Intensive Fruit-zone Leaf Thinning Increases Vitis vinifera L. ‘Cabernet Sauvignon’ Berry Temperature and Berry Phenolics without Adversely Affecting Berry Anthocyanins in Virginia

Abstract

<jats:p>Selective leaf removal in the proximity of grape clusters is a useful practice to manage fruit diseases and otherwise improve fruit composition. The current recommendation in the eastern United States is to create a fruit zone with one to two leaf layers and to focus removal on the “morning sun” side of the canopy. We evaluated a more intense and an earlier application of fruit-zone leaf thinning relative to current recommendations to determine whether additional benefits could be obtained without a penalty of impaired berry pigmentation or other ill effects of abundant grape exposure. Fruit secondary metabolites and berry temperature were monitored in two different field experiments conducted with ‘Cabernet Sauvignon’ in the northern Shenandoah Valley American Viticultural Area (AVA) of Virginia. One experiment evaluated the effects of no leaf removal, prebloom removal of four basal leaves per shoot, and prebloom removal of eight basal leaves per shoot. The other experiment evaluated the effects of no leaf removal and postfruit set removal of six basal leaves per shoot. On average, exposed grapes heated to ≥30 °C for a 126% longer period (53 hours) than shaded grapes in the postveraison period (from color development through harvest). However, postveraison grape temperatures did not remain above provisional, critical temperature thresholds of either 30 or 35 °C for as long as they did in studies conducted in sunnier, more arid climates. There were minimal differences in berry temperature between east- and west-exposed grapes in the northeast/southwest-oriented rows of the experimental vineyard. Regardless of implementation stage, leaf removal consistently increased total grape phenolics measured spectrophotometrically, and either increased or had no impact on anthocyanins relative to no leaf removal. Grape phenolics and anthocyanins were unaffected by canopy side. Berry total phenolics were increased and anthocyanins were at least maintained in fruit zones void of leaf layers—a canopy attribute that reduces bunch rot in humid regions.</jats:p>

Prediction of Grape Berry Temperature Using Wireless Dataloggers Contained Within a Grape Mimic

“…The highest ambient solar radiation occurred between 1000 and 1600 HR and ranged from 79.6 to 1039 W•m À2 during that timeframe. Berry temperature is generally higher as ambient temperature and incident solar radiation increases (Hickey and Wolf 2019;Tarara et al 2008). Exposed grape clusters will be subjected to higher solar radiation compared with shaded clusters, and therefore, internal berry temperature increases throughout the day in exposed clusters (Hickey and Wolf 2019).…”

“…High temperatures over an extended period can reduce grape anthocyanin content and limit color development (Bergqvist et al 2001;Spayd et al 2002;Tarara et al 2008). However, variable cloud cover may moderate radiant heating of berries, relative to conditions experienced in arid, less cloudy growing regions (Faust and Logan 2018), resulting in maintained or increased grape anthocyanins in highly exposed grapes (Hickey and Wolf 2019).…”

Sunburn in Grapes: A Review

“…Higher light exposure also increases the total amount of flavonoids present in the berry ( Pastore et al, 2013 ; Kok and Bal, 2018 ; Würz et al, 2018 ; Brandt et al, 2019 ; Hickey and Wolf, 2019 ). UV-B radiation upregulates genes responsible for the synthesis of a range of phenolic compounds including phenylalanine ammonia-lyase (PAL), flavonoid-3'-hydroxylase (F3'H), flavonoid-3',5'-hydroxylase, flavonol synthase (FLS), MYB transcription factor, and UDP-glucosyl transferases (UFGT; Stracke et al, 2010 ; Czemmel et al, 2012 ; Pastore et al, 2013 ).…”

Missing Links in Predicting Berry Sunburn in Future Vineyards

“…On the other hand, leaf removal is a common management practice (Palliotti et al, 2013;Pastore et al, 2013;Torres et al, 2021), for example, to influence grape composition or to reduce disease pressure (Zenoni et al, 2017;Tóth, 2020;O'Brien et al, 2021;VanderWeide et al, 2021). While timing, extent, and need for leaf removal depend on local environmental factors, opening up the canopy at some point is usually recommended for promoting wine quality (Frioni et al, 2017;Hickey and Wolf, 2019;Satisha and Somkuwar, 2019;Würz et al, 2020;O'Brien et al, 2021). Even though, early leaf removal in the bunch zone can allow berries to better adapt to sunlight and, thereby, reduce their susceptibility to sunburn (Gambetta et al, 2021), leaf removal events just before or during a heatwave might dramatically increase sunburn occurrence due to newly sun-exposed berries being insufficiently adapted to the risky environment (Hayman et al, 2012;Palliotti et al, 2014).…”