Root exudates of the rice plant in relation to akagare, a physiological disorder of rice

Abstract

Twojaponica rice (Oryza sativa, L.) varieties, Norin 36 and Norin 22, susceptible to the physiological disorder of rice known as akagare, and three varieties resistant to this diseaseviz Norin 29, Norin 37, and Norin 32, were chosen for root-exudate studies. Quantitative analysis of carbohydrates and amino acids occurring in sterile root exudates revealed major differences between resistant and susceptible varieties. The major amino acids found in the exudates were lysine, aspartic acid, threonine, serine, glutamic acid, glycine, alanine, methionine, isoleucine, leucine, tyrosine, and phenylalanine. Root exudates of resistant varieties contained ten of these amino acids whereas exudates of susceptible varieties contained only seven. Methionine and glutamic acid were not detected in exudates of the susceptible varieties. Exudates from resistant varieties contained more aspartic acid, threonine, serine and alanine than the exudates of susceptible ones. Major carbohydrates detected were glucose, fructose, arabinose, xylose, and sucrose. Larger amounts of glucose were generally found in exudates from resistant varieties. The exudates from resistant varieties contained two or three times as much carbohydrate as the exudates from susceptible varieties when the total carbohydrates were assayed. The significance of these findings and their possible relation to the rhizosphere microflora and to akagare of the rice plant are discussed.