Comparative effects of NaCl and NaHCO3 stress on photosynthetic parameters, nutrient metabolism, and the antioxidant system in tomato leaves

- Alkali stress caused lower pigments and photosynthesis than neutral salt stress in tomato leaves.
- Alkali stress induced more serious oxidative stress on tomato leaves than neutral salt stress.
- Accumulation of Na in tomato leaves under alkali stress is higher than under neutral salt stress.
- Alkali stress showed higher inhibition in nutrient metabolism than neutral salt stress.

Detrimental effects of NaCl and NaHCO3 stress on tomato plants were compared. With increasing stress intensity, accumulation of biomass in tomato shoots and roots significantly decreased in both NaCl and NaHCO3 treatments. Compared to NaCl stress, NaHCO3 stress showed greater reduction of tomato plant growth, which was in accordance with stronger inhibition of photosynthesis by NaHCO3 stress. Chlorophyll (Chl) contents, the ratio of the variable fluorescence to the maximum fluorescence (Fv/Fm) and the actual quantum efficiency of photosynthetic system II (ФPSII) declined with increasing concentration of NaCl and NaHCO3, and their decreasing degree was more significant in NaHCO3 treatment. On the contrary, higher non-photochemical quenching (NPQ) was observed with increasing concentration of NaCl and NaHCO3, especially in NaHCO3 treatment. Na content in tomato leaves was positively correlated with NaCl and NaHCO3 stress intensity, while contents of K, P, and N as well as nitrogen metabolism-related enzymes including nitrate reductase (NR), glutamine synthetase (GS), glutamate dehydrogenase (GDH) and glutamate synthase (GOGAT) showed significant negative correlation with stress intensity, and their changes were much higher in NaHCO3 treatment than in NaCl treatment. Both NaCl and NaHCO3 stress induced excess reactive oxygen species (ROS) accumulation and obvious lipid peroxidation in tomato leaves, though was much higher in the NaHCO3 treatment. Compared to catalase (CAT) and guaiacol peroxidase (GPX), superoxide dismutase (SOD) and the ascorbate-glutathione cycle components including ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR) and glutathione reductase (GR) showed higher levels, which could play an important role in scavenging ROS in tomato leaves under salt and alkali stress.