Ameliorative Effects of Plant Growth Promoting Bacteria on Water-yield Relationships, Growth, and Nutrient Uptake of Lettuce Plants under Different Irrigation Levels

The aim of this study was to determine the effects of selected plant growth-promoting rhizobacteria (PGPR) on some physiological characteristics, plant growth, yield, and plant nutrient content of lettuce grown under different irrigation levels. Field experiments were carried out as split plot based on randomized complete block design with three replications. Three irrigation levels, I-1 = 100% (control), I-2 = 75%, and I-3 = 50% of the field capacity (FC), were determined at the 0-15 cm soil depth by time-domain reflectometry (TDR), as main plots and three levels of bacterial species consisting of no bacterial inoculation (control), Bacillus megaterium TV 6D (B-1), Bacillus subtilis TV 1211 (B-2) as sub plots in 2012 and 2013. Physiological characteristics, plant growth, yield, and plant nutrient content of lettuce was significantly affected by PGPR and irrigation quantities. Results showed that decreasing irrigation quantities significantly decreased the growth, dry and fresh head weight, and yield of lettuce in both years. Moreover, lower irrigation levels caused a decrease in leaf relative water content (LRWC), stomatal conductance (SC), and plant nutrient element content, but an increase electrolyte leakage (EL) and lipid peroxidation [malondialdehyde (MDA)]. However, PGPR inoculations significantly increased the growth, nutrient element content, LRWC, SC, and yield but decreased EL and MDA of lettuce plants grown under lower irrigation levels. The results of the study suggested that PGPR inoculations could alleviate the deleterious effects of lower irrigation conditions on the growth and yield of lettuce plants.