Experiments were conducted to investigate the acclimation of Palmer amaranth to shading. Plants were grown in the field beneath black shade cloths providing 47 and 87% shade and in full sunlight (no shading). All photosynthetic measurements were taken 4 wk after initiating the shade treatments. Photosynthetic rates of Palmer amaranth grown under 47% shade increased with increasing photosynthetic active radiation (PAR) similar to 0% shade-grown plants. Light-saturated photosynthetic rates were predicted beyond the highest measured PAR of 1,200 µmol m⁻² s⁻¹ for plants grown under 0 and 47% shade. Plants acclimated to increased shading by decreasing light-saturated photosynthetic rates from 60.5 µmol m⁻² s⁻¹ under full sun conditions to 26.4 µmol m⁻² s⁻¹ under 87% shade. Plants grown under 87% shade lowered their light compensation point. Rate of increase in plant height was similar among shade treatments. Plants responded to increased shading by a 13 to 44% reduction in leaf appearance rate (leaf number growing degree days [GDD]⁻¹) and a 22 to 63% reduction in main-stem branch appearance rate (main-stem branch number GDD⁻¹) compared with full sunlight. Palmer amaranth specific leaf area increased from 68 to 97 cm² g⁻¹ as shading increased to 87%. Plants acclimated to 47% shade by increasing total leaf chlorophyll from 22.8 µg cm⁻² in full sunlight to 31.7 µg cm⁻² when shaded; however, the increase was not significant at 87% shading. Thus, it is concluded that Palmer amaranth shows photosynthetic and morphological acclimation to 87% or less shading.

Nomenclature: Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA.