Growth stage–specific water stress effects on Palmer amaranth ( Amaranthus palmeri ) growth and fecundity

Abstract

Palmer amaranth ( Amaranthus palmeri S. Watson) is a highly competitive and drought-tolerant weed that poses significant challenges to U.S. agriculture due to its adaptability, prolific seed production, and herbicide resistance. Understanding how water stress at specific growth stages affects its development and seed production is critical for optimizing management strategies, particularly in water-limited environments. This study evaluated the effects of growth stage–specific water stress on growth and fecundity of A. palmeri under controlled greenhouse conditions. Four irrigation levels (100%, 80%, 60%, and 40% field capacity [FC]) were applied during vegetative and reproductive stages. Increasing water stress significantly reduced plant height, stem diameter, and biomass, with reductions of 17% to 35% under severe stress (40% FC). Total biomass declined by approximately 17% under severe water limitation. Root biomass at 60% FC decreased by 13% to 14% relative to 100% FC, whereas root length density increased by 11% to 16%, indicating adaptive root elongation under severe water deficit conditions. Despite reductions in vegetative growth, A. palmeri maintained high reproductive capacity. Seed production at 100% FC was 48% to 72% higher than at 80% FC and more than 2- to 3-fold greater than at 60% FC. Furthermore, plants subjected to reproductive-stage stress produced approximately 29% more seeds than those stressed during the vegetative stage, highlighting strong stage-dependent reproductive plasticity. Seed germination was highest at 100% FC but declined by approximately 55% at 60% FC. In contrast, germination at 80% FC remained within 10% of optimal levels and was comparable under 40% FC, suggesting a nonlinear response and potential maternal effects enhancing seed resilience under stress. The study revealed the importance of early-season weed control to exploit periods of vulnerability and limit seedbank replenishment. Overall, this study provides valuable insights for integrating weed management strategies in dryland cropping systems, where water scarcity and herbicide resistance present ongoing challenges.