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The effects of two deficit irrigation regimes (50% ETC and 20% ETC) on skin and seed phenolic compounds of the red Vitis vinifera L. cultivar 'Refošk' were studied during the veraison - maturation period. The study was undertaken for two successive seasons in the Karst area of North-East Italy, where rainfall is scarce and subterranean drainage limits surface water. To trace the effect of water deficit on both phenol biosynthesis and berry size reduction, phenols were extracted from lyophilised, cryomilled skins and seeds using organic solvents and evaluated in skin and seeds, per units of both dry and fresh weight. The anthocyanin concentration was higher in 20% ETC early after veraison, whereas at harvest the difference was not significant. A decreasing trend with some fluctuations in proanthocyanidin (PA) concentration in skins and seeds was found during maturation with both water regimes. 20% ETC resulted in a decrease in skin PA concentration and increase in seed PA concentration during ripening. As regards PA structural characteristics, there was a trend for higher galloylation and polymerisation of seeds and skins during ripening with the 20% ETC irrigation regime, whereas the effect on prodelphinidins in skins was inconsistent. In 2018, when most of the differences in vine water status between irrigation regimes occurred during veraison - maturation, the concentrations of PAs were affected more significantly than the mean degree of polymerisation and percentage of galloylation. In contrast, in 2019, when differences in vine water status occurred in the pre-veraison-maturation period, the structural characteristics of PAs were affected more significantly than concentrations, especially galloylation of skins and polymerisation of seeds. The results of the study showed that the biosynthesis of PAs, their polymerization and galloylation can be significantly modulated by water deficit regimes applied in the vineyard, in combination with the meteorological course of the season. [ABSTRACT FROM AUTHOR]

Grapevines store non-structural carbohydrates (NSC) during late summer to sustain plant development at the onset of the following spring’s growth. Starch is the main stored carbohydrate, found in the wood-ray parenchyma of roots and canes. Although the relationship between hydraulic and plant photosynthetic performance is well-recognized, little research has been done on the long-term effects of drought in grapevines adopting different strategies to cope with water stress (i.e. isohydric and anisohydric). We performed our study by exposing two different grape cultivars (Syrah and Cabernet Sauvignon) to a short but severe drought stress, at two stages of the growing season (July and September). No marked differences in the physiological and hydraulic responses of the two varieties were found, probably due to our experimental conditions. However, anatomical and biochemical characterization of overwintering canes pointed out several interesting outcomes. We found a significant and parallel increase of starch and medullar ray number in both cultivars exposed to early water stress. We hypothesize that stressed vines limited their carbon allocation to growth, while shifting it to starch accumulation, with a most evident effect in the period of intense photosynthetic activity. We also speculate that a different aptitude to osmotic adjustment may underlay variation in starch increase and the specific involvement of bark NSC in the two cultivars.

SUMMARY: The leaf of a deciduous species completes its life cycle in a few months. During leaf maturation, osmolyte accumulation leads to a significant reduction of the turgor loss point (ΨTLP), a known marker for stomatal closure. Here we exposed two grapevine cultivars to drought at three different times during the growing season to explore if the seasonal decrease in leaf ΨTLP influences the stomatal response to drought. The results showed a significant seasonal shift in the response of stomatal conductance to stem water potential (gs~Ψstem), demonstrating that grapevines become increasingly tolerant to low Ψstem as the season progresses in coordination with the decrease in ΨTLP. We also used the SurEau hydraulic model to demonstrate a direct link between osmotic adjustment and the plasticity of gs~Ψstem. To understand the possible advantages of gs~Ψstem plasticity, we incorporated a seasonally dynamic leaf osmotic potential into the model that simulated stomatal conductance under several water availabilities and climatic scenarios. The model demonstrated that a seasonally dynamic stomatal closure threshold results in trade‐offs: it reduces the time to turgor loss under sustained long‐term drought, but increases overall gas exchange particularly under seasonal shifts in temperature and stochastic water availability. A projected hotter future is expected to lower the increase in gas exchange that plants gain from the seasonal shift in gs~Ψstem. These findings show that accounting for dynamic stomatal regulation is critical for understanding drought tolerance. [ABSTRACT FROM AUTHOR]