Apple responses to fruit sunburn in Italy

Sunburn is a physiological disorder attributed to high solar radiation and air temperatures. With climate change, this phenomenon will tend to expand to new areas, as the Mediterranean basin. The effects of 2 irrigation restitutions (100 and 70% of evapotranspiration) and 2 training systems (2D and 3D) were evaluated and related to plant water status, in two commercial apple orchards (both Gala varieties) in Italy, during summer 2021. Aluminum foils were applied to a number of fruit, for each treatment, and their temperature was monitored, along with that of control fruit (no aluminum foil), in the last 25 days before harvest. Sunburn occurred only where aluminum foil was applied, where the generally higher temperatures might have been caused by higher reflected irradiance. The results showed that the training system was highly influencing fruit temperature (p 0.0003), rather than deficit irrigation. A 2D-shaped tree will intercept regular quantities of solar radiation, hence fruit will adapt to higher light environments all along the season; in fact, only 1 fruit experienced sunburn. On the contrary, the 3D-shaped trees, having a more irregular canopy, produced 4 sunburned fruit. Assumptions can be made that prolonged exposure of fruit to higher irradiances, since early in the season, and consequently air temperatures, induced higher protection mechanisms against sunburn. An interesting corelation between stem water potential and control fruit temperature, revealed a strong negative relationship (p 0.006). Less stressed plants appeared to have less warm fruit. Suggestions arise that plant water status may play an important role, thus certain training systems that maintain lower water potentials, like a fruiting wall canopy, might help prevent sunburn damage. As fruit were damaged only due to the application of aluminum foils, one could conclude that solar radiation and air temperature levels were insufficient to cause sunburn in the trial.