Performance and water-use efficiency (single-leaf vs. whole-canopy) of well-watered and half-stressed split-root Lambrusco grapevines grown in Po Valley (Italy)

Withdrawing water from one pot triggered a water stress response showing higher stomatal sensitivity to changes in air vapour pressure deficit, relatively low assimilation rates, high intrinsic and extrinsic water-use efficiency (WUE) and earlier cessation of shoot growth. Yet, mid-morning leaf water potential was consistently lower in HS treatment over stress as compared to WW, indicating an anisohydric adjustment. Canopy NCER given on a leaf-area basis showed mean daily rates ranging from 3.9 to 4.9 μmol m2 s−1 in WW canopies against 2.6–3.0 μmol m−2 s−1 in HS. Conversely, canopy transpiration rates varied from 0.915 to 1.157 mmol m−2 s−1 for WW to 0.630–0.714 mmol m2 s−1 in HS. Increased leaf-based intrinsic and extrinsic WUE in HS did not match the canopy response, which to some extent resulted in an opposite outcome, i.e. higher canopy WUE in well-watered vines especially in the morning hours. Likewise, δ13C did not differ between treatments. This suggests caution when point-time determinations of single-leaf-based WUE are extrapolated to the whole-canopy behaviour when assessing the water saving strategies of a given genotype. The stressed vines achieved no variation in yield level and components and had improved grape composition as to soluble solids and total anthocyanins. This optimal behaviour is likely due to earlier shoot growth cessation, enhanced maturity and a buffering leaf-to-fruit ratio (3.61 m2 kg−1) that mitigated the effects of post-veraison stress.