| Plants generally encounter a wide range of abiotic stresses during their growth spam. Rice, a staple food for more than half of the world‘s population, possesses an odd portfolio of tolerance and susceptibility to abiotic stresses compared with other crops. Rice is extremely sensitive to low temperature, drought stress, and salinity particularly at early growth stage. In natural conditions, rice crop is routinely subjected to a combination of different abiotic stresses, therefore, the investigations considering the multiple stress factors are of ecological significance. Response of rice to multiple stress factors is unique and cannot be directly extrapolated by the response to each of the different stresses applied individually. In order to cope with a particular stress conditions, plant require energy and sufficient resources, however, nutrient limitations further hinder the acclimation process of plants to stress conditions. 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. Despite the dearth of literature on the useful effects of seed priming under abiotic stresses, little work has been done regarding the role of seed priming in enhancing the tolerance to multiple stress factors in rice and to explore the underneath mechanisms. Here, a series of experiments were carried out in controlled growth chambers to unravel the physiological, biochemical, metabolic and mineral nutrient responses in primed and non-primed rice seedlings under the interaction of various abiotic stresses viz., chilling(18?C), salinity(10 d S m-1), drought(10% PEG solution), cadmium toxicity(100 μM Cd Cl2) and different nutrient management regimes(Sufficient nutrient supply, N-deprivation, P-deprivation, K-deprivation). It was hypothesized that seed priming might have the potential to confer tolerance to abiotic stresses by regulating various metabolic processes, modulating the production of osmolytes and secondary metabolites, enhancing the antioxidative defense system, and maintaining the plant-nutrient status. The main findings of these studies are summarized below.(1) Exposure of chilling stress at 18?C caused erratic and delayed germination, poor seedling growth, reduced starch metabolism and lower respiration rate, while increased lipid peroxidation and H2O2 accumulation in rice seedlings. Nevertheless, all the tested seed priming treatments effectively alleviated the negative effects of chilling stress. Chemical priming with selenium(Se) and hormonal priming with salicylic acid(SA), remained more effective treatments. The better germination and vigorous growth of primed-rice seedlings under chilling stress was associated with higher starch metabolism, enhanced respiration rate, better membrane integrity, higher metabolite synthesis, and increased activities of antioxidants in these seedlings.(2) Reduced seedling growth of rice was also evident under the interaction of chilling stress and N-, P-, or K-deprivation. Under chilling as well as control temperature, the Ndeprivation caused the maximum reduction in shoot growth, while root growth was only decreased by P- or K-deprivation. The N-deprivation enhanced the root length of rice, nevertheless, root biomass was unaffected. Rate of lipid peroxidation and as well as the production of ROS(O2?-, OH-, H2O2) was generally increased under stress conditions; the K-deprived seedlings recorded significantly lower production of ROS compared with those under N- or P-deprived conditions. Variations in nutrient concentrations of primed and non-primed rice seedlings were also apparent under the influence of chilling stress and N, P, or K deprivation. Chilling stress reduced or had no effect on the concentrations of all measured nutrients, except Zn, and root-P. All the seed priming were found to trigger or at least maintain the antioxidant defense system of rice seedlings. Apparently, the levels of ROS were significantly reduced by seed priming treatments, consistent with the activities of MAO and XOD, suggesting that the reduced activity of ROS-producing enzymes was also the main reason for better tolerance of primed rice seedlings, along with the role of ROS-scavenging enzymes. Seed priming treatments enhanced the concentrations of N, P, and K, while did not alter the concentrations of other elements under chilling stress and different nutrient management regimes. However, total nutrient content per seedlings were considerably higher in primed rice seedlings, and were associated with better and vigorous seedling growth.