**Kaolin Reduces ABA Biosynthesis through the Inhibition of Neoxanthin Synthesis in Grapevines under Water Deficit**

**Tommaso Frioni <sup>1</sup> , Sergio Tombesi 1,\*, Paolo Sabbatini <sup>2</sup> , Cecilia Squeri <sup>1</sup> , Nieves Lavado Rodas <sup>3</sup> , Alberto Palliotti <sup>4</sup> and Stefano Poni <sup>1</sup>**


Received: 4 June 2020; Accepted: 10 July 2020; Published: 13 July 2020

**Abstract:** In many viticulture regions, multiple summer stresses are occurring with increased frequency and severity because of warming trends. Kaolin-based particle film technology is a technique that can mitigate the negative effects of intense and/or prolonged drought on grapevine physiology. Although a primary mechanism of action of kaolin is the increase of radiation reflection, some indirect effects are the protection of canopy functionality and faster stress recovery by abscisic acid (ABA) regulation. The physiological mechanism underlying the kaolin regulation of canopy functionality under water deficit is still poorly understood. In a dry-down experiment carried out on grapevines, at the peak of stress and when control vines zeroed whole-canopy net CO<sup>2</sup> exchange rates/leaf area (NCER/LA), kaolin-treated vines maintained positive NCER/LA (~2 µmol m−<sup>2</sup> s −1 ) and canopy transpiration (E) (0.57 µmol m−<sup>2</sup> s −1 ). Kaolin-coated leaves had a higher violaxanthin (Vx) + antheraxanthin (Ax) + zeaxanthin (Zx) pool and a significantly lower neoxanthin (Nx) content (VAZ) when water deficit became severe. At the peak of water shortage, leaf ABA suddenly increased by 4-fold in control vines, whereas in kaolin-coated leaves the variation of ABA content was limited. Overall, kaolin prevented the biosynthesis of ABA by avoiding the deviation of the VAZ epoxidation/de-epoxidation cycle into the ABA precursor (i.e., Nx) biosynthetic direction. The preservation of the active VAZ cycle and transpiration led to an improved dissipation of exceeding electrons, explaining the higher resilience of canopy functionality expressed by canopies sprayed by kaolin. These results point out the interaction of kaolin with the regulation of the VAZ cycle and the active mechanism of stomatal conductance regulation.

**Keywords:** particle film technology; xanthophylls; VAZ cycle; drought; *Vitis vinifera* L.; abscisic acid
