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

Salinity and flooding are two major production impediments affecting rice cultivation in coastal agro-ecosystems. We investigated how rice plants use two contrasting strategies such as energy conservation (for submergence tolerance) and energy expenditure (for ion exclusion) to adapt 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) QTLs 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 Saltol-containing Na+-excluder could not able to survive. The presence of thicker leaf gas film (LGF) and higher epicuticular wax helped longer underwater retention of LGF in Sub1-lines supporting survival under SWS. Thicker LGF significantly delayed Na+ entry to the leaves under anoxic conditions. Genotypes having good Na+-exclusion potential, but 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 adaptation than ion exclusion under saline water submergence in rice.