Antioxidant Responses of Water-Stressed Cherry Tomato Plants to Natural Biostimulants

Biostimulants’ application to plants can reduce the damage caused by abiotic factors such as drought or salinity and improve crop yield under these stressful conditions. In this work, several biostimulants, namely Terrabion Aminovit^® (a commercial product based on amino acids), potassium fulvate, humic acids, and a seaweed extract, were applied to cherry tomato plants using fertigation at two doses of 0.2 and 1.0 g L⁻¹. The plants were then subjected to a water stress treatment by completely withholding irrigation for 12 days. After the treatments, all plants were harvested to determine several growth and biochemical parameters. Pre-treatment with all biostimulants protected the tomato plants against dehydration, as indicated by a significant increase in leaf water content compared to the non-irrigated controls. Leaf fresh weight and root water content also increased, except in the plants treated with humic acids, by about 2 fold in plants pre-treated with Terrabion Aminovit^® and 1.5 fold in the presence of potassium fulvate and the seaweed extract. The water stress treatment caused a significant increase in leaf proline content, up to 113.6 μmol g⁻¹ DW, approximately 18 fold higher than in well-irrigated control plants; this value was significantly lower in Terrabion Aminovit^® pre-treated plants but even higher, ca. 180 μmol g⁻¹ DW, in those treated previously with the seaweed extract. These results indicate that proline is a suitable water stress biomarker in tomatoes and that the biostimulants probably differ in their mode of action, suggesting that the effect of the seaweed extract is mediated by proline accumulation. A significant activation of antioxidant enzymes, namely superoxide dismutase, ascorbate peroxidase, and glutathione reductase, was also observed in water-stressed plants; application of the biostimulants resulted in all cases, in a significant reduction in the specific activities of the three enzymes, indicating reduced levels of drought-induced oxidative stress in the plants. We conclude that applying these biostimulants, particularly Terrabion Aminovit^®, may help minimise the adverse effects of water stress on tomatoes by maintaining turgor and improving growth through mechanisms still unknown but which appear to involve, at least in part, enhancing the plants’ antioxidant defence responses.