Nitrogen Fixation by Subterranean Clover at Varying Stages of Nodule Dehydration

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Abstract
The nodule water potential (εnod) of subterranean clover (Trifolium subterraneum L.) cv. Seaton Park incubated in a flow-through gas-exchange system was induced to decline independently of leaf water potential (ε1) by passing a continuous dry airstream over the nodulated roots of intact well-watered plants. Reduced transpiration by plants whose nodules had become dehydrated was hypothesized to be related to the decline in nitrogen fixation. Whole-plant and nodule soluble carbohydrates increased as εnod declined. Throughout an 8 d period of continual nodule dehydration, the gaseous diffusion resistance of nodules increased and the optimum pO2 for nitrogenase activity declined from 52 to 28 kPa. Following rehydration of the nodulated roots between days 4 and 5 and between days 7 and 8, nodulated root respiration increased to or above pre-stress levels whereas nitrogenase activity did not recover. Re-establishment of initial rates of nodulated root respiration was due to the stimulation of growth and maintenance respiration, not to the respiration coupled to nitrogenase activity. Although no recovery of nitrogenase activity occurred, the elapsed time from the introduction of acetylene into the gas stream flowing past the nodules until measurement of the acetylene-induced decline in nitrogenase activity, decreased substantially. This was characteristic of an increase in the permeability of the nodules to gaseous diffusion upon rehydration. However, calculated values of nodule diffusion resistance after the 24 h periods of rehydration did not indicate any recovery of gaseous diffusion resistance based on measurements of the respiration coupled to nitrogenase activity. Hence, use of a diffusion analogue (i.e. Fick's Law) in conjunction with nodule respiratory CO2 efflux was unable to predict changes in permeability of the variable barrier of legume nodules following nodule dehydration and recovery.