Effect of Arbuscular Mycorrhizal Inoculation on Salt-induced Nodule Senescence in Cajanus cajan (Pigeonpea)

Abstract

Many physiological and biochemical plant processes affected by salt stress trigger premature nodule senescence and decrease their ability to fix nitrogen. The objective of this study was to evaluate the role of arbuscular mycorrhiza (AM) in moderating salt-induced premature nodule senescence in Cajanus cajan (L.) Millsp. Greenhouse experiments were conducted in which the plants were exposed to salinity stress of 4, 6, and 8 dSm−1. Various parameters linked to nodule senescence were assessed at 80 days after sowing. Nodulation, leghemoglobin content, and nitrogenase enzyme activity measured as acetylene-reducing activity (ARA) were evaluated. Two groups of antioxidant enzymes were studied: (1) enzymes involved in the detoxification of O 2 − radicals and H2O2, namely, superoxide dismutase (SOD), catalase (CAT) and peroxidase (POX), and (2) enzymes that are important components of the ascorbate glutathione pathway responsible for the removal of H2O2, namely, glutathione reductase (GR) and ascorbate peroxidase (APOX). Exposure of plants to salinity stress enhanced nodule formation; however, nodule growth suffered remarkably and a marked decline in nodule biomass, relative permeability, and lipid peroxidation was observed. Leghemoglobin content and ARA were reduced under saline conditions. AM significantly improved nodulation, leghemoglobin content, and nitrogenase activity under salt stress. Activities of SOD, CAT, APOX, POX, and GR increased markedly in mycorrhizal-stressed plants. A synthesis of the evidence obtained in this study suggests a correlation between enhanced levels of antioxidant enzyme activities, reduced membrane permeability, reduced lipid peroxidation, and improved nitrogen-fixing efficiency of AM plants under stressed and unstressed conditions. These factors could be responsible for the protective effects of mycorrhiza against stress-induced premature nodule senescence.