Effects of shade netting on gas exchange of blushed apple cultivars
Shade netting over apple orchards can provide protection against sunburn, as well as hail, wind and bird protection. It reduces total incoming radiation, particularly ultraviolet (UV), and increases the proportion of scattered light. However, light reductions could reduce leaf photosynthesis. Trials were conducted over two seasons in the Western Cape Province, South Africa, on the blushed apple 'Braeburn', 'Fuji', 'Royal Gala' and 'Cripps' Pink' using 20% black shade netting which reduced photo-synthetic photon flux density by 22-31%. Fruit surface temperatures were reduced under nets by 0.7-2.9°C on milder days (Tmax 19.0-23.8°C), and by 4.0-5.3°C on a warm day (Tmax 27.4°C). Leaf temperatures were reduced under nets by 1.7-3.9°C on a milder day (Tmax 23.9°C), and by 4.3-6.2°C on a warm day (Tmax 27.4°C). Gas exchange measurements showed that leaf photosynthetic rate was either not affected or was increased, leaf-to-air vapour pressure deficit was reduced, and stomatal conductance was increased under nets. Pre-harvest leaf water potential was generally improved under nets. Maximum light- and CO2-saturated photosynthetic rates and leaf nitrogen concentration were not affected, indicating no long-term acclimation in photosynthetic capacity under shade netting.