Light intensity modulates antioxidant systems in salt-stressed tomato (Solanum lycopersicum L. cv. Micro-Tom) fruits

- Salt stress reduced ascorbic acid under high and low light but not under dark.
- Upregulation of antioxidant enzymes depended on high light.
- Light intensity modulates antioxidant enzymes in salt-stressed tomato fruit.
- Effects of salt stress and light differed between in vitro and in vivo fruits.

Salt stress has been applied to tomato production for improving fruit quality. Tomato fruit contains ascorbic acid (ASA), also known as vitamin C, which plays an important role in maintaining human health. The ASA content is regulated by several environmental factors including salt stress and light conditions. The aim of this study was to examine the combined effects of salt stress and light intensity on the ASA content and ASA-related antioxidant system in tomato fruits using tissue culture. In the tissue culture experiment, detached fruits were incubated in Murashige and Skoog medium containing 4% sucrose, 0.8% agar, and 100 mM NaCl (in vitro fruit). For comparison with in vitro fruit, hydroponically grown tomatoes were subjected to salt stress by adding 100 mM NaCl to a nutrient solution (in vivo fruit). The fruit were grown under different light conditions: dark (culture jars completely wrapped in aluminum foil), high light (HL, 595 ± 25 μmol m−2 s−1), and low light (LL, 223 ± 12 μmol m−2 s−1) conditions, and were harvested when red-ripe. Salt-induced changes in the net accumulation of ASA [on a dry-weight (DW) basis] were dependent on high light intensity applied to fruit, such that the net accumulation of ASA was lowered by salt stress under HL in both in vitro and in vivo fruits, whereas no such change was observed under the dark in in vitro fruits or LL in in vivo fruits. Only in in vitro fruits under HL, activities of the antioxidant enzymes such as ascorbate peroxidase, dehydroascorbate reductase, and catalase were higher in salt-stressed fruits than in controls. In contrast, oxidative parameters on a DW basis, as indicated by malondialdehyde, remained unchanged in both in vitro and in vivo fruits. In a principal component analysis, salt-induced changes in antioxidant systems showed clear differences between in vitro and in vivo fruits and between light conditions. Thus, we conclude that salt-induced changes in the ASA content and activities of antioxidant enzymes cannot be accounted for by salt stress alone and that high light intensity is also an important factor inducing the upregulation of antioxidant enzymes that reduce salt-induced oxidative stress.