PHYSIOLOGY OF THE LEGUME NODULE AND ITS RESPONSE TO STRESS

A common mechanism is sought for the effect of environmental variables (e.g. light, defoliation, temperature, N: nitrate, ammonium or organic) on nodule metabolism. A decrease in nodule cortical permeability has been suggested to enforce a primary limitation of O-2 supply to the infected zone during periods of environmental stress, but the mechanism by which cortical permeability is altered is controversial. It is proposed that the water requirement of xylem export from the nodule achieves a balance with water delivery to the nodule in the phloem, such that a change in either export or import will affect the turgor of cells surrounding the nodule vascular bundles. Cortical cell turgor may also be influenced by the balance of solutes between the apoplast and symplast. Stress events involving variations of phloem supply or decrease in rate of N-2 fixation (e.g. C2H2 assay) will result in changes in the soluble sugar and nitrogenous solute content of the cortex, with implication to cortical cell turgor. Change in cortical cell turgor may affect the turgor gradient driving phloem supply into the nodule in a form of turgor homeostat, and coincidentally may affect cortical cell shape or the intercellular water content and so alter the permeability of the nodule cortex to gases. It is proposed this regulation of nodule permeability is localized in the vicinity of the vascular bundles. Nodule anatomy and physiology is, however, heterogeneous. For example, nodule cortical anatomy is variable between species. Thus, the response of N-2 fixation to the effect of an environmental variable is species specific. The response will also reflect the duration and severity of stress, the plant growth history and will be complicated by effects on nodulation and anatomical adaptation in the longer term.