Shade Level Affects Microenvironment, Plant Growth, and Plant Function in Bell Pepper (Capsicum annum L.)

The objective of this study was to determine the effects of shade level on the crop microenvironment, bell pepper plant growth, and plant function.  One bell pepper cultivar (‘Heritage’) in Spring 2008 and four cultivars (‘Camelot’, ‘Lafayette’, Sirius’ and ‘Stiletto’) in Spring 2009 and 2010 were grown under the following shading levels: 0%, 30%, 47%, 62% and 80%.  Photosynthetically active radiation, and air, leaf, and root zone temperatures decreased with shade levels. Air temperature decreased less with shading than root zone temperature. Total plant leaf area, individual leaf area, and individual leaf weight increased while the number of leaves per plant and specific leaf weight decreased with increasing shade level. Chlorophyll index (SPAD value) was unrelated to leaf N because of differences in specific leaf weight associated with shade levels. However, chlorophyll index values normalized by specific leaf weight were related to leaf N and increased with increasing shading levels.  Leaf temperature, net photosynthesis, and stomatal conductance decreased and internal CO₂ concentration and Photosystem II efficiency increased with increasing shade levels.  Leaf concentrations of N, P, K, Ca, Mg, S, Al, B, Cu, Mn, and Zn increased with increasing shading levels, while those of Fe, Mo, Na, and Ni were unaffected by shading. There were relatively few differences in plant growth, leaf gas exchange and leaf mineral nutrient concentrations among cultivars. In conclusion, shade levels significantly modified the crop microclimate ameliorating heat stress and resulting in large effects on plant growth and function.  Shading nets reduced light and temperature ameliorating heat stress and improving bell pepper plant growth and function. Intermediate shade levels (30% shade and 47% shade) were the most favorable for bell pepper plant growth and leaf gas exchange.