Responses of photosynthesis, nitrogen and proline metabolism to salinity stress in Solanum lycopersicum under different levels of nitrogen supplementation.

In the present study, effect of different levels of nitrogen (N, deprived; N, sub-optimum; N, optimum and N, supra-optimum) in Solanum lycopersicum L. seedlings under NaCl (NaCl, 0.3 g kg sand and NaCl, 0.5 g kgsand) stress was investigated. Biomass accumulation, pigments, K concentration, nitrate and nitrite contents were declined by NaCl in dose dependent manner. As compared to control (N without NaCl), fresh weight declined by 4% and 11%, and dry weight by 7 and 13% when seedlings were grown under N+NaCl and N+NaCl combinations, respectively. Furthermore, fluorescence parameters (JIP-test): the size and number of active reaction centres of photosynthetic apparatus (F/F), efficiency of water splitting complex (F/F), quantum yield of primary photochemistry (φP or Phi_P), yield of electron transport per trapped excitation (Ψ or Psi_0), the quantum yield of electron transport (φE), and performance index of PS II (PI) and parameters related to energy fluxes per reaction centre (ABS/RC, TR/RC, ET/RC and DI/RC) were also affected by NaCl. However, toxic effect of NaCl on photosystem II photochemistry was ameliorated by N. The lower dose (NaCl) of NaCl exerts damaging effect on oxidation side of PS II, while higher dose (NaCl) damages PS II reaction centre and its reduction side. Moreover, control seedlings (N without NaCl) when exposed to NaCl and NaCl exhibited a significant enhancement in respiration rate by 6 and 16%, Na accumulation by 111 and 169% in shoot, and 141 and 223% in root and ammonium contents by 19 and 34% respectively. Nitrate and ammonium assimilating enzymes such as nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase (GS) and glutamate synthase (GOGAT) were adversely affected by NaCl stress while glutamate dehydrogenase (GDH) showed reverse trend. N addition caused further enhancement in free proline, and activity of Δ-pyrroline-5-carboxylate synthetase (P5CS), while activity of proline dehydrogenase (ProDH) decreased. The results indicate that different levels of N significantly modulated NaCl-induced damaging effects in tomato seedlings. Furthermore, the results suggest that after N addition Na, nitrite, nitrate, ammonium contents, nitrogen metabolic enzymes, proline content, and activity of P5CS are favourably regulated, which might be associated with mitigation of NaCl stress and effect was more pronounced with supra-optimum level of N (N).