Energy conservation or expense? A possible dilemma under combined stresses of salinity and submergence in rice

Salinity and flooding are two major impediments to production affecting rice cultivation in coastal agro-ecosystems. We investigated how rice plants use two contrasting strategies, energy conservation (for submergence tolerance) and energy expenditure (for ion exclusion), to acclimate to the combined stresses of saline water submergence (SWS). Pot and hydroponic experiments were conducted using four selected rice genotypes carrying Sub1 (Submergence1) and/or Saltol (Salinity tolerance) quantitative trait loci in their genetic background and exposed them to salinity and submergence stresses individually and combined under controlled experimental conditions. We found that Sub1-containing submergence-tolerant lines performed better under SWS, where the Saltol-containing Na+ excluder could not survive. The presence of a thicker leaf gas film (LGF) and higher content of epicuticular wax helped longer underwater retention of the LGF in Sub1-lines, supporting survival under SWS. A thicker LGF significantly delayed Na+ entry into the leaves under anoxic conditions. Genotypes having high Na+ exclusion potential, but a thinner LGF, ultimately accumulated more Na+ in the leaf tissue under SWS. The lowest leaf carbohydrate depletion was observed in FR13A, whereas non-Sub1-associated escape coupled with ion exclusion utilized the highest carbohydrate reserve in FL478 under SWS treatment. Overall, this study uncovers that the Sub1-mediated quiescence strategy complemented by higher tissue tolerance ability is a more suitable mechanistic acclimation than ion exclusion under SWS in rice.