Nitrogen mitigates effect of salinity on plant water relations and photosynthesis in indian mustard ( Brassica juncea )

Abstract

The interaction between salinity (8 and 12 dS m −1 ) and three levels (40, 80 and 120 kg ha −1 ) of different forms of nitrogen (NO 3− , NH 4+ and NO 3− + NH 4+ ) were studied in Brassica juncea cv. RH-30. The plants were salinized with 8 and 12 dS m −1 at 35 and 55 days after sowing. The relative water content (RWC), water potential (Ψ w ) and osmotic potential (Ψ s ) exhibited a marked decline under salinity stress. The application of the combined form (NO 3− + NH 4+ ) of nitrogen (120 kg ha −1 ) considerably improved the water status and mitigated the adverse effect of salinity on growth. The salinity-induced osmotic effect led to stomatal closure and caused a substantial reduction in net photosynthetic rate (P N ), stomatal conductance (g s ) and transpiration rate (E) at the pre-flowering and flowering stages (45 and 65 DAS). Salinity effects were considerably moderated by additional nitrogen supply, which varied with the source of nitrogen, the level of salinity/fertilizer and the stage of plant growth. The inhibition in photosynthesis was relatively greater in ammonium-fed (NH 4+ ) than in nitrate-fed (NO 3− ) plants, while the transpiration rate was relatively lower in nitrate-fed plants grown either with or without saline water irrigation. The nitrate form of nitrogen @ 120 kg ha −1 proved best in alleviating the adverse effect of salinity on photosynthesis and transpiration at both the growth stages.