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 (g_s) and substomatal internal CO₂ concentration (C_i) were significantly higher, by 94%, 120% and 13%, respectively, than in controls. On days with regular fall temperatures, PPFD, T_air and T_leaf 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, g_s was similar and C_i 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.

Summary: Little is known about long‐distance mesophyll‐driven signals that regulate stomatal conductance. Soluble and/or vapor‐phase molecules have been proposed. In this study, the involvement of the gaseous signal ethylene in the modulation of stomatal conductance in Arabidopsis thaliana by CO₂/abscisic acid (ABA) was examined. We present a diffusion model which indicates that gaseous signaling molecule/s with a shorter/direct diffusion pathway to guard cells are more probable for rapid mesophyll‐dependent stomatal conductance changes. We, therefore, analyzed different Arabidopsis ethylene‐signaling and biosynthesis mutants for their ethylene production and kinetics of stomatal responses to ABA/[CO₂]‐shifts. According to our research, higher [CO₂] causes Arabidopsis rosettes to produce more ethylene. An ACC‐synthase octuple mutant with reduced ethylene biosynthesis exhibits dysfunctional CO₂‐induced stomatal movements. Ethylene‐insensitive receptor (gain‐of‐function), etr1‐1 and etr2‐1, and signaling, ein2‐5 and ein2‐1, mutants showed intact stomatal responses to [CO₂]‐shifts, whereas loss‐of‐function ethylene receptor mutants, including etr2‐3;ein4‐4;ers2‐3, etr1‐6;etr2‐3 and etr1‐6, showed markedly accelerated stomatal responses to [CO₂]‐shifts. Further investigation revealed a significantly impaired stomatal closure to ABA in the ACC‐synthase octuple mutant and accelerated stomatal responses in the etr1‐6;etr2‐3, and etr1‐6, but not in the etr2‐3;ein4‐4;ers2‐3 mutants. These findings suggest essential functions of ethylene biosynthesis and signaling components in tuning/accelerating stomatal conductance responses to CO₂ and ABA.

The worldwide demand for avocados has resulted in the planting of millions of young plants each year. However, global warming, resulting in high temperatures, sensed as heat stress, may severely damage these new plantings. The objective of this study was to assess the risks of heat stress on young avocado plants. We aimed to characterize different physiological parameters of young ‘Hass’ plant leaves following exposure to high temperatures under low light (LL) intensity and to pinpoint the temperature threshold for significant heat stress damage in these plants. To this end, young potted plants were subjected to different temperature gradients in a controlled-climate chamber. Minor and severe leaf damage was apparent in plants subjected to the 51 °C and 53 °C treatments, respectively. Minor and vast reductions in optimal quantum yield efficiency of photosystem II (Fv/Fm) values were observed in plants subjected to 51 °C and 53 °C, respectively. Heat stress treatments significantly reduced CO₂ assimilation in plants subjected to 49 °C and higher temperatures. Stomatal conductance to water vapour and substomatal internal CO₂ concentration were less sensitive to the heat treatments. These results imply that the heat damage threshold for young avocado plants under LL conditions is between 49 °C and 51 °C, whereas at 53 °C, severe and irreversible leaf damage occurs.