Pools of non‐structural carbohydrates in soybean root nodules during water stress

Abstract

The aim of this study was to examine how the pools of non‐structural carbohydrates in soybean nodules are affected under water stress conditions depending on the nature of the symbiont strains with particular emphasis on the plant‐borne carbohydrates sucrose and pinitol, and on trehalose, a compatible solute synthesized by the bacteroids. Soybean (Glycine max [L.] Merr. cv. Maple Arrow) plants were inoculated with the nitrogen‐fixing strains Bradyrhizobium japonicum 61‐A‐101 or USDA 110 spc4 and cultivated axenically under conditions in which nodules formed in an upper soil compartment while roots for water supply grew into a compartment filled with nutrient solution. When the nodules were well established (1 month post inoculation), 10% (w/v) PEG 6000 was added to the nutrient solution. This led to a slowly progressing, moderate water stress, as determined by measuring the decrease of transpiration, and to a decrease in nitrogen fixation. The pool sizes of the major non‐structural nodule carbohydrates changed during progression of water stress. Sucrose, the major soluble carbohydrate in nodules of unstressed plants (2 and 4%, respectively of nodule dry weight depending on symbiont strain), strongly increased in nodules of stressed plants, reaching nearly 10% of dry weight. The activities of two major sucrose‐consuming enzymes, sucrose synthase and alkaline invertase, decreased markedly in nodules of stressed plants. Starch decreased only transiently upon water stress. Pinitol, a cyclitol serving as compatible solute in many plants, increased more than 4 times, reaching about 1% of nodule dry weight during the stress. Trehalose, the major soluble carbohydrate synthesized by the bacteroids, increased in nodules colonized by USDA 110 spc4 from about 0.2 to 0.8% of nodule dry weight, while in nodules colonized by 61‐A‐101 it amounted to more than 1.5% of dry weight both with and without stress.