Palmer amaranth is a very problematic weed in several crops in the southern USA due to its competitive ability and resistance to herbicides representing different mechanisms of action. Variation in growth and subsequent interference of North Carolina Palmer amaranth accessions has not been examined. A greenhouse experiment determined response of 15 North Carolina Palmer amaranth accessions to drought stress beginning 15 days after seedling emergence (DAE) for a duration of 3, 5, 7, and 9 days. Following exposure to drought, plants were grown under optimal moisture conditions until harvest at 30 DAE. Five accessions each of glyphosate-resistant (GR), acetolactate synthase inhibitor-resistant (ALSR), and acetolactate synthase inhibitor-susceptible and glyphosate-susceptible (ALSS/GS) were compared. Variation in response to drought stress, based on height and dry weight reduction relative to nonstressed controls, was noted among accessions. Stress for 3 or more days affected height and dry weight. Height and dry weight of GR and ALSR accession groups were reduced less by drought than the ALSS/GS accession group. Results suggest a possible relationship between herbicide resistance and ability of Palmer amaranth to withstand drought stress and thus a possible competitive advantage for resistant accessions under limited moisture availability. 1. Introduction The ability of crops and weeds to extract water from soil and their response to moisture stress are key factors in determining the outcome of crop-weed interference under drought [1–6]. Ability to absorb water from soil under limited water availability, water use efficiency, and transpiration vary among crop and weed species [7–11]. For example, water use efficiency of genotypes of vegetable amaranth (Amaranthus tricolor L., A. blitum L., and A. cruentus L.) was not affected by drought stress. However, stress significantly reduced total plant dry mass and leaf area per unit root dry mass and increased root dry mass ratio differently in genotypes . Under limited water conditions, plants respond differently and show a wide range of drought tolerance mechanisms both in terms of morphology and physiology . In another experiment involving vegetable amaranth, significant variation existed among genotypes for transpiration and stomatal conductance which was positively correlated with relative decrease in dry weight across four genotypes . The critical period for crop-weed interference and the extent of crop losses to weed competition can be influenced by soil moisture availability . In some
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