Abstract
It has already been reported that oxygen requitement of crop roots in the soil varied with the difference of crops or varieties. The author found a close relation between reducing power and oxygen requirement of roots during the water-culture of various plants, and then cultured young plants in a solution containing nitrate as nitrogen source (cf. table I) and investigated the amount of nitrous acid formed in the solution. Soybean (12 varieties), barley (14 varieties), wheat (7 varieties), sweet potato (12 varieties), rape (14 varieties), other two or three crops and weeds were employed in this experiments. All these plants excluding sweet potato were sown in the quartz sands until they bears one or two leaves, then the seedlings were water-cultured in the 300cc flasks covered by blackpapers, and the plants were supported with cotton stoppers. On sweet potato, the top of vine was cut in 15 cm and then water-cultured. The solution was changed every 3-4 days and nitrous acid in the residual solution was measured, and the total amount of nitrous acid formed during the term of water-culture (generally one month) was calculated. The results of the experiments are as follows : Nitrous acid was not formed in the culture solutions of paddy rice, Alopeculus aequalis, and Beckmannia erucaeformis, but it was found in those of barley, wheat, soybean, corn and rape having strong reducing power in their roots (cf. table 2). Considering from these facts upland plants generally have the strong reducing power of nitrate in the culture solution, but marsh plants have the very weak reducing power. Further the varietal difference of nitrate reducing power was investigated with soybean, sweet potato, barley, wheat and rape, and the following results were obtained (cf. table 3-6): Drought-resistant varieties form more nitrous acid than drought-susceptible varieties in the same species, but dump-resistant varieties form less nitrous acid on the contrary, i. e., have less reducing power, than the susceptible varieties. According to the results, it may certainly be concluded that the reducing power is closely correlated with oxygen requirement in roots, i. e., species or varieties having the stronger reducing power in their roots require more oxygen in the soil.
