Abstract:
Many climatic factors influence grape berry composition including nutrient
status, water availability, biotic stress, sun exposure and temperature. Previous
research examined the effects of many factors listed above and much progress has
been made. It is often difficult, however, to separate effects that typically confound
each other, such as sun exposure and temperature. Increasing exposure of a berry to
the sun will lead to some degree of heating unless the temperature is otherwise
maintained. In this study berry temperatures where manipulated independent of sun
exposure, necessarily separating the two effects.
The objective of this study was to assess the impact of fruit temperature on the
phenolic metabolism of grape berries (Vitis vinifera L. cv. Merlot) grown under field
conditions with controlled exposure to sunlight. While similar studies have focused
on production and accumulation of anthocyanins our primary focus was on
proanthocyanidins or ‘tannins’. Here we report the effects of modulating daytime and nighttime temperatures as well as damping the diurnal temperature range.
Furthermore, research was broken into two phases: berry set to véraison (phase I) and
véraison to commercial harvest (phase II). This was to assess the effects of
treatments during two discrete phases of berry development characterized by
accumulation of distinct phenolic metabolites.
Samples collected at véraison indicated that damping the diurnal temperature
fluctuation advanced the onset of ripening. Those berries were larger (doubledamped:
0.753±0.015 vs control: 0.512±0.034 g/berry) and more colored than all
others. Phenolic material from grape seed and skin was quantified and characterized
using three chromatography methods. Proanthocyanidin accumulation at véraison
was linearly related to heat summation over the developmental period with nighttime
heating yielding the highest concentration and daytime cooling yielding the lowest
(night-heat: 1.46±0.13 vs day-cool: 0.97±0.09 mg/berry). Damping the diurnal
temperature fluctuation reduced proanthocyanidin mDP (double-damped: 21.8±1.0 vs control: 28.0±1.7). Day-Cooling resulted in an increase in the concentration of
flavonols at the end of phase I yet a decrease at the end of phase II. The goal of this work is to provide researchers with additional information regarding climatic factors influencing phenolic biosynthesis and to provide grape growers with tools to better manage their crop.
