Microorganism-Based Biostimulants for Alleviating Water Deficit in ‘Formosa’ Papaya: Physiological Indices and Growth

Papaya is an economically important crop, but its production in semiarid regions is severely limited by water scarcity. However, microorganism-based biostimulants have been studied as a promising strategy to mitigate water stress and support plant growth. Therefore, the objective of this study was to evaluate the effect of microorganism-based biostimulants on gas exchange, photochemical efficiency, and growth of ‘Formosa’ papaya under water deficit in a semiarid area. The experimental design was a randomized complete block design with split plots. The plots considered three irrigation depths (100, 75, and 50% of crop evapotranspiration-ETc) and the subplots the application of four biostimulants (control (no biostimulant application); Trichoderma harzianum; Ascophyllum nodosum; Bacillus aryabhattai), with three plants per plot and four replicates. B. aryabhattai mitigated the effects of deficit irrigation at 50% ETc on ‘Formosa’ papaya, increasing transpiration, CO2 assimilation rate, and instantaneous carboxylation efficiency. Under irrigation at 50% ETc, T. harzianum provided beneficial effects on water use efficiency, instantaneous carboxylation efficiency, and photosystem II quantum efficiency. A. nodosum stimulated chlorophyll a synthesis in ‘Formosa’ papaya plants irrigated at 75% ETc during the fruiting stage, but reduced the absolute and relative growth rate in stem diameter under 50% ETc. Irrigation at 50% ETc reduced stomatal conductance and growth of ‘Formosa’ papaya plants 235 days after transplanting. We conclude that the application of B. aryabhattai and T. harzianum is a viable strategy to increase the tolerance of ‘Formosa’ papaya to the adverse effects of water deficit in semiarid regions.