Effects of moderate soil salinity on osmotic adjustment and energy strategy in soybean under drought stress

Under drought and soil salinity, plants usually respond to accumulate inorganic and organic osmolytes for adaptation, that would induce changes in energy consumption strategy of plants. Moderate soil salinity would enable plants to lower energy consumption for osmotic adjustment by passively absorbing more Na⁺. This action would keep more energies for growth of drought-stressed plants. Thus, Na⁺ accumulation might be an energy-efficient strategy for plants to cope with drought was speculated. To support this speculation, we assessed the effects of soil salinity on osmotic adjustment and energy utilization under drought in this study. Our results indicated that the ratio and content of inorganic osmolytes was significantly higher under drought-saline stress (D + S) than those under single drought stress (D), while the osmolarity and contents of organic osmolytes of D + S were significantly lower than those of D. This indicated that moderate soil salinity could enable soybean seedlings to consume relatively lower energies to produce less organic osmolytes and accumulate more inorganic ions for osmotic adjustment coping with drought. Meanwhile the water content, cell turgor, ash content, and specific leaf area and biomass of D + S were significantly higher than those of D, but the leaf construction cost of D + S was significantly lower than those of D. This suggested that moderate soil salinity could enhance water retention, and reduce the photoassimilate and energy consumption of droughty soybean seedlings. This work would help to understand the positive effects of moderate soil salinity on plant growth on the level of osmotic adjustment and energy consumption strategy.