Role of putrescine (Put) in imparting salt tolerance through modulation of put metabolism, mycorrhizal and rhizobial symbioses in Cajanus cajan (L.) Millsp.

Abstract

Salt stress is a major environmental constraint that limits growth and nitrogen-fixation in legumes. Role of putrescine (Put) and arbuscular mycorrhiza (AM) in improving functional efficiency of legumes has gained importance in recent years. Present investigations assessed the impact of Put (1 mM) seed priming and/or Rhizophagus irregularis inoculation on growth, mycorrhizal and rhizobial symbioses with an emphasis to correlate the same with nodular Put metabolism in pigeonpea (Cajanus cajan L.) genotypes (Tolerant-Pusa 2001 and Sensitive-Pusa 991) under salt stress. Salinity declined plant biomass, with greater negative effects on roots than shoots which reduced the mycorrhizal colonization as well as nodulation ability of Sinorhizobium fredii AR-4. The decline in nitrogen-fixing efficiency could be correlated with increase in Na+ concentrations in roots as well as nodules. Salinity reduced endogenous Put by increasing diamine oxidase (DAO) as well as decreasing arginine decarboxylase (ADC) as well as ornithine decarboxylase (ODC) activities in nodules. Put priming enhanced per cent mycorrhizal colonization which further increased the rhizobial symbiotic efficiency, more in Pusa 2001 than Pusa 991. Both Put priming and AM inoculations reduced Na+ uptake and improved nutrient status especially P in both underground organs, with AM more effective than Put. The further decline in Na+ uptake was recorded when both amendments were given together which enhanced nitrogen-fixing ability of nodules by modulating anabolic and catabolic enzyme activities responsible for Put biosynthesis. Hence, +Put+AM can be used as an effective strategy to improve symbiotic potential and arrest nodule senescence in pigeonpea under salt stress.