Optimal Ranges and Thresholds of Grape Berry Solar Radiation for Flavonoid Biosynthesis in Warm Climates

Abstract

In commercial wine grape production, canopy management practices are applied to control the source-sink balance and improve the cluster microclimate to enhance berry composition. The aim of this study was to identify the optimal ranges of berry solar radiation exposure (exposure) for upregulation of flavonoid biosynthesis and thresholds for their degradation, to evaluate how canopy management practices such as leaf removal, shoot thinning, and a combination of both affect the grapevine (Vitis vinifera L. cv. Cabernet Sauvignon) yield components, berry composition, and flavonoid profile. Three experiments were conducted in Oakville, CA, USA. First experiment assessed changes in the grape flavonoid content driven by four degrees of exposure. In the second experiment, individual grape berries subjected to different exposures were collected from two cultivars (Cabernet Sauvignon and Petit Verdot). The third experiment consisted of an experiment with three canopy management treatments (i) LR (removal of 5 to 6 basal leaves), (ii) ST (thinned to 24 shoots per vine), and (iii) LRST (a combination of LR and ST) and an untreated control (UNT). Berry composition, flavonoid content and profiles, and 3-isobutyl 2-methoxypyrazine were monitored during berry ripening. Although increasing canopy porosity through canopy management practices can be helpful for other purposes, this may not be the case of flavonoid compounds when a certain proportion of kaempferol was achieved. Our results revealed different sensitivities to degradation within the flavonoid groups, flavonols being the only monitored group that was upregulated by solar radiation. Within different canopy management practices, the main effects were due to the ST. Under environmental conditions given in this trial, ST and LRST hastened fruit maturity; however, a clear improvement of the flavonoid compounds (i.e., greater anthocyanin) was not observed at harvest. Methoxypyrazine berry content decreased with canopy management practices studied. Although some berry traits were improved (i.e. 2.5°Brix increase in berry total soluble solids) due to canopy management practices (ST), this resulted in a four-fold increase in labor operations cost, twofold decrease in yield with a 10-fold increase in anthocyanin production cost per hectare that should be assessed together.

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“…Stem water potential ( stem ) was greatly affected by doubling the amount of water applied. Customarily, vineyard operators would apply 40-50% of estimated ET c in the study area (Torres et al, 2020). Contrary to our expectations, 80%ET c did not incur higher stomatal conductance, transpiration or carbon assimilation for the instances measured.…”

“…Therefore, the spatiotemporal variation in cluster temperature differences among treatments can be anything from similar (e.g., at night or clusters occluded by leaves) to + 12 • C (i.e., at 1530 h in Southwest exposed clusters). This finding provided evidence that the greater berry temperature experienced by berries of Exposed vines-only experienced by some clusters on the Southwest aspect for a period of less than 4 h-has a milder impact on overall grape must pH and TA (Zarrouk et al, 2016) than persistent changes in ambient temperature reported elsewhere (Sweetman et al, 2014;Rienth et al, 2016;Torres et al, 2020).…”

“…The berries were collected from bunch-closure to harvest. A total of seventy-five berries were collected from each experimental unit on six dates, on both sides of the canopy in equivalent numbers, avoiding severely dehydrated or raisined berries as reported previously (Martínez-Lüscher et al, 2017b;Torres et al, 2020). Each set of seventy-five berries were weighed for individual berry mass calculations.…”

“…Grapevine is a rather resilient perennial crop, tolerating long periods of drought and extreme temperatures. However, as in many other fleshy fruits, grape berry is sensitive to exposure to solar radiation, causing damage on the surface of the fruit or even fruit abortion (Tinyane et al, 2018;Torres et al, 2020). Fruit exposure to solar radiation was highlighted for decades as a key factor to enhance ripening of fruits and their composition and is a very relevant concept for cultural practices in grapes (Jackson and Lombard, 1993;Cook et al, 2015).…”

“…However, solar radiation, especially infrared, transmits thermal energy to exposed objects with an intrinsic increase in their temperature. High temperature may play repressive role in the synthesis of anthocyanins, eventually inducing their degradation (Mori et al, 2007;Martínez-Lüscher et al, 2017b;Torres et al, 2020). In addition, high temperatures may be responsible for a desynchronization between the accumulation of sugars and anthocyanins leading to lower anthocyanin contents at harvest (Sadras and Moran, 2012).…”