Validation of vine canopy density measurements by using an ultrasonic-based method.

Quantification of leaf area in grapevine has always been a critical point in researches focused on irrigation management, source-sink interrelationships and efficiency of spray application to canopies. In this work we propose the use of ultrasonic sensors as a fast and accurate tool for the estimation of large portions of leaf canopy area. One ultrasonic sensors were assembled on a tractor to calculate a ultrasonic-based leaf density index that was correlated with measured leaf area, with point quadrat outputs and with estimations of canopy surface obtained by using the LAI2000 (Li-Cor) technology. The measurements were assessed on three vineyard rows consisting of 6 cultivars replicated in a vertical trellis system. A linear regression between different methods has been tested to understand the most reliable and quick approach to validate the ultrasonic index. The relation among ultrasonic elaborations, point quadrat, and direct leaf area measurement were significant, with R2 ranging between 0.80 and 0.85. On the contrary, a lower correlation subsisted with LAI2000 outputs. These results were obtained by averaging all the values belonging each cultivar (10.5 m along the row); however, when we analyzed a lower distance (3.5 m, i.e. four contiguous vines) inside the 10.5 m parcel the reliability of the ultrasonic-based method decreased. This results point out that the LAI2000-derived method underestimates the canopy surface in grapevines trained on vertical trellis system and show that ultrasonic sensors can represent a powerful tool for estimate leaf canopy area. At the moment, however, another limit in the application of this instrument is the needing of a reference value (e.g. leaf area measured through point quadrat measurements) to obtain absolute and not only relative results.