THE EFFECT OF IRRADIANCE ON THE RECOVERY OF SOYBEAN NODULES FROM SODIUM CHLORIDE-INDUCED SENESCENCE

Soybean (Glycine max L. Merr) cv. Clarke plants inoculated with Bradyr hizobium japonicum strain RCR3407 were grown either in a greenhouse wi th a low irradiance (200-400 mumol m-2 s-1) or in a controlled-environ ment growth cabinet with a higher irradiance (600 mumol m-2 s-1). At 4 2 d plants were given a nitrogen-free nutrient solution containing 50 mol m-3 sodium chloride for 2 weeks and then allowed to recover from s alt-stress for a further 2 weeks. Salt treatment reduced plant growth by at least half in both growth regimes. however. the controlled envir onment-grown (CEG) plants were five times larger than the greenhouse-g rown (GG) plants in terms of dry weight and number weight of nodules p er plant, regardless of treatment. The structure of nodules, from both growth regimes, harvested at the end of the 2 week salt-stress was si milar to unstressed control nodules. However, nodules harvested 1 week later from both CEG and GG plants had structural changes including de gradation of bacteria in vacuoles around host cell nuclei, particularl y in the outer cell layers of the infected tissue. In addition, merist ematic activity was seen in the cortex of some nodules from GG plants. Young cells here contained infection threads and newly-released bacte ria. Nodules harvested 2 weeks after removal of the salt-stress from C EG plants showed an apparent recovery from the stress. However, there was a very marked increase in the amount of starch in the cortex which was not seen in equivalent GG noduleS. In contrast, nodules from GG p lants contained many vacuolate infected cells and, consequently, a low ered bacteroid population. Further, meristematic activity was seen in a zone concentric to the infected zone, newly-formed cells contained m any large infection threads and were interspersed with intercellular b acteria. The meristematic activity increased the relative volume of co rtical to infected cells in these nodules. Growth conditions did not a ffect control nodule specific nitrogenase activity or oxygen diffusion resistance (R) and these parameters were also not altered in CEG nodu les exposed to salt plus the 14 d recovery period. However, nitrogenas e activity was greatly reduced, and R increased by more than eight tim es in equivalent GG noduleS exposed to salt plus recovery. It is hypot hesized that the gross morphological changes were an attempt to counte r salt toxicity and or oxygen damage under conditions of reduced photo synthate supply to the nodules due to the poor light levels in the gre enhouse. However, soybean nodules supplied with adequate photosynthate were able to withstand and recover from long-term salt-stress little alteration to their structural integrity