Limitation of Acetylene Reduction (Nitrogen Fixation) by Photosynthesis in Soybean Having Low Water Potentials

Abstract

The role of photosynthesis and transpiration in the desiccation-induced inhibition of acetylene reduction (nitrogen fixation) was investigated in soybean (Glycine max [L.] Merr. var. Beeson) using an apparatus that permitted simultaneous measurements of acetylene reduction, net photosynthesis, and transpiration. The inhibition of acetylene reduction caused by low water potentials and their aftereffects could be reproduced by depriving shoots of atmospheric C02 even though the soil remained at water potentials that should have favored rapid acetylene reduction. The inhibition of acetylene reduction at low water potentials could be partially reversed by exposing the shoots to high C02 concentrations. When transpiration was varied independently of photosynthesis and dark respiration in plants having high water potentials, no effects on acetylene reduction could be observed. There was no correlation between transpiration and acetylene reduction in the C02 experiments. Therefore, the correlation that was observed between transpiration and acetylene reduction during desiccation was fortuitous. We conclude that the inhibition of shoot photosynthesis accounted for the inhibition of nodule acetylene reduction at low water potentials.

Nodule Activity and Allocation of Photosynthate of Soybean during Recovery from Water Stress

“…Several studies of soybeans during brief periods of water stress have focused on effects on N2-fixation. As *W decreases in response to drought conditions, the decline in photosynthetic rate (2,3,13,17) is accompanied by decreases in adenylate levels (17) and acetylene reduction activity (10,12,13,17) (10,20) and nodules (10, 21) has been observed in response to decreased *I', the association of changes in photosynthesis and nodule activity during onset and recovery of water stress is cited (10, 13, 17, 21) as evidence that N2-fixation during episodes of water stress is limited by availability of carbohydrate. Unfortunately, in none of these studies were carbohydrate contents of nodules measured.…”

“…Several studies of soybeans during brief periods of water stress have focused on effects on N2-fixation. As *W decreases in response to drought conditions, the decline in photosynthetic rate (2,3,13,17) is accompanied by decreases in adenylate levels (17) and acetylene reduction activity (10,12,13,17) of nodules, and the recovery of adenylate levels and acetylene reduction activity lags only slightly behind the recovery of *I,, and photosynthesis after rewatering (13,17). Although enhanced carbon reallocation from shoot to roots (10,20) and nodules (10,21) has been observed in response to decreased *I', the association of changes in photosynthesis and nodule activity during onset and recovery of water stress is cited (10,13,17,21) as evidence that N2-fixation during episodes of water stress is limited by availability of carbohydrate.…”

Response to Drought Stress of Nitrogen Fixation (Acetylene Reduction) Rates by Field-Grown Soybeans

“…While some researchers have concluded that this loss of nodule activity is due to an inhibition ofphotosynthesis (8,9,13), Sprent (22) concluded that nitrogen fixation is more sensitive than photosynthesis to drought stress. This is supported by the findings of Bennett and Albrecht (1) who found that nitrogen fixation was closely correlated with nodule water potential which, in turn, was more sensitive to drought stress than was either leaf water potential or diffusive conductance.…”

Effects of Soil Moisture Around Nodules on Nitrogen Fixation by Well Watered Soybeans

“…In recent field trials near Guelph there were significant negative correlation's between soybean water stress and N2 fixation (Azu 197 5). Decreases in fixation under moisture stress may occur because reduced photosynthate supply lowers nodule activity (Huang et al 1975). However, Kuo and Boersma (197 l) found that the amount of N2 fixed by soybeans, per unit of CO2 absorbed, decreased with increasing soil water suction, indicating that fixation was more sensitive than photosynthesis to water stress.…”