Comparative response to salinity of the growth and nodulation of Macroptilium atropurpureum cv. Siratro and Neonotonia wightii cv. Cooper seedlings

Abstract

Seedlings of two tropical legumes, Macroptilium atropurpureum cv. Siratro (siratro) and Neonotonia wightii cv. Cooper (glycine), were grown in nitrogen-free sand culture, and 12 days from germination were exposed to 14 days of salt treatment by addition of 0, 18, 36,72 or 144 mM NaCl to the base culture solution. Cowpea Rhizobium CB756 was applied either at germination (early inoculated) or at the start of salt treatment (late inoculated). Plants were sampled during salt exposure and during a 21-day recovery period in the absence of salt. Siratro was more tolerant of salt than glycine. Both species had similarly high chloride concentrations in their tops, up to 8-9% of dry weight of the leaf tissue. Siratro tolerated this high chloride with less injury and fewer plant deaths than glycine. Sodium was effectively excluded from leaves except in glycine at 72 and 144 mM NaCl. Chloride concentration of the leaves was more closely related to plant growth rate during recovery than was sodium concentration. Late-inoculated plants despite their small size and nitrogen deficiency were not more sensitive to salt than the early-inoculated plants. It is suggested that sodium and chloride accumulation in the late-inoculated plants may have been somewhat restricted by nitrogen deficiency, thereby limiting tissue injury in these small, slow-growing seedlings. Nodulation occurred during salt exposure, but was minimal at 144 mM NaCl. Rhizobia, however, survived this concentration and nodulation occurred during the recovery phase. Nitrogen fixation per unit nodule weight was not greatly restricted by salt, except where injury to the host was severe. Concentrations of salt ions in the nodules were generally low compared with those for the plant tops.