Trichomes affect grapevine leaf optical properties and thermoregulation

In the context of climate change, the optical properties of grapevine leaves have been used in imaging technologies to screen grapevine phenotypes tolerant to water deficit and heat waves in field conditions. Image-based plant phenotyping is challenging because the adaxial and the abaxial sides of the leaf present different morphology. This study investigated the effect of trichomes of the abaxial epidermis on the spectral responses in the two grapevine leaf sides. It was also examined the effect of pubescence on leaf thermoregulation, either during water deficiency or not. For this study, 99 Vitis spp. genotypes were categorised for their prostrate trichome density between main veins on the abaxial side of the leaves (using the descriptor OIV 084). In the first week of 2022, August, reflectance spectra from 400 to 700 nm and color indexes CIELAB and RGB were recorded in five leaves (both abaxial and adaxial sides) per genotype. During three days in midsummer, crop water stress index (CWSI) was also determined in each genotype. The abaxial leaf side was more reflective than the adaxial one in all visible color bands. Values of CIELAB (except for component a*) and RGB were higher for the abaxial leaf side than the adaxial one. The different spectral and color responses between leaf sides positively correlated with the descriptor OIV 084. Trichomes, which mainly occur on the abaxial side in grapevine, significantly contribute to the different optical properties between the two leaf sides. Correlation between OIV 084 and CWSI values was significantly positive (p < 0.05) only when vines were under drought, indicating that trichomes rise leaf temperature by probably reducing evaporative cooling under dry conditions. Therefore, pubescence could be a promising trait to consider when selecting varieties for drought tolerance.