Leaf water content: an indicator of drought stress and glyphosate efficacy in two morphologically distinct weed species

Abstract Improving herbicide efficacy under drought stress is essential for improving weed management and limiting the associated challenges. Leaf water content (LWC), a vital ecophysiological trait, can be used to indicate drought stress and improve herbicide efficacy. To investigate how water stress–induced changes in LWC affect glyphosate efficacy across diverse weed types, the broadleaf weed Santa Maria feverfew ( Parthenium hysterophorus L.) and the narrow-leaf giant foxtail ( Setaria faberi Herrm.) were selected. A controlled greenhouse study was conducted with two treatment factors including three moisture levels (well watered at 100% WHC, moderate drought at 75% WHC, and severe drought at 50% WHC) and glyphosate doses (0, 180, 360, 540, 720, and 900 g ae ha⁻¹). Drought stress significantly reduced LWC and stomatal conductance in both species; while the reduction in LWC was more pronounced in P. hysterophorus (26%) than in S. faberi (23%), the reduction in stomatal conductance was severe and similar for both species (82% and 83%, respectively). Severe drought stress drastically reduced shikimic acid concentration (41% to 59%) in both species. Severe drought stress significantly reduced glyphosate efficacy, suppressing mortality by up to 63% and biomass reduction up to 58%. Shikimic acid concentration and weed mortality both showed a strong positive correlation with change in LWC under all tested water-stress levels. Glyphosate applications when LWC falls below 70% resulted in poor weed control at recommended field rates. Therefore, LWC can be used as a real-time predictive biomarker to monitor drought stress, schedule irrigation before glyphosate application, optimize the dose, and ultimately improve its efficacy.