Growth behavior of tomato (Solanum lycopersicum L.) under drought stress in the presence of silicon and plant growth promoting rhizobacteria

Among abiotic stresses, drought is considered as the most important growth limiting factor, particularly in arid and semiarid regions. Plant tolerance to drought is mainly associated with the maintenance of plant water status either by reducing water loss through decreasing transpiration or improving plant root capacity to extract more water through osmotic adjustment. In this regard, adequate regulation of plant nutrients may help to maintain or even improve plant water status and hence plant tolerance to drought stress. In the present study, silicon (Si) and plant growth promoting rhizobacteria (PGPR) containing ACC-deaminase activity were evaluated as a tool to improve drought tolerance of tomato (Solanum lycopersicum L.) grown in pots under natural environmental conditions. Nine treatments including, control (irrigation at 60% of field capacity); drought-1 (irrigation at 45% of field capacity); drought-2 (irrigation at 35% of field capacity); drought-1+50 ppm Si; drought-2+50 ppm Si; drought-1+PGPR; drought-2+PGPR; drought-1+50 ppm Si+PGPR; drought-2+50 ppm Si+PGPR were arranged in completely randomized design with 5 replications. Results revealed that both levels of drought stress caused a significant (P <= 0.05) reduction in plant growth and yield, K+, Ca2+ and Mg2+ accumulation as well as relative water content (RWC) while increase in Na+ concentration and electrolyte leakage. However, supplementation of Si and PGPR inoculation increased plant K+ accumulation, RWC while reduced Na+ uptake and electrolyte leakage, and subsequently improved fruit yield by 18.34% with Si, 22.80% with PGPR and 30.44% with Si+PGPR at drought-1 while 31.13% with Si, 35.32% with PGPR and 42.36% with Si+PGPR at drought-2 compared to drought stressed plants without any amendment. Ameliorative effects of Si or/and PGPR were mostly more pronounced at drought-2 compared to drought-1. Furthermore, Si and PGPR were more effective to alleviate drought stress effects when applied in combination compared to their individual application. Silicon and PGPR-induced increase in K+, Ca2+ and Mg2+ accumulation, and RWC while decrease in electrolyte leakage were the main factors associated with drought tolerance of tomato.