Biostimulants Alleviate Heat Stress in Organic Hydroponic Tomato Cultivation: A Sustainable Approach

Heat stresses significantly reduce crop yield by 50% to 70%, irrespective of stress exposure time, plant species, or cultivars, posing substantial challenges for organically growing hydroponic tomatoes in controlled environment conditions. Low productivity in tomatoes is attributed to the inadequate stress tolerance of existing cultivars, which hinders their ability to optimize fruit set and yield. In this study, three biostimulants—Liquid Seaweeds, MycoApply ® , and MycoLife—were applied to the leaf surface of tomato plants at 7-day intervals starting 4 weeks after planting. Their effects on plant growth, physiology, phenology, fruit yield, and quality were assessed. Each biostimulant was applied at intervals of 4, 8, 12, and 16 weeks to optimize application frequency. The experimental design employed a strip plot, with the biostimulants (Liquid Seaweeds, MycoApply ® , MycoLife) as the main plot treatments, and application frequency (4, 8, 12, 16 weeks) randomly assigned to the subplots. We observed that the foliar application of biostimulants promoted plant vigor to varying degrees under heat stress conditions compared with the control. Specifically, MycoApply ® applied for 12 weeks enhanced chlorophyll synthesis and photosynthesis rates, thereby boosting plant productivity. Tomato (‘Valdeon RZ’) plants treated with MycoApply ® demonstrated an increased net assimilation rate (20%), and stomatal conductance (40%), along with reduced transpiration loss (28%) and electrolyte leakage (31%), while maintaining intercellular CO 2 concentrations. Flowering occurred 5 days earlier in tomato plants treated with MycoApply ® compared with untreated plants. MycoApply ® -treated tomato plants also exhibited superior fruit set (19%), pollen viability (37%), and fewer incidences of flower drop (10%) compared with the control. Among the application frequencies, MycoApply ® applied for 12 weeks exhibited superior plant growth and tomato productivity compared with the control. MycoApply ® treated for 12 weeks outperformed the control in terms of marketable fruit yields, with a significantly higher yield (30%). In addition, ‘Valdeon RZ’ tomato plants treated with MycoApply ® demonstrated superior postharvest quality, including firmness, soluble solids, acidity, and color dynamics. Correlogram, heat map, and cluster analysis further confirmed that under heat stress, biostimulants had various promotional effects on tomato growth and productivity. Therefore, MycoApply ® emerged as a promising biostimulant with strong heat stress tolerance capacity in organic hydroponic systems.