Effects of Covering Mature Avocado `Pinkerton` Trees with High-Density Shading Nets during Cold Winters on Microclimate, Chlorophyll Fluorescence, Flowering and Yield

Abstract

Avocado (Persea americana Mill.) is a subtropical fruit tree of high commercial value with increasing global demand. Most avocado cultivars are vulnerable to cold climates, which may reduce yields and restrict their geographical expansion. This includes the green-skinned avocado cv. Pinkerton, which accounts for 45% of the avocado cultivated in northeastern Israel. Shading nets can protect agricultural crops from cold environments. We therefore evaluated the effect of covering mature Pinkerton trees with high-density shading nets during the winter. Trees were covered with Silver-coloured 50% or 70% shading nets during three consecutive winters, while uncovered trees served as controls. Photosynthetically active radiation in plots covered with the Silver 50% or 70% nets was significantly lower than for the control by 52% and 90%, respectively. Minimum air temperature was similar between treatments. Maximum air temperature was generally lower under the shading nets compared to the control. The ratio of variable to maximum fluorescence (Fv/Fm) measured in February 2019 and 2020 was 0.72 and 0.8 in the control trees, 0.79 and 0.83 in the Silver 50% trees and 0.81 and 0.84 in the Silver 70% trees, respectively. Flowering intensity was lower in the net-covered trees compared to the control, by up to 42%. Interestingly, the three-year average yield of trees covered with the Silver 50% or 70% nets was insignificantly higher by 27% and 38%, respectively, compared to the control trees. These results suggest that the reduction of daytime solar irradiance in the winter by the shading nets may mitigate cold stress and increase yield. Additional long-term studies should examine the effects of shading nets and other shading strategies on different avocado cultivars.