Optimizing deficit irrigation strategies to manage vine performance and fruit composition of field-grown 'Sangiovese' (Vitis vinifera L.) grapevines

Vineyard irrigation practices in viticultural regions with substantial fluctuations in summer precipitation require the development of efficient water management strategies focused on time of application and irrigation volumes, especially for the widely planted 'Sangiovese' grape variety in Central Italy (Marche region). In this research, key to each irrigation strategy was the monitoring of soil water content (SWC) underneath the vine canopy by capacitance probe. The probe was utilized as a tool for scheduling irrigation and for evaluating vine physiological performance under different environmental conditions. Control non-irrigated vines (I0) were compared to deficit irrigation (DI) treatments based on replacement of 33% (I1) or 70% (I2) of crop evapotranspiration of the previous week, applied between pea-size (4-5mm fruit diameter) and veraison phenological stages. Vines were irrigated when midday leaf water potential (Ψl) was near -0.9MPa, net photosynthesis (Pn) was still high (about 11μmol CO2 m-2s-1), and soil water content (SWC) in the first 0.6m depth beneath the vine canopy had reached the wilting point (WP) threshold. The driest season (2007) resulted in low yield (8t/ha) and excessive total soluble solids (TSS; 25.7°Brix) in the grapes at harvest for the I0 treatment. The limited amount of water supplied to I1 treatment benefited yield (12t/ha) and lowered TSS (about 22°Brix) without reducing anthocyanins and phenols concentration, while saving 25mm of water as compared to I2 treatment. Consequently, a strategy providing irrigation with a DI approach was a useful tool to limit excessive alcohol levels in wines produced from berry grown in 2007. However, DI treatments did not influence vine yield and grape composition in the 2008 and 2009 seasons. Those seasons were characterized by moderate and slight water scarcity, respectively. In addition, the I2 treatment did not improve any of the parameters measured in this experiment. When compared to I1, midday Ψl and Ψstem were strongly correlated to SWC in the first meter of soil depth below the vine canopy, measured by a capacitance probe. SWC and midday Ψl were also positively correlated to leaf photosynthesis. Further, SWC can be easily monitored in the vineyard, and therefore could be potentially used as an effective indicator of grapevine water status and photosynthetic performance.