| Abstract: | Soybean (Glycine max L. Merr) cv. Clarke plants inoculated withBradyrhizobium japonicum strain RCR3407 were grown either ina greenhouse with a low irradiance (200–400)µmolm–2 s–1) or in a controlled-environment growth cabinetwith a higher irradiance (600 µimol m–2 s–1).At 42 d plants were given a nitrogen-free nutrient solutioncontaining 50 mol m–3 sodium chloride for 2 weeks andthen allowed to recover from salt-stress for a further 2 weeks. Salt treatment reduced plant growth by at least half in bothgrowth regimes, however, the controlled environment-grown (CEG)plants were five times larger than the greenhouse-grown (GG)plants in terms of dry weight and number/weight of nodules perplant, regardless of treatment. The structure of nodules, from both growth regimes, harvestedat the end of the 2 week salt-stress was similar to unstressedcontrol nodules. However, nodules harvested 1 week later fromboth CEG and GG plants had structural changes including degradationof bacteria in vacuoles around host cell nuclei, particularlyin the outer cell layers of the infected tissue. In addition,meristematic activity was seen in the cortex of some nodulesfrom GG plants. Young cells here contained infection threadsand newly-released bacteria. Nodules harvested 2 weeks after removal of the salt-stress fromCEG plants showed an apparent recovery from the stress. However,there was a very marked increase in the amount of starch inthe cortex which was not seen in equivalent GG nodules. In contrast,nodules from GG plants contained many vacuolate infected cellsand, consequently, a lowered bacteroid population. Further,meristematic activity was seen in a zone concentric to the infectedzone, newly-formed cells contained many large infection threadsand were interspersed with intercellular bacteria. The meristematicactivity increased the relative volume of cortical to infectedcells in these nodules. Growth conditions did not affect control nodule specific nitrogenaseactivity or oxygen diffusion resistance (R) and these parameterswere also not altered in CEG nodules exposed to salt plus the14 d recovery period. However, nitrogenase activity was greatlyreduced, and R increased by more than eight times in equivalentGG nodules exposed to salt plus recovery. It is hypothesized that the gross morphological changes werean attempt to counter salt toxicity and/or oxygen damage underconditions of reduced photosynthate supply to the nodules dueto the poor light levels in the greenhouse. However, soybeannodules supplied with adequate photosynthate were able to withstandand recover from long-term salt-stress with little alterationto their structural integrity. Key words: Soybean, sodium chloride, nitrogen fixation, light intensity, oxygen diffusion resistance |
