Modeling grapevine leaf reflectance and physiological response to increasing kaolin doses and varying light and water availability

Kaolin is a white clay powder available in various formulations, commonly used to mitigate the negative effects of water and heat stress on a range of crops. According to existing literature, while minimum doses are required to ensure sufficient photoprotection, excessive applications may reduce net carbon assimilation, but such thresholds are yet undefined. In this study, potted grapevines were treated with foliar applications of kaolin at concentrations of 3 % w/v, 5 % w/v, and 7 % w/v, and compared to untreated controls. Vines were then subjected to progressive water stress up to the achievement of a midday stem water potential of -1.3 MPa, and then fully rewatered until the end of the experiment. The objective was to evaluate the effects of different kaolin dosages on leaf physiological performances under varying light and water availability conditions. Under full water supply, no significant differences in leaf net photosynthetic rate or transpiration were observed between control and vines sprayed with kaolin 3 % w/v. In contrast, a 7 % w/v treatment significantly reduced leaf net photo-assimilation rates at photosynthetically active radiation levels between 50 and 900 μmol m^-2 s^-1 compared to other treatments. At a stem water potential of -1.3 MPa, all vines exhibited stomatal closure. However, following rewatering, vines treated at 7 % w/v displayed higher leaf photosynthetic rates under light-saturating conditions, exceeding the control by +1.31 μmol m^-2 s^-1. Notably, control vines showed significantly lower photosynthesis than all other treatments at irradiance levels between 300 and 900 μmol m^-2 s^-1. Additionally, median leaves were more responsive to kaolin treatments than basal or apical leaves. Increasing kaolin doses progressively enhanced leaf reflectance in both the visible and, more notably, the infrared wavelengths. The 5 % w/v and K7 w/v treatments significantly increased the leaf Photochemical Reflectance Index compared to control vines, while 3 % w/v doses exhibited intermediate values. Our results demonstrate that a 3 % w/v kaolin application provides some photoprotection under stress conditions, with higher doses offering limited additional benefit in preserving canopy functionality. In contrast, increasing the dose to 7 % w/v can reduce maximum photosynthetic capacity under water non-limiting conditions, likely due to enhanced reflectance that lowers light-use efficiency and raises the light saturation point. Therefore, high doses should be considered only when the specific goal is to limit photosynthetic activity, such as to reduce sugar accumulation in grapes and delay ripening.