Isohydric and anisohydric behavior of 18 wine grape varieties grown in an arid climate

Abstract

Context and purpose of the study – The interest in understanding the water balance of terrestrial plants under drought has led to the creation of the isohydric/anisohydric terminology. The classification was related to an implication-driven framework, where isohydric plants maintain a constant and high leaf water potential through an early and intense closure of their stomata, hence risking carbon starvation. In contrast, anisohydric plants drop their leaf water potential to low values as soil drought is establishing due to insensitive stomata and thus risk mortality through hydraulic failure, albeit maximizing carbon intake. When applied to grapevines, this framework has been elusive, yielding discrepancies in the classification of different wine grape varieties around the world. There is a need to assess different wine grape varieties under the same growing conditions to enable conclusions on the differences in their response to drought and facilitate variety-specific irrigation management.

Material and methods – The vineyard was located in the ROZA irrigation district in the Yakima valley, Washington. Varieties were grown side by side and replicated 8 times. Spacing was 1.8 m x 2.7 m in a North-South orientation. The vines were on their own-roots, double-trunked, trained to a bi-lateral cordon. 12-18 varieties of wine grape grown were studied for this experiment. Access tubes were installed for soil moisture measurements using a neutron probe, and irrigation was independently controlled for each row. Dry-down cycles were applied pre- and post-veraison from 2016 to 2018. On the same day, predawn (Ψpd) and midday leaf water potential (Ψmd) were measured with a pressure chamber, stomatal conductance (gs) was measured with a porometer at midday and on the same leaf in 2016 and 2017 and with an infrared gas analyzer in 2018. Soil moisture measurements were taken on the same day for each vine.

Results – The results show that there may be three distinctive major patterns of midday leaf water potential response to soil water availability: Linear drop across the entire soil moisture range such as for Cabernet franc and Semillon, linear drop below a threshold of soil moisture such as for Gewurztraminer and Grenache, and an insensitive to soil moisture such as for Lemberger and Riesling. Meanwhile, the stomatal sensitivity did not always mirror the Ψmd behavior; for example some varieties like Cabernet franc show a linear drop of Ψmid while having a tight stomatal control during soil drought (r=0.76) while other varieties like Riesling have an insensitive response of Ψmid (r=0.33) without necessarily having sensitive stomata (r=0.56). Finally, the slope of the linear Ψmd:Ψpd, studied as an the indicator of the internal regulation of water status, varied between 0.4 for Grenache and 1 for Semillon. This shows that for our vineyard, transpiration sensitivity was always higher than hydraulic sensitivity. Since intense yellowing of leaves has been recorded in varieties like Cabernet franc, Muscat blanc and Malbec, these results direct us to inspect if the sensitivity of gs in those varieties is leading to carbon starvation during drought. These results may eventually be used by growers to devise variety-specific irrigation management strategies.