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

Abstract

The effects of drought stress on soybean nodule conductance and the maximum rate of acetylene reduction were studied with in situ experiments performed during two seasons and under differing field conditions.In both years drought resulted in decreased nodule conductances which could be detected as early as three days after water was withheld. The maximum rate of acetylene reduction was also decreased by drought and was highly correlated with nodule conductance (r = 0.95). Since nodule conductance is equal to the nodule surface area times the permeability, the relationship of these variables to both whole-plant and unit-nodule nitrogenase activity was explored. Drought stress resulted in a decrease in nodule gas permeability followed by decreases in nodule surface area when drought was prolonged. Under all conditions studied acetylene reduction on a unit-nodule surface area basis was highly correlated with nodule gas permeability (r = 0.92). A short-term oxygen enrichment study demonstrated nodule gas permeability may limit oxygen flux into both drought-stressed and well-watered nodules of these field-grown soybeans.Drought stress decreases nitrogen fixation rates as measured by both acetylene reduction (1,7,(17)(18)(19)(20)(21)(22) and 'sN uptake (16). 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. Pankhurst and Sprent (11, 12) postulated that the primary effect of drought stress on nitrogen fixation was to depress oxygen uptake and therefore respiration, causing a depletion of ATP inside the nodule. They presumed that the depressed oxygen uptake was caused by either a loss ofoxygen conductance through the nodule or by inhibition of oxygen requiring reactions, or perhaps by both. Restricted oxygen conductance has been shown by theoretical arguments to be an important feature of viable, well-watered nodules (16).With the development of an in situ system to make measurements of acetylene reduction rates under field conditions (3) and an analysis scheme to calculate the maximum rate of acetylene reduction, the nodule gas conductance, and the Michaelis-Menten constant (4), quantitative observations of the potential limitation of nodule conductance on acetylene reduction rates could be made. Further, conductance could be broken into its two components, permeability and surface area, k= P*A,where k = nodule conductance (mm3/s), P = the mean nodule gas permeability of all the nodules in the root chamber (mm/s), and A = total nodule surface area in the root chamber (mm2). Ifthe nodule gas permeability was found to influence acetylene reduction rates, this would indicate that 0...

Measurement of Legume Nodule Respiration and O2 Permeability by Noninvasive Spectrophotometry of Leghemoglobin

“…The gas permeability of legume nodules is of considerable current interest because nodule responses to drought, nitrate, and photosynthate supply all apparently involve changes in nodule 02 permeability (6,7,11,14,16,20,22). Similar permeability changes occur in response to acetylene exposure (12) or with increased or decreased external 02 concentration (23,27).…”

“…The steady-state acetylene method (4,26) is somewhat inaccurate and too slow to monitor rapid changes in nodule permeability. It is, however, the only method that has been used under field conditions (5,22) or with an individual nodule (26). Permeability estimates based on an entire modulated root represent an average for a number of nodules.…”

Phosphorylation of Soybean Nodulin 26 on Serine 262 Enhances Water Permeability and Is Regulated Developmentally and by Osmotic Signals

“…In addition, nodule development and maintenance are extremely sensitive to environmental and metabolic stresses. Osmotic stress results in morphological and biochemical changes that lead to the rapid cessation of nitrogen fixation (Pankhurst and Sprent, 1975;Weisz et al, 1985;Serraj et al, 1994), the induction of genes involved in stress adaptation (Delauney and Verma, 1990), and the accumulation of compatible solutes such as Pro and sugars, presumably to restrict water loss (Kohl et al, 1991;Irigoyen et al, 1992). Changes in membrane permeability coordinated with the accumulation and compartmentation of compatible solutes may be part of the osmotic adjustment, aiding in the maintenance of cell turgor and redirecting tissue water flow toward critical cells (Bohnert et al, 1995).…”

“…Because calcium signaling has been implicated in plant responses to osmotic signals (Knight et al, 1997), and considering the high sensitivity of nodules to osmotic stress signals (Pankhurst and Sprent, 1975;Weisz et al, 1985;Serraj et al, 1994), the effects of salinity stress on nodulin 26 phosphorylation in mature nodules were addressed. Exposure of 33-dayold nodulated soybeans to saline conditions (0.3 M NaCl) resulted in no detectable change in the overall levels of nodulin 26 protein ( Figure 9A, anti-nodulin 26 signal), whereas the phosphorylation levels of the protein ( Figure 9B, anti-GC12P signal) were enhanced as early as 4 h after treatment and remained high at 24 h after stimulus.…”

Mathematical Modeling of Oxygen Diffusion and Respiration in Legume Root Nodules

“…The responses of nodules to such seemingly unrelated factors as drought, nitrate, and defoliation all apparently involve changes in the gas permeability of this barrier (8,10,14,17,23,24 Variable gas permeability of nodules might result from changes in (a) the number or size of gas-filled radial pores through the barrier, (b) the thickness of water plugs in radial pores, (c) the extent of flooding between layers of cells, or (d) flooding of airspaces in the central zone.…”