Relationships between vine isohydricity and changes of fruit growth and metabolism during water deficit

Abstract

The frequency of water deficits is increasing in many grape-growing regions due to climate change. Grapevine varieties differ in stomatal behavior and their ability to regulate water potential, as well as differ in changes to fruit metabolites and quality during water deficits. However, it remains unclear whether stomatal behavior and water potential regulation, often described as the degree of plant isohydricity, has any relationships with changes to fruit growth and metabolism. This study was conducted on four varieties (`Cabernet Franc`, `Semillon`, `Grenache`, and `Riesling`) subjected to both short-term, severe water deficit and long-term, moderate water deficit. Water deficits were applied at both pre-veraison and post-veraison. Stomatal conductance and leaf gas exchanges were tracked using LI-600 and LI-6800 systems, respectively, and pre-dawn and mid-day stem water potentials were tracked using a Scholander-type pressure chamber. Fruit samples were taken during water deficits and primary metabolites (sugars, acids) and hormone (abscisic acid) levels were analyzed using HPLC-RID-DAD and HPLC-QTOF. The results indicate that `Cabernet Franc` and `Riesling` are more resistant to changes in fruit growth, sugar levels, and abscisic acid levels compared to `Semillon` and `Grenache` during both short- and long-term water deficit, especially at the pre-veraison stage. `Cabernet Franc` and `Riesling` exhibited a greater ability to maintain stomatal conductances, pre-dawn water potentials, and mid-day water potentials as compared to `Semillon` and `Grenache`. Lower stomatal conductances and water potentials coincided with earlier and larger increases in fructose in fruit for `Semillon` and `Grenache`. A relationship between fruit abscisic acid and sugar levels was found at pre-veraison, but not post-veraison. Common metrics of describing isohydricity, including stomatal conductance responses to pre-dawn water potentials and the slopes of the relationship between pre-dawn and mid-day water potentials, did not allow for a clear classification of the varieties based on the isohydricity level. The results of this study show that there is a relationship between variety-specific water relations and fruit growth and metabolism, but the common metrics used to describe isohydricity alone are not helpful for predicting changes to fruit during water deficits.