PEANUT ACCLIMATION TO SIMULATED SHADING BY WEEDS

Research was conducted to assess the acclimation responses of 'Florunner' peanut (Arachis hypogaea L.) to shading and the possible role of this acclimation in determining pod yield loss due to shading by weeds. The PNUTGRO model is being used to develop a model of competition for light between peanut and broadleaf weeds. This research was undertaken to validate the treatment of peanut response to shading in PNUTGRO. Peanut plants were grown in two matched growth chambers. At 2 wk after planting, light in one chamber was reduced to 51% of total daily photosynthetic photon flux density (PPFD) received by the full-light treatment (900 mumol m-2 s-1). Photosynthetic rates acclimated to shading within 3 d. Apparent photosynthesis (AP) rates of shaded vs. control plants were not different at 350, 500, or 1000 mumol photons m-2 s-1, but at 2000 mumol m-2 s-1 shaded plants had AP rates 76% of those of controls. Specific leaf area (SLA) of shaded plants was not statistically different from the controls. There were no observed differences in chlorophyll content or chlorophyll a:b ratios within 3 d after imposition of shading. Field studies were conducted in 1991 and 1992 with shading imposed by shade cloth attached to computer-controlled shelters. Shading reduced total daily PPFD by 40%, from about 40 d after planting to harvest. Leaf photosynthesis measurements confirmed laboratory results, indicating that shade-acclimated peanut plants had greater efficiency at low light levels. Shaded plants accumulated less dry weight and were taller than controls. Shaded plants produced fewer roots, with root length density remaining nearly constant after shade imposition. Over the two years, the shaded plants averaged 72% of the full-light yield on 60% of the PPFD, even though they produced fewer pods. Shaded peanut plants maintain photosynthetic capacity at low light and change partitioning to optimize utilization of available assimilate. Reproductive changes included production of fewer pegs, but a larger proportion of pegs produced harvestable pods. PNUTGRO performed well in simulating dry matter accumulation, but consistently overestimated SLA. It predicted 30% yield loss in both 1991 and 1992. Observed yield losses were 33 and 26% in the 2 yr, respectively. PNUTGRO performed sufficiently well in simulating shade response by peanut to justify its use in a light competition model.