Silvina Dayer, Sunita Ramesh, Johannes Scharwies, Stephen Tyerman, and Vinay Pagay*  
*University of Adelaide, PMB 1, Glen Osmond, SA 5064, Australia
(vinay.pagay@adelaide.edu.au)

Cultivar-specific differences in stomatal responses to soil moisture deficits, known as isohydry, are well documented in grapevine; however, the basis for these differences is less well understood. In 2016, we conducted a study to investigate the basis of isohydry using potted Vitis vinifera Shiraz and Grenache on a Drought-Spotter automated gravimetric platform. This platform allowed precise control of soil moisture and quantification of the daily dynamics of canopy transpiration of individual potted vines. Leaf and stem water potentials, leaf stomatal conductance (gs), root hydraulic conductance (Lo), and leaf hydraulic conductance (Kleaf) were measured at several timepoints over the course of soil dry-down and rehydration. At the same timepoints, xylem sap and leaf and root samples were collected to determine abscisic acid (ABA) concentrations and aquaporin (AQP) gene expression levels, respectively. We hypothesized that the putatively more isohydric Grenache regulates its stomata under water stress by decreasing root and/or leaf hydraulic conductance, while the relatively anisohydric Shiraz maintains L0 and Kleaf under water stress. Withholding irrigation lowered gs to near stomatal closure in both cultivars within three days. Well-watered Grenache vines increased L0, possibly in response to high gs; this response was not observed in water-stressed vines. In contrast, L0 tracked gs closely in both well-watered and water-stressed Shiraz vines, indicating a high degree of hydraulic coupling between roots and shoots in this cultivar. Under water stress, AQP gene expression levels decreased in Grenache leaves but increased in roots, while water stress increased both leaf and root AQPs in Shiraz. Xylem ABA concentrations were higher in Grenache, which could explain the downregulation of specific leaf AQPs. These findings suggest that Grenache may be relatively isohydric due to regulation of both hydraulic and chemical signals while Shiraz is relatively anisohydric due predominantly to hydraulic regulation.

Funding Support: Australian Research Council Centre of Excellence in Plant Energy Biology