Salicylic Acid Increases Photosynthesis of Drought Grown Mustard Plants Effectively with Sufficient-N via Regulation of Ethylene, Abscisic Acid, and Nitrogen-Use Efficiency

An essential approach to reduce drought in plants is to maximize the use of most limited available resources. The increase in water-use efficiency (WUE) is important to maximally utilize the available water to increase photosynthesis and growth of plants under water-deficit stress. Both WUE and photosynthetic nitrogen-use efficiency (PNUE), as the indices of resource-use efficiency were studied in mustard ( Brassica juncea L.) plants grown under limited water conditions with low-N (100 mg N kg −1 soil) and sufficient-N (200 mg N kg −1 soil) and sprayed with 0 or 0.5 mM salicylic acid (SA). Application of SA increased water potential, osmotic potential, WUE, and the incorporation of soil N into photosynthetic machinery by enhancing PNUE. It also increased photosynthesis of plants maximally by increasing stomatal conductance and intercellular CO 2 concentration under water-deficit stress. This increase was greater in the presence of sufficient-N where 0.5 mM SA maximally enhanced the N metabolism, redox ratio that mitigated the oxidative stress. The application of SA on plants supplemented with N reduced ethylene and abscisic acid (ABA) synthesis. It could be inferred that SA enhanced N utilization through increase in NUE when it is sufficiently available, increasing the GSH content to maintain redox ratio and inhibit ABA-mediated stomatal closure to enhance the resource utilization and photosynthesis. SA also enhanced glucose utilization which prevented glucose-mediated photosynthetic repression under stress. Thus, SA application may impart a potential management tool for increasing photosynthetic NUE, WUE, and photosynthesis under drought.