Wangchu, L.Khongla, LakidonHazarika, B.N.2019-07-182019-07-182019-07-01http://krishikosh.egranth.ac.in/handle/1/5810115307Abiotic stress poses major problems to agriculture and the understanding of the responses and mechanisms of plant stress tolerance is key to the successful selection of tolerant crops in growth areas affected by changes in the abiotic environment. The condition of the soil plays a major role in determining the success or failure of the crop grown. Soil fertility constraints, like soil salinity, soil acidity and nutrient deficient soils, are one of the critical components by which plants are subjected to abiotic stress. Under these circumstances, identification of tolerant plant species is fundamental to improve crop production and sustainability.The effects of Aluminium and Manganese toxicity, applied singly or in combination, on the growth, photosynthetic attributes, oxidative stress and anti-oxidative capacities of 5 citrus species were evaluated in this study. This experiment was taken up to understand the tolerance mechanism of citrus to the toxicities which occur in acid soils and to identify species better suited for such soils. In this experiment, Rough lemon (Citrus jambhiri), Rangpur lime (Citrus limon), Indian wild orange (Citrus indica) and two local citrus species viz., Tasi and Samphola were evaluated for a period of 20, 40 and 60 days in a sand culture experiment wherein 3 doses of Mn (0, 300 µM, 600 µM) and 4 doses of Al (0, 600 µM, 1200 µM, 1800 µM) were incorporated, either singly or in combination, to their nutrient solutions. Here, morpho-physiological parameters indicate a significant negative impact of the treatments on all the citrus species with growth inhibition by Mn treatments more pronounced in shoots while Al affected root growth more than shoot. Chlorophyll content and total soluble protein also declined with the increase in treatment concentration and exposure time although the least decline was observed in Samphola followed by Rough lemon. The peroxidation of lipids and the accumulation of reactive oxygen species (ROS) viz., superoxide anion (O2•-), hydroxyl free radical (-OH) and hydrogen peroxide (H2O2) revealed that Tasi followed by Rangpur lime was the most sensitive as both species recorded high amounts of ROS when compared with other species indicating that they may show lower resistance to the stresses induced. Al content in citrus shoots and roots also elevated with the increase in their concentrations and exposure time. Mn content also increased in both shoots and roots, however, addition of Al (600 µM) resulted in lower Mn accumulation (300 µM) than compared to the same treatment applied singly and this reduction was also reflected in lowered negative impact of physical parameters, particularly, shoot length, shoot weight (fresh and dry), relative water content and other parameters including lipid peroxidation and ROS accumulation. Antioxidants, both enzymatic and non-enzymatic, showed heightened activities in all the citrus species with the increase in time exposure although non-ezymatic antioxidants (ascorbic acid and tocopherol) progressively declined with time. The comparatively low antioxidant capacity of Tasi and Rangpur lime reflects their low resistance as evident from their high ROS production as well as more severe decline in their physical growth. Therefore, on considering the tolerance levels of the citrus species by their antioxidative defense, this experiment revealed that Samphola, followed by Rough lemon and Indian wild orange exhibited better tolerances to both Mn and Al toxicity while Tasi and Rangpur lime exhibited comparatively lower resistance to these stresses. It can also be concluded that antagonistic relationship is observed between Al and Mn although the effect is dose responsive as it is seen only in the lowest doses of both Al and Mn.ennullThesis