Appraisal of kinetin spraying strategy to alleviate the harmful effects of UVC stress on tomato plants

Increasing ultraviolet (UV) radiation is causing oxidative stress that accounts for growth and yield losses in the present era of climate change. Plant hormones are useful tools for minimizing UV-induced oxidative stress in plants, but their putative roles in protecting tomato development under UVC remain unknown. Therefore, we investigated the underlying mechanism of pre-and post-kinetin (Kn) treatments on tomato plants under UVC stress. The best dose of Kn was screened in the preliminary experiments, and this dose was tested in further experiments. UVC significantly decreases growth traits, photosynthetic pigments, protein content, and primary metabolites (proteins, carbohydrates, amino acids) but increases oxidative stress biomarkers (lipid peroxidation, lipoxygenase activity, superoxide anion, hydroxyl radical, and hydrogen peroxide) and proline content. Treatment of pre-and post-kinetin spraying to tomato plants decreases UVC-induced oxidative stress by restoring the primary and secondary metabolites' (phenolic compounds, flavonoids, and anthocyanins) status and upregulating the antioxidant defense systems (non-enzymatic antioxidants as ascorbate, reduced glutathione, α-tocopherol as well as enzymatic antioxidants as superoxide dismutase, catalase, ascorbate peroxidase, glutathione peroxidase, glutathione-S-transferase, and phenylalanine ammonia-lyase). Thus, the application of Kn in optimum doses and through different modes can be used to alleviate UVC-induced negative impacts in tomato plants.

Tomato plants use non-enzymatic antioxidant pathways to cope with moderate UV-A/B irradiation: A contribution to the use of UV-A/B in horticulture

Plants developed receptors for solar UV-A/B radiation, which regulate a complex network of functions through the plant's life cycle. However, greenhouse grown crops, like tomato, are exposed to strongly reduced UV radiation, contrarily to their open-field counterparts. A new paradigm of modern horticulture is to supplement adequate levels of UV to greenhouse cultures, inducing a positive mild stress necessary to stimulate oxidative stress pathways and antioxidant mechanisms. Protected cultures of Solanum (cv MicroTom) were supplemented with moderate UV-A (1h and 4h) and UV-B (1min and 5min) doses during the flowering/fruiting period. After 30days, flowering/fruit ripening synchronization were enhanced, paralleled by the upregulation of blue/UV-A and UV-B receptors' genes cry1a and uvr8. UV-B caused moreover an increase in the expression of hy5, of HY5 repressor cop1 and of a repressor of COP1, uvr8. While all UV-A/B conditions increased SOD activity, increases of the generated H₂O₂, as well as lipid peroxidation and cell mebrane disruption, were minimal. However, the activity of antioxidant enzymes downstream from SOD (CAT, APX, GPX) was not significant. These results suggest that the major antioxidant pathways involve phenylpropanoid compounds, which also have an important role in UV screening. This hypothesis was confirmed by the increase of phenolic compounds and by the upregulation of chs and fls, coding for CHS and FLS enzymes involved in the phenylpropanoid synthesis. Overall, all doses of UV-A or UV-B were beneficial to flowering/fruiting but lower UV-A/B doses induced lower redox disorders and were more effective in the fruiting process/synchronization. Considering the benefits observed on flowering/fruiting, with minimal impacts in the vegetative part, we demonstrate that both UV-A/B could be used in protected tomato horticulture systems.