Photosynthetic response of citrus grown under reflective aluminized polypropylene shading nets

High temperatures and high atmospheric vapor pressure deficits (VPDs) are usually encountered in greenhouses in hot climates. For citrus, these environmental conditions can lead to decreases in photosynthetic activity with detrimental effects on plant growth. The aim of this study was to evaluate the use of reflective aluminized polypropylene shading nets on photosynthetic performance of citrus plants, by measuring CO2 assimilation, transpiration rate, stomata] conductance and chlorophyll a fluorescence. Incident photosynthetically active radiation (PAR) levels and leaf temperatures were reduced when the reflective nets were used. Higher stomata] conductance and higher CO2 assimilation rates were observed in shaded plants, so that integrated daily net CO, uptake was approximately 20% higher than in exposed plants. The better performance of shaded plants, however, was observed only during the middle of the day, being PAR-limited in early morning and late afternoon. The reflective net was also effective in preventing photoinhibition of photosynthesis in shaded plants, which sustained higher maximum (F-v/F-m) and effective (DeltaF/F'(m)) quantum yield with higher apparent electron transport rates (ETRs) than exposed plants. Observed photoinhibition in exposed plants was transient, probably reflecting photosynthetic regulatory responses to excess absorbed light energy. Therefore, the results clearly showed that photosynthetic performance of citrus cultivated in greenhouses can be improved by the use of reflective nets. Favorable effects comprised not only the maintenance of proper stomatal aperture for gas exchange but also a better functioning of the photosystem II (PSII) under non-photoinhibitory conditions. (C) 2002 Elsevier Science B.V. All rights reserved.