Leaf Gas Exchange of Bell Pepper (Capsicum annum L.) As Affected by Shading Level Under Field Conditions

Bell pepper is an important crop in Georgia and is grown primarily on plastic film mulches.  There is currently no commercial production of bell pepper under shade in Georgia, although preliminary studies show that shaded conditions may improve fruit yield and quality.  The objective of this study was to determine the effects of shading level on leaf gas exchange of bell pepper plants grown under field conditions.  Bell pepper (‘Camelot’, ‘Lafayette’, ‘Sirius’, and ‘Stiletto’) plants were grown on silver reflective plastic film mulch in the spring of 2009 and 2010 under different shading levels (0%, 30%, 47%, 60% and 80%). Results showed that  leaf temperature declined quadratically with increasing shading level from 34.5 ºC at 0% shade to 32.0 ºC at 80% shade.  Internal CO₂ concentration was similar at 0% shade and 30% shade and then increased with increasing shade levels.  Leaf photosynthesis was similar at 0% shade and 30% shade (mean = 24.7 µmol m^-2 s^-1) and declined with increasing shade level to 11.7 µmol m^-2 s^-1 at 80% shade.  Stomatal conductance was lowest at 80% shade (0.474 mol m^-2 s^-1) and showed no consistent differences among the other shade levels (mean = 0.633 mol m^-2 s^-1).  Leaf transpiration declined quadratically from 10.7 mmol m^-2 s^-1 at 0% shade to 7.6 mmol m^-2 s^-1 at 80% shade with increasing shading level.  Water use efficiency was similar at 0% shade and at 30% shade (mean = 2.63 µmol/mmol) and then declined quadratically with increasing shading level to 1.8 µmol mmol^-1 at 80% shade.  Leaf fluorescence (Photosystem II efficiency) increased with increasing shading level from 0.19 µmol/mmol at 0% shade to 0.52 µmol mmol^-1 at 80% shade.  In conclusion, bell pepper gas exchange variables were significantly affected by shading level.  Compared to the control (0% shade), moderate shading (30%) had no deleterious effect on photosynthesis, stomatal conductance, or water use efficiency, but resulted in some benefits, such as, reduced leaf temperature and increased Photosystem II efficiency.  This improved gas exchange under moderate shading may benefit plant growth and yield in bell pepper.