ABSTRACT
Once a seed has been sown in soil, it has to pass through various biotic and abiotic stress factors. Abiotic stress such as drought, salinity, nutrient excess and deficiency, temperature extremes and submergence are major limiting factors to achieve potential crop yield. Overall, almost all the cultivable land area is affected by one or more abiotic stresses. As major constraints limiting crop production worldwide, water scarcity, nutrient imbalance, salinity and submergence have resulted in a decline or even complete inhibition of seedling emergence and stand establishment. Such inhibitions are mainly due to a decrease in water potential, which led to lower water uptake and also to excessive production of reactive oxygen species under abiotic stress. Priming is the cellular state in which the harmful effects of abiotic stress factors on plants are hindered by pre-exposure to a stimulus, which results in greater survival. It has become increasingly evident that priming techniques (e.g., external application of natural or synthetic compounds in plants) can enhance the tolerance of crops to environmental stresses. In this situation, seed priming helps to alleviate various abiotic stresses on seed germination. Seed priming varies with the type and intensity of abiotic stresses and needs to be adopted in order to counter them. It improves the germination percentage, promotes uniformity in seedling emergence and enhances subsequent seedling growth. At a sub-cellular level, seed priming was found to improve germination and seedling vigor by protecting cellular protein, repairing DNA damage during seed storage, improving the functioning of the protein synthesis machinery and increasing energy levels by mitochondrial integrity. During priming, metabolic activities proceed to repair and build up nucleic acids, increase synthesis of proteins as well as repair membranes and enhance the activities of anti-oxidative enzymes. As a result, the germination capability and tolerance of unfavorable conditions of seeds can be promoted. The positive effects of priming are associated with a wide range of metabolic and physiological improvements. Among them, activating protective enzymes such as SOD, POD and CAT and accumulating osmoprotectants such as proline, soluble sugar and soluble protein are the typical stress-avoidance responses. Activation of enzymatic antioxidants can reduce ROS-induced oxidative damages. Osmoprotectants help to enhance water uptake by improving the water status. The aim of this chapter is to provide an up-to-date overview demonstrating responses of plants towards alleviation of abiotic stresses.
