A study of kaolin effects on grapevine physiology and its ability to protect grape clusters from sunburn damage

Sunburn (SN) damage on grape berries has become more prevalent in the Mediterranean region in recent years. Especially clusters on the west side of the vine canopy are more affected due to higher temperatures in the afternoon, in the north-south oriented vineyards. The radiation-reflecting inert substance kaolin particle film (KPF) is a well-known method for its ability to lower fruit temperature. However, there is limited evidence that this method prevents sunburn damage to grape berries. This study was undertaken to determine how KPF influences photosynthesis (A), stomatal conductivity (gs), transpiration rate (E), and midday leaf water potential (Ψmd) of vines. In addition, KPF was also examined for its ability to reduce berry temperatures and protect berries from SN damage during both morning and afternoon sun exposure. Apart from the experimental control, KPF was applied three times between berry set and veraison to the whole vine canopy as a dose of 5% (w/v) once and twice as a half dose of 2.5% (w/v). KPF did not lead to significant changes in A, gs, E, and Ψmd (P ≤ 0.05). However, the KPF sprayed leaves tended to have higher Ψmd values than the control throughout the two seasons. No differences in berry composition were observed among treatments. KPF significantly reduced leaf and berry temperatures throughout growing seasons. According to the measurements made throughout the season, the highest leaf temperature difference reached 2.84 °C but berry temperatures also reached 2.04 °C among treatments. Despite the control group being the most severely affected, SN damage occurred in KPF treatment as well. SN damage level (%) was higher on the west side of the canopy than on the east side in consecutive years, 46.25% and 27.27% of clusters in KPF treatment were affected by SN damage respectively. Consequently, KPF reduced leaf and berry temperatures but did not totally protect berries from SN damage compared to the control.