Abstract
Grape berry composition in terms of sugars and titratable acidity is strongly dependent on vine water status. The objective of this study was to evaluate the feasibility of using airborne derived Plant Cell Density (PCD) and canopy reflectance derived Simple Ratio (SR) measurements – both remote indicators of leaf area and intercepted light – at estimating berry yield and composition in vineyards experiencing water deficits. In addition, the capability of the Water Index (WI) – a hyperspectral reflectance based indicator of water status – derived from canopy reflectance at estimating berry yield and composition was tested. The study was conducted in vineyards of Vitis vinifera L. ‘Chardonnay’. Remote optical imagery was acquired at veraison and PCD derived. Field measurements of fractional intercepted Photosynthetic Active Radiation (fIPAR), predawn water potential (Ψp), canopy to air temperature difference at midday (ΔT(midday)) and canopy hyperspectral reflectance were also conducted at the stage of veraison. Yield, Total Soluble Solids (TSS), Titratable Acidity (TA) and maturation index (IMAD) were determined at harvest. Predawn water potential ranged from -0.85±0.06 MPa to -0.50±0.07 MPa among the vineyards studied indicating moderate to severe water stress at the stage of veraison. Berry quality attributes did not show significant relationships against fIPAR but were related to ΔT(midday) in a variable extend. Consistently, PCD derived from airborne imagery, as well as the SR, did not show significant correlation against berry quality attributes. On the other hand, the WI derived from canopy reflectance showed significant correlation against ΔT(midday) (r = -0.82, P<0.05) and successfully estimated TA and IMAD (r = 0.70, P<0.05, for both indices). The results obtained suggest that, in vineyards experiencing water deficits, remote estimates of vine water status through the WI might be more appropriate to characterize berry composition than remote estimates of vine vigor such as SR or PCD.
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