The geoscience component of terroir in wine grape production continues to be
criticized for its quasi-mystical nature, and lack of testable hypotheses.
Nonetheless, recent relational investigations are emerging and most involve
water availability as captured by available water capacity (AWC, texture) or
plant available water (PAW) in the root zone of soil as being a key factor.
The second finding emerging may be that the degree of microscale variability
in PAW and other soil factors at the vineyard scale renders larger regional
characterizations questionable. Cimatic variables like temperature are well
mixed, and its influence on wine characteristic is fairly well established.
The influence of mesogeology on mesoclimate factors has also been
characterized to some extent. To test the hypothesis that vine water status
mirrors soil water availability, and controls fruit sensory and chemical
properties at the vineyard scale we examined such variables in a iconic,
selectively harvested premium winegrape vineyard in the Napa Valley of
California during 2007 and 2008 growing seasons. Geo-referenced data vines
remained as individual study units throughout data gathering and analysis.
Cartographic exercises using geographic information systems (GIS) were used
to vizualize geospatial variation in soil and vine properties. Highly
significant correlations (P < 0.01) emerged for pre-dawn leaf water
potential (ΨPD), mid-day leaf water potential (ΨL) and
PAW, with berry size, berry weight, pruning weights (canopy size) and
soluble solids content (°Brix). Areas yielding grapes with
perceived higher quality had vines with (1) lower leaf water potential (LWP)
both pre-dawn and mid-day, (2) smaller berry diameter and weight, (3) lower
pruning weights, and (4) higher °Brix. A trained sensory panel
found grapes from the more water-stressed vines had significantly sweeter
and softer pulp, absence of vegetal character, and browner and crunchier
seeds. Metabolomic analysis of the grape skins showed significant
differences in accumulation of amino acids and organic acids. Data vines
were categorized as non-stressed (ΨPD ≥ −7.9 bars and
ΨL ≥ −14.9 bars) and stressed (ΨPD ≤ −8.0 bars
and ΨL ≤ −15.0 bars) and subjected to analysis of
variance. Significant separation emerged for vines categorized as
non-stressed versus stressed at véraison, which correlated to the areas
described as producing higher and lower quality fruit. This report does not
advocate the use of stress levels herein reported. The vineyard was planted
to a vigorous, deep rooted rootstock (V. rupestris cv. St. George), and from years of
management is known to be able to withstand stress levels of the magnitude
we observed. Nonetheless, the results may suggest there is not a linear
relationship between physiological water stress and grape sensory
characteristics, but rather the presence of an inflection point controlling
grape composition as well as physiological development.
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