Physiological Basis for Palmer Amaranth Competitiveness in Cotton

Palmer amaranth (Amaranthus palmeri) is one of the most troublesome weeds in the southeast.  A single plant left in a row of cotton (Gossypium hirsutum) has been documented to reduce cotton lint yields by 10-30%.  Previous studies in other crops have found light to be the most important factor in Palmer amaranth-crop competition.  However, water is thought to be the main factor driving competition in the sandy soils of the southeast.  Relative soil water content and stomatal conductance were measured during the 2011, 2012 and 2013 cotton growing seasons to determine the impact of Palmer amaranth on soil available water.  Differences in soil relative water content were observed at a depth of 30 cm when Palmer amaranth reached 30 cm in height.  This trend continued as Palmer amaranth plants reached >100 cm and soil water differences were found up to 100cm in depth. In addition to water availability relative to soil depth, water deficit at various distances away from the Palmer amaranth plant was also tested.  To do this, stomatal conductance of cotton was measured 90 DAP with a leaf porometer at distances of 0 to 1000 cm away from the Palmer amaranth plant.  Palmer amaranth plants were greater than 200cm in height.  It was observed that stomatal conductance was reduced by 37.8 to 22.1% ranging from 0 to 200 cm away from a single Palmer amaranth plant, respectively.  These studies indicate that water is indeed an important factor driving competition between Palmer amaranth and cotton in Florida.  Current research focuses on more in-depth photosynthetic measurements to evaluate the zone of influence Palmer amaranth has on cotton.