Impact of Foliar Biostimulants on Wheat Yield under Variable Drought Stress Conditions

Thesis Abstract

Wheat production in semiarid cropping systems is increasingly challenged by episodic drought events that constrain yield and undermine input efficiency; foliar biostimulants offer a potential means to mitigate drought-induced yield losses by enhancing nutrient use efficiency, stress signaling, and membrane stability, yet empirical evidence under field conditions remains fragmented. The aim of this study is to evaluate the effectiveness of foliar biostimulants on grain yield and agronomic performance of wheat under variable drought stress, with objectives to quantify yield response across rainfall- and irrigation-derived drought gradients, assess impacts on physiological traits (photosynthetic rate, stomatal conductance, chlorophyll content) and root–shoot biomass balance, examine interactions with soil moisture and nutrient status, and identify predictive indicators of yield response using multivariate models. The research adopts a split-plot field experiment arranged in a randomized complete block design with four irrigation regimes (adequate, moderate deficit, severe deficit, and rainfed) as main plots and four foliar treatments (control, seaweed-based biostimulant, amino acid–based biostimulant, and humic–oftanolide blend) as subplots, replicated three times across a 1.2 ha experimental site located in the Barossa Valley region over two consecutive growing seasons. The population comprises commercially grown hexaploid wheat (Triticum aestivum L.) cultivars commonly used by commercial producers, with a sample size determined by 12 complete blocks total across both seasons to ensure sufficient power for interaction effects. Data collection instruments include portable spectroradiometers for leaf chlorophyll and NDVI, a portable photosynthesis system for gas exchange parameters, soil moisture probes (theta sensors), SPAD chlorophyll meter, grain yield harvests, thousand-kernel weight, above- and below-ground biomass measurements, and soil nutrient analyses (N, P, K) pre- and post-treatment. Validity and reliability will be ensured through calibration standards, standardized measurement protocols, and pilot testing of instruments prior to full-scale deployment. Data analysis will employ ANOVA to test main and interaction effects, followed by Tukey’s HSD for post hoc comparisons; regression analysis will model yield as a function of physiological traits, soil moisture, and treatment, while mixed-effects models will account for block and year effects. Multivariate approaches, including principal component analysis and partial least squares regression, will identify key predictors of yield response, and structural equation modeling will be used to test causal pathways linking foliar biostimulant application, plant physiology, and grain yield. The study hypothesizes that foliar biostimulants improve yield under drought by enhancing photosynthetic efficiency, reducing transpiration costs, and improving nutrient uptake, with greater benefits under moderate drought than severe drought. Expected findings include higher grain yield and thousand-kernel weight in biostimulant-treated plots relative to control under moderate deficits, improved chlorophyll content and net photosynthetic rate, higher NDVI values during grain filling, and a positive correlation between root biomass and yield under limited soil moisture. The research is expected to contribute to knowledge by delivering field-based evidence on the conditional efficacy of foliar biostimulants under drought gradients, clarifying interactions with soil moisture and nutrition, and providing a predictive framework for farmers to optimize biostimulant use in water-limited environments. The main conclusion anticipated is that foliar biostimulants can be a valuable component of integrated drought management for wheat, particularly under moderate deficit conditions, though efficacy is contingent on timing, formulation, and baseline soil fertility. Recommendations will target optimization of application timing, formulation selection, and integration with irrigation scheduling, with guidance for policymakers and extension services on technology adoption and risk mitigation under climate variability.