Mitigation of High Solar Irradiance and Heat Stress in Kiwifruit during Summer via the Use of Alleviating Products with Different Modes of Action—Part 1 Effects on Leaf Physiology and Biochemistry

Kiwifruit is a significant fruit crop species for many countries around the world. Due to climate change, it undergoes significant heat stress during the summer months in the Mediterranean area. Heat stress, along with high irradiance, generally imposes significant reductions in leaf photosynthetic activity and changes in leaf antioxidant status. In order to ameliorate these impacts, three alleviating commercial products (the osmoprotectant glycine betaine—commercial product BlueStim SP, the antioxidant mixture of tocopherol and phenolic compounds—commercial product Sun Protect, and reflectance calcium carbonate—commercial product Pureshade) were tested. In a fully mature kiwifruit orchard (‘Hayward’ cultivar), the prementioned products were foliarly applied during the summer months, and three assessments took place (in early and late August and late September) to assess their effects on photosynthetic activity, leaf carbohydrate concentration, the leaf sclerophylly indexes, leaf phenolic compound concentration, and antioxidant capacity. The three products induced various effects on leaf physiology and biochemistry, alleviating stress impact to some extent. Glycine betaine proved to be more efficient in alleviating the negative effects on the photosynthetic machinery, while leaf relative water content and, therefore, succulence remained at high levels. The reflectance calcium carbonate product resulted in lower leaf temperatures during the August measurements and in relatively high leaf carbohydrate concentrations. The discriminant analysis, which took place regarding all the measured parameters per assessment, resulted in distinct differences among the treatments, revealing the different modes of action and the effects of the products used. The alleviating products ameliorated the effects of heat and high irradiance stress in the kiwifruit leaves in terms of photosynthetic activity and hydration status, with glycine betaine being more effective than the others, especially under unfavorable conditions in mid-summer.