Drought Effects on the N2‐Fixing (Acetylene Reducing) Ability of Vetch and Sweetclover Growing under Saline Conditions1

Abstract

AbstractDrought and salinity are prominent environmental stresses which limit N2 fixation by legumes. Both drought stress and salinity stress induce physiological adjustments which allow plants to compensate for decreasing available water, although the mechanisms may not be the same for the two types of stress. This study was designed to compare the effects of drought stress on N2 fixation for plants in nonsaline and saline soils.The acetylene reducing (N2‐fixing) activity of hairy vetch (Vicia villosa Benth) and yellow sweetclover [Melilotus officinalis (L.) Lam.] was determined after repeated cycles of drying and rewetting. Soil in pots was artificially salinized by adding NaCl and CaCl2 to achieve nominal electrical conductivity values of 0 (control), 4, 8, and 12 mmhos/cm. Day temperatures ranged between 24 and 30 C and night temperatures between 15 and 18 C. Soil water potential was measured by using thermocouple psychrometers. Drought was induced by withholding water and plants were rewatered when soil water potentials at the 6 cm depth dropped below −45 bars. Both vetch and sweetclover had decreasing acetylene reduction rates with increasing salinity levels when water was not limiting. After two drying and rewetting cycles, all plants had decreased levels of acetylene reducing activity regardless of salinity level. However, vetch plants in salinized soils retained a greater proportion of their pre‐drought acetylene reducing activity than plants in non‐saline soil. Sweetclover plants had little acetylene reducing activity after two drought cycles and vetch plants lost most activity after five drought cycles.