The Influence of Irrigation Levels on the Phenology and Physiological Responses of Apple (Malus Domestica Borkh.) Genotypes under Tropical Highland Conditions

The study examines the effects of different irrigation levels on the phenology and physiological responses of apple trees (Malus domestica Borkh) cultivated in tropical highland environments. Tropical highlands pose unique challenges to temperate crops like apple due to fluctuating temperature and water availability. Instead of optimizing irrigation per se, the study evaluates the influence of four irrigation treatments—70%, 90%, 110%, and 130% of crop evapotranspiration (ETc)—on key growth and physiological parameters using a split-plot design with two apple cultivars (Anna and Dorsett Golden). ETc was estimated using local reference evapotranspiration (ETo) adjusted with crop-specific coefficients (Kc). Phenological data were recorded across developmental stages: bud break, flowering, fruit set, and harvest. Physiological measurements included chlorophyll index, relative water content, net photosynthesis, stomatal conductance, transpiration, internal CO₂ concentration, leaf temperature, and instantaneous carboxylation efficiency. Biochemical analyses included reducing sugars, total soluble sugars, non-reducing sugars, and protein contents in leaves. Findings indicate that moderate irrigation (around 90–110% ETc) supported optimal phenological development and physiological efficiency, contributing to improved fruit yield and quality. In contrast, deficit irrigation delayed development and suppressed physiological performance, while excessive irrigation caused root zone saturation and health issues. These results highlight the need for balanced irrigation management to support apple cultivation in tropical highlands.