Germination, survival and growth of the coleoptile of rice under combined NaC1 and anoxia when submerged

[Truncated abstract] In many tidal lowland and coastal areas, direct seeded rice is often exposed to flooding in saline water. Understanding the physiological adaptations of submerged (non-transpiring) rice seedlings in these unfavorable conditions will provide new information on salt tolerance mechanism under submergence. In aerated salinity, submerged rice seeds germinated and tolerated at least 200 mM NaCl. Na+ and Cl- were used by seedlings as osmotica, by maintaining tissue concentrations approximately similar to those in the external medium during the quasi-steady state. With less organic solutes available and decreases of Na+ and Cl- concentrations with time, osmotic pressure in roots was probably maintained by restricted growth (i.e. less volume expansion). At 200 mM NaCl, supplemental Ca2+ at 5 and 10 mM alleviated~24% of the growth inhibition in roots, but had no effects in shoots. The different responses of shoots and roots to the different addition of Ca2+ might be due to the increase of K+/Na+ ratio in roots but not in shoots. In a combination of NaCl and anoxia, intact rice seedlings tolerated at least 100 mM, whereas excised coleoptile tips tolerated only 50 mM NaCl. In this energy starvation, Na+ and Cl- concentrations in shoots of intact seedlings were~30% less than concentrations in shoots of seedlings submerged in aerated solutions. In anoxic intact seedlings, an increase of ethanol production at 50 mM NaCl compared to that in 0.3 mM NaCl indicated that more energy was required for cell maintenance. Despite a 25% shoot growth inhibition, anoxic rice seedlings were able to survive at 100 mM NaCl by maintaining K+/Na+ ratio and also turgor pressure.