Germination Stage Oxygen Deficiency (GSOD): An Emerging Stress in the Era of Changing Trends in Climate and Rice Cultivation Practice.

Abstract

Rice (Oryza sativa) is probably the most important crop to ensure world's food security. Adapted to semi-aquatic ecology, it has got an innate ability to tolerate submergence to some extent. Nevertheless, long period of submergence invites unavoidable negative consequences starting from yield and quality reduction of the produce to complete crop failure. Under submergence plants are deprived of oxygen either completely (anoxia) or partially (hypoxia) which is well-recognized as a potent abiotic stress factor. Significant research efforts have been directed globally toward understanding the mechanism of submergence tolerance particularly during vegetative stage of rice; as the crop frequently encounters flash flood or water stagnation after transplanting and during its initial growth stage. Thanks to these scientific endeavors which resulted in identification, cloning, characterization, and deployment of genes like SUB1 (Xu et al., 2006; imparts quiescence during submergence) and SNORKEL1 and SNORKEL2 (Hattori et al., 2009; induce quick elongation of internodes) which help rice plants either to tolerate or to escape submergence during its vegetative growth stage. However these genes, unfortunately, have little or no consequences when seed germination under submergence is concerned. Seed germination under oxygen deprived condition is gradually becoming an area of active research owing to the shifting trend of direct seeding by abandoning traditional transplanting method, in order to intensify as well as economize rice cultivation (Kumar and Ladha, 2011). Though, direct seeding can potentially reduce cultivation cost, it also makes the crop vulnerable to the fluctuations of monsoon rains which are quite frequent in South and South-East Asia (the rice bowl of the world) dominated by low land rain-fed ecosystem. Unpredicted heavy downpour immediately after direct seeding can call upon flash flood. Low land situation can also result in continuous water stagnation for several days. Stagnant water can condition typical stress situation by restricting free diffusion of oxygen from air to germinating seeds (Narsai et al., 2015). But grossly being adapted to aquatic ecology, rice has developed the unique mechanism to germinate and elongate its coleoptile under water (nearly at the rate of 1 mm h−1) even in complete absence of oxygen (Magneschi and Perata, 2009; Narsai et al., 2015)—a phenomenon termed as anaerobic germination (AG). However, anaerobic germination potential (AGP) varies greatly among different rice cultivars which ultimately provide an edge to a few cultivars to perform better under oxygen deprived conditions over others. In recent literatures, the rice cultivars having better germination potential under oxygen deprived condition and hence are capable of withstanding the stress have been deemed as anaerobic germination tolerant, while the cultivars having the contrasting character have been termed as anaerobic germination susceptible (Angaji et al., 2010; Baltazar et al., 2014; Kretzschmar et al., 2015). Since, this is an upcoming area of research, it is necessary to set the terminologies right at the very beginning as once established these terminologies will stand for future and can even percolate and perpetuate in related field of studies. In this article, we would try to critically analyze the aptness of the terminologies used so far by logical arguments and present our view where we feel a need for change/rectification.