Barley responses to combined waterlogging and salinity stress: separating effects of oxygen deprivation and elemental toxicity

: Salinity and waterlogging are two major factors affecting crop production around the world and often occur together (e.g. salt brought to the surface by rising water tables). While physiological and molecular mechanisms of plant responses to each of these environmental constraints are studied in details, mechanisms underlying plant tolerance to their combine stress are much less understood. In this study, whole-plant physiological responses to individual/combined salinity and waterlogging stresses were studied using two barley varieties grown in either vermiculite (semi-hydroponics) or sandy loam. Two weeks of combined salinity and waterlogging treatment significantly decreased shoot and root fresh and dry weights, chlorophyll content, maximal quantum efficiency of PSII, and water content in both varieties, while the percentage of chlorotic and necrotic leaves and leaf sap osmolality increased. The adverse effects of combined stress were much stronger in waterlogging-sensitive variety Naso Nijo. Compared with salinity stress alone, combined stress resulted in 2-fold increase in leaf Na+ but 40% decrease in leaf K+ content. Importantly, effects of combined stress were strongly influenced by the growth media composition and were much pronounced in plants grown in sandy loam compared with those grown in vermiculite. Thus, it appears that hypoxia alone was not a major factor determining differential plant growth under adverse stress conditions, and that elemental toxicities resulting from changes in soil redox potential had a major impact on genotypic differences in plant physiological and agronomical responses. These results are further discussed in the context of plant breeding for waterlogging stress tolerance.