Salicylic acid and kaolin effects on pomological physiological and phytochemical characters of six hazelnuts Corylus avellana cultivars at an arid warm summer condition

Nowadays, nuts are accounted more important in the human diet because of their nutritional values 1,2 and hazelnut is one of the world’s major nut crops. The world market of hazelnut witnesses an upward trend, such that it reached nearly 1,05 Million tons all over the world in 2017 3. This is because of its nutritional value which causes increasing the sales and consumption of hazelnut4. Nearly 90% of haszelnut products have been used in the confectionery and chocolate industry, and remaining are sold in-shell for fresh consumption5. Turkey and Italy are leading producer countries, with approximately 50% of the whole global production3. Iran produced nearly 18,000 T of hazelnuts in 2017, ranking 11th in world production3.
In general, Hazelnut (Corylus avellana L.) production has been restricted to humid temperate areas with a moderate winter and summer. Hazelnut trees are sensitive to heat stress in warm summers and freezes at low temperatures less than -15ºC6. In these climatic conditions, new production management practices are needed, as well as new genotypes to overcome problems 7. As expected, the temperature in the future will be raised approximately 1.5ºC until 2030, according to prediction obtained of weather modeling 8. To reduce the impacts of high temperatures on horticultural products, strategies such as the selection of tolerant cultivars, technical management practices, and the use of exogenous protectants for mitigating heat-induced damages are essential. Technical management methods include several ways like shading and irrigation system, which reduce the effects of intense light and high temperatures, but these techniques are costly, with high demand technology and need of abundant water, which is along with the complications by the spread of some diseases due to high humidity 9.
Currently, the application of particle films is one of the inexpensive and facile ways for decreasing the temperature effects in orchards and field crops 10. Kaolin spray, an aqueous formulation made from inert clay chemical (Al4Si4O10(OH)8),11 is specially formulated for mitigating the solar damage by coating the surface of leaf and fruit for protecting from UV injury as well as the heat stress due to high light intensities, insects, and diseases. Besides, it enables enhancement, photosynthesis functions, gas exchange, and net CO2 assimilation, also improves fruit criteria such as color, yield and post-harvest quality as presented in olive, walnut, apple, mango, pomegranate, grape, tomato, and berry12–19. Kaolin also has effects on increasing the antioxidant capacity, secondary metabolites such as phenolic content and phenylpropanoids and flavonoids 20–23.
Salicylic acid plays a crucial role in response to abiotic stresses, and its external application at a suitable concentration can improve the plant's antioxidant system 24. Salicylic acid significantly affects the biosynthesis of supportive compounds such as polyamines, proline, and heat stress proteins 25–28. Moreover, SA pretreatment revealed to alleviate the oxidative stress as an effective protectant under heat, UV, drought, and salinity stresses 29. The SA signaling affects photosystem function and maintaining higher Rubisco activation level and acceleration of the recovery of net photosynthesis rate (Pn) at heat stress, mainly by influencing the PSII function and increased heat shock proteins (HSPs) 30–33. SA treatments were also stimulatory for antioxidant systems like proline, phenolic content, and antioxidant enzyme activities (SOD, APX, CAT) and inhibitory on H2O2, MDA, and electrolyte leakage (EL) under heat stress in pea, strawberry, olive, grape, cotton, and rapeseed 34–36.
Considering all the merits mentioned above, the experiment was set out to understand the effects of kaolin, salicylic acid, and their interaction on hazelnut trees. A wide range of pomological and physicochemical properties of hazelnut was studied. This study aimed to protect hazelnut orchards under heat stress conditions, considering the climate change trends in recent years in many parts of the world.
1. Results and Discussion:
The pomological characters of nuts and kernels
Mean comparisons showed the Merveille de Bollwiller cultivar had the highest weight of nut and kernel; conversely, Segorbe had the lowest values in these traits (Table 1 ). The application of salicylic acid and kaolin did not affect the pomological characteristics, such as sphericity, geometric mean diameter, and the volume of nut. Conversely, the kernel properties like sphericity of the kernel (%), geometric mean diameter, and the volume of the kernel were affected compared to control under SA and KA treatments (supplementary data, No.1). As can be seen in Table 2, the results of this study indicate that kaolin treatment at a concentration of 6% increased the mean weight of nut and kernel nearly 5.6% and 11.2%. Kaolin effects on weight and yield improvement also reported in crops like mango 12, apple 14,37, walnut 18, olive 38,39 and tomato 16, and in red-skinned wine grape 19 showing increase in weight and yield. This may be due to the increased light reflection, reduced leaf temperature, less water stress, and evapotranspiration increasing of the canopy by the coating of kaolin resulting in increased photosynthesis efficiency reduced oxidative stress and supplying of more energy to improve the material transfer to the fruits as sink 14,37,40,41.
Salicylic acid treatment increased the weight of nuts and kernels 6% and 7.2% respectively, compared to control. Increasing in dry weight of nuts and kernels under SA treatment may be due to the reduction of the effects of summer heat stress, especially in July and August, by improving the antioxidant system and enhancing photosynthesis. These results are in accordance with the studies on peas and olives under the stress of ultraviolet radiation, and high temperature 32,38,42. Also, the results are in accordance with the study on strawberry, when 1 mM SA applied at high temperatures under greenhouse conditions reduced the oxidative stress of treated plants by 10% 43–45. Plant response to salicylic acid seems to be related to concentration, stress intensity, plant species; however, the response of herbaceous plants may vary with the woody perennial plants. Table 2 shows that 1 mM salicylic acid and 6% kaolin had a meaningful influence on the mean weight of nut and kernel by approximately 7.1% and 12.7% increase respectively. In general, the application of SA and KA on the leaves under these treatments conducted to enable more photosynthesis and, consequently, more reserves in nut and kernel dry weights.