Iso-/anisohydric behavior in wine grapes may be a matter of soil moisture

Abstract

Context and purpose of the study

There are claims that wine grape cultivars are either isohydric or anisohydric; the former maintaining, and the latter decreasing, their plant water status as soil moisture declines. However, available information is inconsistent. There are those that show an existence of a continuum in cultivar response to soil moisture rather than a distinct categorization. Others even show both behaviors in the same cultivar grown in different environments. In this study we investigated the behavior of 30 own rooted Vitis vinifera cultivars during successive drydown and rewatering cycles over two growing seasons in arid eastern Washington (<200 mm annual precipitation).

Material and methods

A field trial was conducted in 2021 and 2022 in a drip-irrigated research vineyard in Washington’s Yakima Valley. All cultivars were fully irrigated through bloom, then the soil was subjected to two drydown cycles to create a gradual soil water deficit. The first cycle began at fruit set, and the second at veraison following irrigation to replenish soil moisture to near field capacity. Fortnightly measurements of soil moisture, predawn and midday leaf water potential (Ψleaf), and gas exchange were conducted throughout the season. Volumetric soil moisture data were converted to relative extractable soil water (ESW) to normalize the influence of soil texture. These measurements were supplemented with measurements of shoot growth and canopy size.

Results

Findings showed that the cultivars differed in shoot growth, and this was consistent in the two growing seasons. In both seasons, there was a two-fold difference in shoot growth between the most and the least vigorous cultivars. Moreover, the unusually wet spring in 2022 caused most cultivars to grow more vigorously than in 2021. Regarding soil moisture, all cultivars initially behaved isohydrically as the soil dried down from ESW ≥ 1.0 but became anisohydric at ESW < 0.4. Even cultivars with “known” contrasting responses to water stress (Grenache and Sémillon) and those with varied shoot growth responded similarly. In addition, there was a steeper decline in cultivars’ midday Ψleaf with moisture stress in 2022 than in 2021. These findings imply that wine grape cultivars are both isohydric and anisohydric, depending on soil moisture. They are isohydric under no and mild soil moisture deficit conditions and become anisohydric as moisture stress worsens. Canopy size may be a poor predictor of a vine’s physiological behavior under drought stress. But bigger canopies can dry the soil more quickly, leading to a greater decline in Ψleaf.