Leaf Gas Exchange and Growth Responses of Tomato Plants to External Flavonoids Application as Biostimulators under Normal and Salt-Stressed Conditions

The exogenous application of natural metabolites, such as phenolic compounds, is a useful strategy to stimulate growth and reduce the adverse effects of abiotic stress on crops, such as salinity. Salinity stress is one of the most damaging abiotic stresses to plants, causing reductions in growth by changes in the physiology, biochemistry, and gene expression. In this work, we investigated the effect of the foliar application of flavonoids (CropBioLife, CBL) on control and salt-stressed (NaCl 60 mM) tomato plants grown in controlled conditions. The results showed that CBL mainly influenced the stimulation of photosynthesis, increasing CO2 fixation and promoting growth. Furthermore, a higher stomata number in an open state was found in CBL-treated plants in relation to the higher CO2 fixation, which also resulted in a higher H2O uptake due to increasing stomatal conductance and nutrient uptake that plants need for growth. The results were due to the increase of phenolic metabolism and the expression of most of the aquaporins, which could be the triggering signal for the rest to the changes observed. The effect of the biostimulation of CBL under salinity was related to higher levels of photosynthesis, the increase of some mineral nutrients, and the increase of some PIP aquaporins expression, although no effect on growth was observed. The results of this work showing the mechanism of action of flavonoids in tomato plants open a new line of investigation with great importance for the future of agronomy.