Leaf water content and physiological recovery influence glyphosate efficacy in drought‐stressed and re‐watered weeds

BACKGROUND: Glyphosate efficacy is highly dependent on plant physiological processes that are sensitive to drought stress. Leaf water content (LWC) can be used as a proxy to indicate drought stress and improve herbicide efficacy. A greenhouse study was conducted to understand the relationship between LWC, physiological processes, and glyphosate efficacy in two weed species, Parthenium hysterophorus (C₃) and Setaria faberi (C₄) under high soil moisture [90-100% water holding capacity (WHC)], re-watering at 21 days after weed emergence (DAWE), re-watering at 25 DAWE, and low soil moisture (40-50% WHC). RESULTS: Soil moisture stress caused significant reduction in LWC (27-32%), stomatal conductance (81-84%), photosynthetic rate (76-82%), chlorophyll content index (46-55%) and shikimic acid accumulation (55-57%) in both species. Both re-watering treatments recovered LWC to well-watered levels; however, significant recovery of other physiological traits occurred only in the re-watering at 21 DAWE (5-day recovery period), where glyphosate efficacy remained high (94% mortality) in both species. Re-watering at 25 DAWE (1-day recovery period) failed to recover stomatal conductance and photosynthesis, resulting in significantly reduced glyphosate efficacy (81% and 69% mortality for P. hysterophorus and S. faberi, respectively). The LWC showed strong correlation with glyphosate efficacy for all moisture regimes except for the re-watering at 25 DAWE. CONCLUSION: The time allowed for physiological recovery after re-watering is critical. Therefore, LWC with the advantage of real-time remote estimation, can be used as a practical proxy for scheduling glyphosate applications, but the history of drought-breaking irrigation must be considered to ensure that weeds are physiologically susceptible.