EFFECTS OF NITRATE ON ACCUMULATION OF TREHALOSE AND OTHER CARBOHYDRATES AND ON TREHALASE ACTIVITY IN SOYBEAN ROOT-NODULES

Soybean (Glycine max cv. Maple Arrow) plants were infected with Bradyr hizobium japonicum (strain 61-A-101), grown in sterilized Leonard jars , and exposed to various amounts of nitrate either from the beginning or after completion of nodulation. The presence of 5 mM and more nitra te during nodulation caused a considerable reduction of the number and biomass of nodules per plant, and of nitrogenase activity per nodule fresh weight. The carbohydrate content of nodules was determined on a dry weight basis. The level of the disaccharide trehalose, produced by the microsymbiont, was 50% lower in nodules formed in the presence of 20 mM nitrate than in control nodules formed in its absence. With reg ard to the non-structural carbohydrates produced by the plant, nodules formed in the presence of high amounts of nitrate contained about 75% less starch but three- to fourfold higher levels of sucrose and pinit ol than control nodules. Sucrose was the most abundant non-structural carbohydrate in nodules formed in the presence of 20 mM nitrate, accou nting for 4-5% of the dry weight. When plants with fully established n odules, grown in the absence of nitrate, were shifted to 20 mM nitrate , the levels of trehalose and starch decreased over a period of 3 week s while the level of sucrose increased, until the carbohydrate levels attained similar values as found in nodules established in the presenc e of nitrate. The activity of trehalase, an enzyme known to be induced in nodules, was about 75% lower in nodules formed in the presence of nitrate than in control nodules. However, trehalase activity did not c hange in established nodules during a 3-week exposure to 20 mM nitrate . Similarly, the number of colony- forming bacteria recovered from the nodules and the activities of endochitinase and endoglucanase, two pl ant defense hydrolases, were not affected during a 3-week exposure to nitrate.