Shading Nets Reduce Canopy Temperature and Improve Photosynthetic Performance in ‘Pinkerton’ Avocado Trees during Extreme Heat Events

Abstract

Frequent extreme heat events cause major financial losses for the avocado industry. Shading nets are used to protect crops from harsh environmental hazards. To determine their ability to improve photosynthetic performance under extreme heat in avocado, we examined the impact of a resilient high-density silver 60% shading net on mature ‘Pinkerton’ avocado trees during such conditions. We hypothesized that reduced solar irradiation will lower canopy temperature and improve tree performance. During extreme heat events, photosynthetic photon flux density (PPFD), air temperature (Tair) and leaf temperature (Tleaf) reached 1188 μmol m−2 s−1, 43.1 °C and 43.8 °C in the control plots, respectively. In the net-covered plots, these parameters significantly decreased to 401 μmol m−2 s−1, 40.3 °C and 39.8 °C, respectively. Interestingly, leaf CO2 assimilation, stomatal conductance to water vapor (gs) and substomatal internal CO2 concentration (Ci) were significantly higher, by 94%, 120% and 13%, respectively, than in controls. On days with regular fall temperatures, PPFD, Tair and Tleaf reached 814 μmol m−2 s−1, 31.2 °C and 31.6 °C in the control plots, respectively. In the net-covered plots, these parameters significantly decreased to 291 μmol m−2 s−1, 29.5 °C and 29.4 °C, respectively. However, leaf CO2 assimilation was significantly (20%) lower, gs was similar and Ci was 10% higher than in control trees. These findings suggest that silver 60% shading nets may potentially reduce heat stress during extreme heat events, calling for long-term studies on their effects on flowering and fruit set, load, size and quality.