 10 h) glasshouse under reduced light conditions by shading. The other half of the pots was left in the previous glasshouse for the control treatment. At 0, 7, 16, and 28 d after the start of the treatment, the growth parameters and the Nand P contents of the plants were determined. The low temperature and shading (LT) treatment reduced the dry matter production by soybean irrespective of the inoculation. The relationship between plant dry weight and N accumulation was not affected by the LT treatment, while that between plant dry weight and P accumulation was altered by the LT treatment, which increased the ratio of P accumulation per unit amount of dry matter production. Field experiment:. Soybean cv. Nanbushirome was grown in an experimental field (Morioka City, Japan) with different rates of applications of fused magnesium phosphate and manure in 1980 and 1981. The weather conditions were in sharp contrast in these years: Warm spring and cool summer in 1980, and cool spring and warm summer in 1981. The difference in the weather conditions did not affect the relationship between N accumulation and dry matter production of top irrespective of the application rates of the soil amendments. However, the relationship between P accumulation and dry matter production before the flowering stage was altered as observed in the pot experiment under the cool weather conditions. The data obtained from both pot and field experiments suggested that P accumulation per unit amount of dry matter production in soybean plant was higher under cool weather conditions than under warm ones. These findings are discussed in relation to the high efficacy of P fertilizer application under cool weather conditions.
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