(3) High salinity and deprivation of N, P, or K induced the production of ROS and caused lipid peroxidation in the rice leaves. The negative effects of salinity were more with the interaction of K-deprivation, although the K-deprivation in non-saline conditions recorded the lower ROS production than N- or P-deprivation. Salinity stress also disrupted the ionic balance in all the nutrient management regimes. The symptoms of stress-induced oxidative damage were not apparent in priming rice seedlings. The priming-induced coordinated changes in the activity of various antioxidative enzymes and the levels of non-enzymatic antioxidant efficiently scavenged the overproduction of ROS, thereby protecting the membrane integrity as evidenced from the unchanged level of membrane lipid peroxidation. Moreover, seed priming was helpful in decreasing the salinity-induced ion toxicity, and maintained the nutrient balance in rice seedlings. Higher content of essential mineral nutrients in primed rice seedlings might have played important part in activating a number of biochemical and physiological events associated with stress adaptations. The beneficial effect of seed priming treatments on nutrient uptake were more prominent under the combined stress of K-deprivation and salinity, where primed rice seedlings recorded 18-102% higher nutrient content compared with respective non-primed seedlings.(4) Interaction of drought and nutrient deprivation also caused pronounced changes in the oxidative metabolism and subsequent growth of rice seedlings. The marked increase in the accumulation of ROS(O2?-, OH-, H2O2) and activities of ROS-producing enzymes under the individual as well as interactive effect of drought and N-, P-, or K-deprivation led to higher lipid peroxidation. The interaction of drought stress and N-deprivation caused the maximum oxidative damage, and recorded poor antioxidant activity, suggesting that N-supply is more crucial under drought stress. The N-deprivation also significantly decreased the levels of GSH, Vc and Ve, which are crucial for the drought tolerance of plants. Under all the nutrient management regimes, drought stress was generally found to decrease the concentrations of macro- and micro-nutrients in rice seedlings except K and Zn. The oxidative stress evoked by drought or/and nutrient deprivation, was effectively alleviated after seed priming. The leaves of rice seedlings emerged from primed seeds(particularly Se- and SA-priming), recorded significantly lower accumulation of ROS and MDA, and lower activities of MAO and XOD. These attributes were well linked to priming-induced enhancements in the activities/levels of SOD, POD, GR, GPX, GSH and Vc in the rice leaves. The total content of different mineral nutrients in the primed rice seedlings were also increased in the range of 10-47%, clearly indicating the higher acquisition ability of primed rice seedlings.(5) Exposure of Cd stress enhanced the oxidative stress particularly under N-deprivation. However, values of ROS accumulation, and MDA content in the leaves of primed rice seedlings were lower compared with than non-primed seedlings under Cd stress and different nutrient management regimes, suggesting that seed priming alleviated the oxidative damage caused by excessive generation of ROS. Seed priming strengthen the antioxidative defense system of rice seedlings by regulating the activities of antioxidant enzymes and levels of non-enzymatic antioxidants. The exposure of Cd stress severely reduced the uptake and accumulation of N, P, K, Zn, Mn, and Cu, irrespective of the nutrient management regimes. In the roots, Cd stress considerably enhanced the ratios of Ca/K, Ca/Mg, and Fe/Mn. Interestingly, P- as well as K-deprived seedlings recorded lower concentrations of Cd in both plant parts. It might be suspected that the protective effect of K or P-deprivation against Cd toxicity was also due to the inhibition of Cd uptake along with enhanced antioxidant defense system. Vigorous growth performance of primed rice seedlings under normal as well as stress conditions led to higher uptake of essential macro- and micro-nutrient. Better nutrient status(Ca, Mg, Zn, Fe, and Mn) in primed rice seedlings restricted the entry and transport of Cd, as evident by lower shoot Cd accumulation in these seedlings. Obviously, the lowered root to shoot translocation and less accumulation of Cd in primed rice seedlings reduced the harmful effect evoked by Cd and exerts a beneficial effect on the growth. Taken together, these lines of evidence could explain, at some extent, the protective role of seed priming against the adversities caused by Cd stress and nutrient deprivation in rice seedlings. |
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