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Arbuscular mycorrhizal (AM) fungi are known to alleviate heavy-metal stress in plants. The intent of the present work was to analyze accumulation of heavy metals (Cd and Zn) in nodules of two Cajanus cajan (L.) Millsp. genotypes and their subsequent impact on nitrogen fixation, oxidative stress, and non-protein thiols (glutathione and phytochelatins) with and without AM fungus Glomus mosseae. Accumulation of Cd and Zn in nodules resulted in sharp reduction in nodule number, nodule dry mass as well as nitrogen fixation (leghemoglobin and nitrogenase (N₂ase)), although Cd had more pronounced effects than Zn. Cd-induced lipid peroxidation, H₂O₂ accumulation, and electrolyte leakage were largely reversed by Zn supplementation. Zn application significantly altered the negative effects of Cd on the synthesis of non-protein thiols, suggesting antagonistic behaviour of Zn. Higher concentration of Zn was more effective in lessening the negative effects of Cd than its lower concentration. Remarkable genotypic variation was found, with more severe effects of both the metals in P792 than Sel 85N. Glomus mosseae attenuated the phytotoxic effects of metals in nodules by decreasing metal uptake, oxidative stress, and by enhancing defense system ultimately leading to better nitrogen-fixing potential of pigeonpea nodules.
Salinity has plagued soil fertility and drastically affected growth and survival of glycophytes in irrigated regions of the world since the beginning of recorded history. It is particularly common in arid and semi-arid areas where evapotranspiration exceeds annual precipitation, and where irrigation is therefore necessary to meet crop water needs. Salt buildup in the soils and groundwater has threatened its productivity and sustainability. Plant responses to salt stress include an array of changes at the molecular, biochemical and physiological levels. Salt stress involves a water deficit induced by the salt concentration in the rhizosphere, resulting in disruption of homeostasis and ion distribution in the cell and denaturation of structural and functional proteins. As a consequence, salinity stress often activates cell signaling pathways including those that lead to synthesis of osmotically active metabolites, specific proteins, and certain free radical scavenging enzymes that control ion and water flux and support scavenging of oxygen radicals or chaperones. ROS detoxification forms an important defense against salt stress. Legumes are a key component of sustainable agriculture and can offer many economic and environmental benefits if grown more widely in crop rotations because of their ability to fix nitrogen in the root nodules in a symbiotic interaction with soil rhizobia. Due to their capacity to grow on nitrogen-poor soils, they can be efficiently used for improving saline soil fertility and help to reintroduce agriculture to these lands. However, in legumes, salt stress imposes a significant limitation of productivity related to the adverse effects on the growth of the host plant, the rootnodule bacteria, symbiotic development and finally the nitrogen fixation capacity. This paper reviews responses of legumes to salinity stress with emphasis on physiological and biochemical mechanisms of salt tolerance.
Salinity is an ever-increasing constraint limiting crop production in arid and semi-arid regions. Arbuscular mycorrhiza (AM) helps host plant to cope with detrimental effects of salinity. Experiments were aimed to examine the hypothesis that emergence is a better stage to determine salt tolerance of chickpea genotypes than germination and genotypic variability in their tolerance ability at emergence and subsequent vegetative growth is the manifestation of differential benefits imparted by mycorrhiza. Investigations were carried out at germination and emergence stage of genotypes (PBG 5, GPF 2, PBG 1, BG 1053, L 550) at 0, 40, 60, 80 mM NaCl. Significant genotypic variations in salt tolerance were observed at emergence rather than germination because of greater inhibitory effects on seedling emergence. Percent mycorrhizal colonization (MC) and its resulting impact on respiration rate (RR) and salt tolerance index (STI) at emergence indicated that PBG 5, with lowest RR, highest STI and mycorrhiza benefit percentage was the most tolerant whereas, L 550 the most sensitive genotype. Genotypic variability recorded at 30 days was consistent with that at emergence stage. Superior salt tolerance of PBG 5 than L 550 could be attributed to higher correlation between MC and physio-biochemical traits (RWC, chlorophyll a/b, proline accumulation, antioxidant activities). The study supported the hypothesis that both emergence stage and mycorrhizal effectiveness are important determinants of salt tolerance in chickpea genotypes. Evaluation of genotypes for relative adaptation to salinity should include estimation of their differential salt tolerance at different growth stages and symbiotic effectiveness of AM.
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