Repeated Drought Events in Sauvignon Blanc (V. vinifera): Exploring Physiological Responses

Drought events represent a growing challenge for agriculture in the Mediterranean region, particularly for Vitis vinifera, a species with relevant economic and cultural significance. This study focuses on evaluating the effects of two subsequent drought stress on grapevine cv. Sauvignon Blanc combining physiological and molecular approaches. The trial was conducted in a semi-controlled tunnel at the “L. Toniolo” Experimental Farm of the University of Padua in Legnaro, northeast Italy, from mid-June to mid-July 2024. Fifty vines of Sauvignon Blanc clone 108, grafted onto Kober 5 BB rootstock, were grown in 30 L pots and divided into two groups: well-watered control plants and water-stressed plants. Control plants were maintained at 85-90% of field capacity throughout the experiment, which began just before véraison (June 18th) and continued until ripening (July 22nd). In water-stressed plants watering was stopped until a stem water potential (SWP) of -1.3 MPa (First stress) was reached, followed by a six-day recovery period when vines were watered at well-watered conditions. After recovery, a second drought stress cycle was applied, with half the stressed plants experiencing similar conditions to First stress (Second short-stress) and the other half subjected to a more prolonged stress (SWP of -2.3 MPa, Second long-stress). Both groups underwent a subsequent six-day recovery at well-watered conditions. Throughout the experiment, stomatal conductance was daily monitored using a porometer (LI-COR 600), while chlorophyll fluorescence and leaf gas exchange measurements were measured at peak stress and recovery phases using the LI-COR 6800 system. Photosynthesis was assessed by measuring the net CO2 assimilation rate along with chlorophyll fluorescence parameters. Additionally, leaf water potential (Ψleaf) and SWP were recorded using a Scholander-type pressure chamber. On the same days as the eco-physiological measurements, leaf samples were collected in three replicates for RNA sequencing and non-structural carbohydrate content analyses. From the onset of véraison, berry diameter, soluble solids content, tartaric acid, and malic acid levels were monitored. At harvest, grape bunches were collected for the determination of the C13/C12 isotope ratio. Results confirmed the negative impact of water deficiency on photosynthesis and stomatal conductance. Significant differences were observed between the second long-stress and both the short first- and second-stresses, indicating a grapevine's resilience in recovering from water stress. However, after the prolonged second-stress cycle, this recovery capacity diminished. This was evident in the photosynthesis and fluorescence measurements suggesting a reduced photosynthetic efficiency. Additionally, RNA sequencing results identified key differentially expressed genes associated with different stress responses. Differences in ripening parameters further underscore the impact of repeated drought stress on grapevine development. Further studies are needed to explore these effects in subsequent growing seasons.