Assessment of Grape Yield and Composition Using Reflectance-Based Indices in Rainfed Vineyards

Spectral vegetation indices such as the normalized difference vegetation index (NDVI) have proved to be useful in estimating berry yield and composition in irrigated vineyards that usually experience mild to moderate water deficits. However, their performance is not clear under rainfed conditions. This study explores the relationships between NDVI and berry yield and composition in rainfed vineyards experiencing mild to moderate water stress with respect to the timing of water deficits. The study was conducted on grape (Vitis vinifera L. 'Chardonnay'). Biophysical variables such as predawn water potential (Psi(p)), canopy minus air temperature difference (Delta T-m), fractional intercepted photosynthetic active radiation (fIPAR) and canopy reflectance measurements were acquired at veraison and were related to berry yield, total soluble solids (TSS), titratable acidity (TA), and maturity index (IMAD) at harvest. In our study, while NDVI provided consistent estimates of TSS (r(2) = 0.81) and IMAD (r(2) = 0.89) when water deficit conditions had a larger incidence before veraison, the water index (WI)-a reflectance-based index of water status-was related to TA (r(2) = 0.62) and IMAD (r(2) = 0.67) under post-veraison water deficits. Since early and late water deficits (i.e., before and after veraison) have a differential impact on the determinants of vine photosynthetic activity (i.e., leaf area and stomatal conductance), the capability of NDVI to estimate berry yield and composition might be subject to the extent and timing of water deficits in these rain-fed vineyards.