Acclimation of Palmer Amaranth (Amaranthus palmeri) to Shading

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−2s−1for plants grown under 0 and 47% shade. Plants acclimated to increased shading by decreasing light-saturated photosynthetic rates from 60.5 µmol m−2s−1under full sun conditions to 26.4 µmol m−2s−1under 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 cm2g−1as shading increased to 87%. Plants acclimated to 47% shade by increasing total leaf chlorophyll from 22.8 µg cm−2in full sunlight to 31.7 µg cm−2when 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.