Impact of Supplemental Irrigation on Total Chlorophyll Content, Chlorophyll Fluorescence and Water Use Efficiency in Barley ( <i>Hordeum vulgare</i> L.) in Semi‐Arid Environments

ABSTRACT In Jordan and other arid and semi‐arid Middle Eastern countries, water scarcity and unpredictable rainfall patterns make crop productivity difficult. Thus, breeding strategies aim to improve the stability of drought‐stressed barley ( Hordeum vulgare L.) production. This research examined the impact of four irrigation regimes: rainfed (0%) and supplementary irrigation (SI) at 50%, 75% and 100% of field capacity (FC) on the yield, yield components and physiological parameters of two barley cultivars (Acsad176 and Yarmouk) during two growing seasons. A three‐replicate split‐plot design was used with irrigation treatments as the main factor and cultivars as the sub‐factor. SI outperformed rainfed conditions in grain yield and water‐use efficiency (WUE), especially at 75% FC, where yield and resource use were optimised. Improved physiological characteristics like chlorophyll fluorescence (F v /F m ), total chlorophyll content (measured by SPAD meter) and harvest index (HI) increased crop production. Acsad176 produced higher grain yield and WUE across all treatments, supported by its superior physiological performance and more grains per spike. At the same time, Yarmouk's earlier maturity allowed partial avoidance of terminal heat stress but resulted in lower yields. Rainfed conditions caused substantial yield reductions, highlighting SI's importance in mitigating the effects of drought. These findings demonstrate that Acsad176 is a promising cultivar for enhancing barley production in semi‐arid regions and emphasise the potential of traits like SPAD and F v /F m as selection parameters in breeding programs to develop drought‐resistant cultivars.