Enhancing Water Productivity, Biomass Production and Drought Tolerance of Passion Fruit (Passiflora edulis Sims) Seedlings Using Chitosan and Spirulina Biostimulants Under Deficit Irrigation

Water scarcity is a major constraint to sustainable horticultural production, particularly in arid and semi-arid regions. This study evaluated the effectiveness of two natural foliar biostimulants, chitosan and Spirulina extract, in improving water productivity, growth performance, biomass production, and drought tolerance of passion fruit (Passiflora edulis Sims) seedlings under deficit irrigation. A factorial experiment was conducted using three irrigation regimes (100%, 85%, and 70% of field capacity, FC) combined with three foliar spray treatments: distilled water (control), chitosan (1 g L−1), and Spirulina extract (1 g L−1). Deficit irrigation significantly reduced chlorophyll content, vegetative growth, biomass production, and water productivity, with the greatest reductions observed at 70% FC. Foliar application of both biostimulants effectively alleviated these adverse effects by enhancing plant growth, biomass production, chlorophyll retention, and water productivity under water-limited conditions. Chitosan consistently outperformed Spirulina, particularly under moderate water deficit, by reducing irrigation water requirements while maintaining seedling growth and physiological performance comparable to those under full irrigation. Multivariate analyses, including Pearson correlation, principal component analysis, and response surface methodology, revealed strong positive relationships among biomass production, chlorophyll content, vegetative growth traits, and water productivity. Among all treatments, foliar application of chitosan at 1 g L−1 combined with 85% FC was identified as the optimum strategy, providing substantial irrigation water savings while maintaining vigorous seedling growth, high biomass production, and superior water productivity. These findings demonstrate that integrating moderate deficit irrigation with chitosan application is an effective and sustainable strategy for improving passion fruit nursery production under conditions of limited water availability.