L-proline alleviates heat stress of tomato plants grown under protected environment

Tomato (Solanum lycopersicum L.) grown in the state of Sao Paulo represents 21.5 % of the total production in Brazil, making it the top table tomato-producing state in the country. Tomato cultivation in greenhouses is becoming increasingly popular in Brazil, as crop plants are protected against pests and unfavorable weather. However, issues regarding elevated greenhouse temperatures are common and can severely impact tomato yield due to the deregulation of vital plant functions, leading to the generation of toxic reactive oxygen species (ROS) and oxidative stress. Since endogenous proline accumulation has been correlated with heat stress relief due to ROS elimination and protection of cell structures, we evaluated the effects of foliar applications of L-proline at rates ranging from 0 to 1600 mg L-1 onto photosynthetic parameters, antioxidant system enzymes, malonaldehyde and hydrogen peroxide contents, post-harvest characteristics, and productivity of tomato plants subjected to high temperatures during the growth cycle. Regardless of rate, L-proline applications resulted in lower hydrogen peroxide contents despite no increase in antioxidant enzymes' activity, and improved titratable acidity (TTA) as well as the overall fraction of average total soluble solids (TSS) by TTA (i.e. TSS/TTA). L-proline applied at 50 and 100 mg L-1 increased commercial and total fruit yield per plant. The 100 mg L-1 rate also enhanced the plants' water use efficiency while decreasing malonaldehyde content, further indicating that exogenous applications of L-proline at the optimal rate of 100 mg L-1 can aid in alleviating high temperaturerelated damage to greenhouse-grown tomato plants.