Biostimulants have the potential to enhance plant growth and improve resilience to environmental stresses such as drought. This study investigated the effects of biostimulant application on the growth and physiological responses of two lettuce cultivars, ‘Green Oakleaf’ and ‘Red Oakleaf’, grown in a greenhouse at Mississippi State University. Plants were grown in containers under three substrate field capacities (40%, 70%, and 100%) and treated with one of four biostimulant treatments: Tribus®, Huma Pro®16, Kelpak®, or an untreated control. Data collected included plant growth index (PGI), leaf SPAD readings, photosynthetic activity, fresh and dry weights, and leaf color. Water availability and cultivar significantly affected PGI, biomass accumulation, and SPAD readings. The 70% and 100% field capacities resulted in the highest fresh and dry weights, while ‘Red Oakleaf’ exhibited greater dry weight and SPAD values than ‘Green Oakleaf’. Biostimulant treatments had no significant effect on any of the measured parameters. These results indicate that water availability plays a critical role in greenhouse lettuce growth, while the biostimulants tested did not enhance plant performance under the conditions of this study.
