Physiological responses of leaves, tree growth and fruit yield of grapefruit trees under reflective shade screens

Understanding the influence of reductions in solar radiation on crop physiology, growth and yield is important because of the widespread practice of protected cultivation under various light reducing covers. Parts of a commercial bearing grapefruit orchard (Citrus paradisi L.) were covered with aluminized reflective screens during three consecutive summers in southern Israel. Screens were installed shortly after fruit set and removed a week or 2 before harvest. Screens were of two types, 30 and 60% shade, erected horizontally over the rows. Shading reduced mid-day direct radiation reaching the trees and reduced exposed leaf temperatures, but increased their net CO2 uptake (A) and conductance (gl). The increased gl was reflected by decreases in carbon isotope ratios (δ13C) of leaves from under both shades and of fruit flavedo from the 60% shade. Final leaf area index (LAI), determined by gap fraction inversion, was 10% higher in 30% shade than in the control and canopy volume increased similarly. Trunk cross-section, total yield and size distribution of fruit were little affected by shade. During the second year, however, early shading presumably induced more than normal fruit abscission, leading to fewer but larger fruit. Fruit in the middle of the canopy of shaded trees were consistently heavier and also larger in the first 2 years, but peel width was not affected by shade. Juice content was reduced in the second and third years, but the ratio of sugar to acid was not affected. Mid-day leaf water potentials in August and September under 60% shade increased by 13%. Shading did not significantly reduce fruit yield.