Water availability is arguably the most important environmental factor limiting crop growth and productivity. Erratic precipitation patterns and increased temperatures resulting from climate change will likely make drought events more frequent in many regions, increasing the demand on freshwater resources and creating major challenges for agriculture. Addressing these challenges through increased irrigation is not always a sustainable solution so there is a growing need to identify and/or breed drought-tolerant crop varieties in order to maintain sustainability in the context of climate change. Grapevine (Vitis vinifera), a major fruit crop of economic importance, has emerged as a model perennial fruit crop for the study of drought tolerance. This review synthesizes the most recent results on grapevine drought responses, the impact of water deficit on fruit yield and composition, and the identification of drought-tolerant varieties. Given the existing gaps in our knowledge of the mechanisms underlying grapevine drought responses, we aim to answer the following question: how can we move towards a more integrative definition of grapevine drought tolerance?

Grapes are one of the major fruit crops and they are cultivated in many dry environments. This study comprehensively characterizes the metabolic response of grape berries exposed to water deficit at different developmental stages. Increases of proline, branched-chain amino acids, phenylpropanoids, anthocyanins, and free volatile organic compounds have been previously observed in grape berries exposed to water deficit. Integrating RNA-sequencing analysis of the transcriptome with large-scale analysis of central and specialized metabolites, we reveal that these increases occur via a coordinated regulation of key structural pathway genes. Water deficit-induced up-regulation of flavonoid genes is also coordinated with the down-regulation of many stilbene synthases and a consistent decrease in stilbenoid concentration. Water deficit activated both ABA-dependent and ABA-independent signal transduction pathways by modulating the expression of several transcription factors. Gene-gene and gene-metabolite network analyses showed that water deficit-responsive transcription factors such as bZIPs, AP2/ERFs, MYBs, and NACs are implicated in the regulation of stress-responsive metabolites. Enrichment of known and novel cis-regulatory elements in the promoters of several ripening-specific/water deficit-induced modules further affirms the involvement of a transcription factor cross-talk in the berry response to water deficit. Together, our integrated approaches show that water deficit-regulated gene modules are strongly linked to key fruit-quality metabolites and multiple signal transduction pathways may be critical to achieve a balance between the regulation of the stress-response and the berry ripening program. This study constitutes an invaluable resource for future discoveries and comparative studies, in grapes and other fruits, centered on reproductive tissue metabolism under abiotic stress.

Adapting agriculture to climate change is driving the need for the selection and breeding of drought-tolerant crops. The aim of this study was to identify key drought tolerance traits and determine the sequence of their water potential thresholds across three grapevine cultivars with contrasting water use behaviors, Grenache, Syrah, and Semillon. We quantified differences in water use between cultivars and combined this with the determination of other leaf-level traits (e.g. leaf turgor loss point, π _TLP), leaf vulnerability to embolism (P₅₀), and the hydraulic safety margin (HSM P₅₀). Semillon exhibited the highest maximum transpiration (E_max), and lowest sensitivity of canopy stomatal conductance (G_c) to vapor pressure deficit (VPD), followed by Syrah and Grenache. Increasing E_max was correlated with more negative water potential at which stomata close (P_gs90), π _TLP, and P₅₀, suggesting that increasing water use is associated with hydraulic traits allowing gas exchange under more negative water potentials. Nevertheless, all the cultivars closed their stomata prior to leaf embolism formation. Modeling simulations demonstrated that despite a narrower HSM, Grenache takes longer to reach thresholds of hydraulic failure due to its conservative water use. This study demonstrates that the relationships between leaf hydraulic traits are complex and interactive, stressing the importance of integrating multiple traits in characterizing drought tolerance.

Background and aims: In recent years, increasing summer temperatures coupled with reduced and erratic rainfall during the growing season have induced accelerated fruit ripening in several regions, resulting in an undesirable increase in wine alcohol content. This study was designed to evaluate the impact of canopy and water management on grape sugar and flavonoid accumulation, with the goal of reducing wine alcohol content while conserving or enhancing flavonoid concentration.\ud Methods and Results: In 2011 and 2012, two irrigation treatments (I–irrigated and DI–deficit irrigated) and two canopy heights (HC–high canopy and SC–short canopy) were applied in a ‘Merlot’ vineyard. No interactions between treatments were observed, and thus, independent results were obtained; DI berries had significantly higher sugar concentration (+5%) than I in both years and higher wine alcohol content only in 2012. SC berries had lower sugar concentration (-4%) than HC and lower wine alcohol (-8%) only in 2011. Anthocyanins and tannins in berry and wine were increased by water deficit and not affected by severe trimming.\ud Conclusion: DI did not reduce berry sugar concentration and wine alcohol content but did enhance desirable wine attributes. SC reduced berry sugar concentration and alcohol content in wine in one of the two seasons. Water deficit and severe trimming showed independent effects on berry composition.\ud Significance of the Study: Severe canopy reduction performed at early stages of ripening can reduce sugars without affecting anthocyanin accumulation in ‘Merlot’. Conversely, deficit irrigation applied before veraison, despite promoting anthocyanin accumulation, may also increase berry sugar concentration at harvest

Main conclusion Drought-acclimated vines maintained higher gas exchange compared to irrigated controls under water deficit; this effect is associated with modified leaf turgor but not with improved petiole vulnerability to cavitation. A key feature for the prosperity of plants under changing environments is the plasticity of their hydraulic system. In the present research we studied the hydraulic regulation in grapevines (Vitis vinifera L.) that were first acclimated for 39 days to well-watered (WW), sustained water deficit (SD), or transient—cycles of dehydration–rehydration—water deficit (TD) conditions, and then subjected to varying degrees of drought. Vine development under SD led to the smallest leaves and petioles, but the TD vines had the smallest mean xylem vessel and calculated specific conductivity (k _ts). Unexpectedly, both the water deficit acclimation treatments resulted in vines more vulnerable to cavitation in comparison to WW, possibly as a result of developmental differences or cavitation fatigue. When exposed to drought, the SD vines maintained the highest stomatal (g _s) and leaf conductance (k _leaf) under low stem water potential (Ψ_s), despite their high xylem vulnerability and in agreement with their lower turgor loss point (Ψ_TLP). These findings suggest that the down-regulation of k _leaf and g _s is not associated with embolism, and the ability of drought-acclimated vines to maintain hydraulic conductance and gas exchange under stressed conditions is more likely associated with the leaf turgor and membrane permeability.