| Heavy metals are important environmental pollutants, and their toxicity is a problem of increasing significance for nutritional, evolutionary, ecological and environmental reasons. Rice crop is highly susceptible to the toxic levels of lead(Pb), mercury(Hg), and nickel(Ni), particularly at initial growth stages. Moreover, rain-fed lowland rice in the Asia commonly encounters the deficiencies of primary major nutrients due to fluctuation in water in the root zone and intensive cropping rotations. Rice growth and development is controlled by optimum supply of nutrients like N, P, and K, and the sub optimal levels of these nutrients is the key limiting factor of rice production throughout the world. The heavy metal toxicity as well as N-, P-, or K-deprivation may lead to reduced growth, nutrient imbalance, hormonal changes, oxidative damage, and the alterations of various stress-related metabolites. Seed priming is a controlled hydration technique, used for rapid and uniform emergence, high seedling vigor, and better growth in many field crops, particularly under unfavorable environmental conditions. Regardless of the shortage of literature on the significant effects of seed priming under abiotic stresses, little work has been done regarding the role of seed priming in enhancing the tolerance to the interaction of heavy metal toxicity and nutrient deprivation in rice. Here, a series of experiments were carried out in controlled growth chambers to unravel the role of seed priming on seedling growth, physiology, reactive oxygen species, antioxidants and nutrient status in rice seedlings under the interaction of various heavy metals(viz., Ni, Hg, Pb) and nutrient management regimes(Sufficient nutrient supply, N-deprivation, P-deprivation, K-deprivation). The main findings of current project are summarized below.(1) Nickel toxicity and deprivation of N, P, or K posed negative effects on the establishment of rice seedlings. The shoot length and biomass was severely reduced by Ni toxicity and nutrient deprivation, the poorest shoot growth was recorded for N-deprived and Ni-stressed seedlings. The root length and root biomass of N-deprived seedlings were increased, Ni stress reduced the root biomass but did not affect the root length of N-deprived seedlings. The P or K-deprived seedlings generally recorded similar root biomass compared with All Nut. Ni toxicity alone or in combination with nutrient deprivation triggered the production of ROS and caused lipid peroxidation in rice seedlings. Among antioxidants, only GR and Ve were found to significantly increase by Ni stress under all the nutrient management regimes. Ni stress also reduced the concentrations of N, Ca, Mg, Fe, and Moparticularly in shoot of rice seedlings.The N-deprived seedlings recorded maximum Ni concentration in shoot, while K-deprived seedlings showed higher Ni root concentrations. Seed priming of rice counteracted the stress-induced adversities in rice seedlings. The better growth and greater stress tolerance of primed rice seedlings was coordinately attributed to lower ROS production and accumulation, higher membrane stability, strong antioxidative defense system, and maintenance of mineral nutrient status.(2) The Hg toxicity induced several toxicity symptoms on rice seedlings that were documented by decreased shoot length, root length and their fresh and dry biomasses. Moreover, the rice seedlings under Hg stress were not adapted to the deprivation of N, P or K, the factor most limiting to plant growth despite the elevated total concentrations of Hg. The deprivation of any macronutrient, particularly N, exacerbated the toxicity of Hg in rice. Hg stress increased oxidative stress measured as MDA, and ROS accumulation in leaves under all the nutrient management regimes. The activities of key enzymes(SOD, POD, CAT, GPX, GR, GST) and the contents of non-enzymatic antioxidants(GSH, Vc) were also altered under the influence of Hg stress and nutrient deprivation. The rice seedlings grown under the interaction of Hg and N-deprivation showed poorest antioxidative defense system. The Hg stressed seedlings recorded lower concentrations of N, P, and Ca in their shoot. Interesting, K translocation from root to shoot was triggered under Hg stress. Seed priming was found to increase the rice growth, biomass and nutrient content under stress conditions. Seed priming also decreased the oxidative stress and enhanced the activities/levels of enzymatic and nonenzymatic antioxidants in rice leaves. It was concluded that seed priming may alleviate the ill effects caused by Hg toxicity and N, P or K deprivation in rice by decreasing oxidative stress and enhancing the activities/levels of antioxidants that positively affected the seedling growth and biomass.(3) Lead(Pb) toxicity also severely reduced the shoot growth of rice in N- or P-deprived seedlings, however, root growth was unaffected. Rice seedlings grown with sufficient supply of nutrients(All Nut) or K-deprivation showed no growth reduction under Pb toxicity. Exposure of Pb stress also triggered the oxidative stress under all the nutrient management regimes particularly under N- or P-deprivation. Pb toxicity was found to hamper the shoot accumulation of N, P, K, Ca, Mg, Cu, Mn, and Mo compared with control. However, concentrations of Zn and Pb in shoot were unaffected or increased. The Pb-induced reductions of P and Mn in the shoots were more severe than rest of elements. K-deprivation was found to restrain the uptake of Pb, which led to better growth performance of K-deficit seedlings under Pb toxicity. Seed priming treatments were found to be significant inalleviating the undesirable effects of Pb toxicity on rice growth. The primed rice seedlings showed minimal oxidative damage caused by excessive generation of ROS under Pb stress and/or nutrient deprivation. Seed priming strengthen the antioxidative defense system of rice seedlings by regulating the activities of antioxidant enzymes and levels of GSH in rice leaves. Moreover, seed priming prevented the cumulative damage development in response to Pb, as evident by the lowered shoot accumulation of Pb in primed rice seedlings. Obviously, the lowered root to shoot translocation and less accumulation of Pb in primed rice seedlings reduced the harmful effect evoked by Pb. Conclusively, these findings suggests the effective role of rice seed priming in enhancing tolerance to Pb toxicity and/or nutrient deprivation. |
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