Understanding kaolin effects on grapevine leaf and whole-canopy physiology during water stress and re-watering

Kaolin applications have been investigated in grapevines to understand cooling effects on leaves and clusters and the relative impact on gas exchange, leaf biochemistry, water use efficiency, glyco-metabolism and hormonal patterns. Several Almost all previous contributions have relied upon single-leaf measurements, leaving uncertainty on whole canopy performances, depending on the complexity of a canopy system vs. individual leaves. In our study, kaolin was sprayed at pre-veraison (DOY 204) on potted mature vines (cv. Sangiovese) and washed off a month later (DOY 233), while control vines were left unsprayed. Within control (C) and kaolin (KL) treated vines, well-watered (WW) and water stress (WS) treatments were also imposed over a 10-day period (DOY 208-217) and all vines were re-watered when the WS reached its peak (stem water potential between -1.3 and -1.6 MPa). Single leaf measurements included leaf surface temperature by thermal imaging (Leaf T_mean), assimilation (Leaf A), transpiration (Leaf E), stomatal conductance (Leaf g_s) rates, F_v/F_m fluorescence ratio, pre-dawn and stem water potential. Concurrently, whole canopy gas exchange was monitored continuously from DOY 200-259 using a vine enclosure system and daily net CO₂ exchange rate (NCER) and canopy transpiration (E_canopy) were calculated and then normalized vs. leaf area per vine. Results report that for any of the parameters recorded at both levels (single leaf and whole canopy), there was good agreement in terms of relative changes. In absence of water stress, KL was able to improve leaf cooling, while slightly reducing photosynthetic and water loss rates. More interestingly, data taken under water deficit and upon re-watering support the hypothesis that KL can turn into a protective agent for leaf function. In fact, the lack of photo-inhibition and the maintenance of leaf evaporative cooling found in KL-WS at the peak of water-stress (F_v/F_m > 0.7, Leaf T_mean < 38°C and E_canopy > 0.5mmol m^-2 s^-1) warranted a prompter recovery of leaf functions upon re-watering that did not occur in C-WS vines.