Measurement of grapevine canopy leaf area by using an ultrasonic-based method

Aim: Measurement of leaf area in grapevine has always been a critical point in researches focused on irrigation management, training systems, 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. Methods and results: Through outputs of ultrasonic sensors installed on a tractor moving along vineyard rows, we calculated an ultrasonic-based leaf density index that we correlated with three measurements or estimates of canopy area: I) direct measurement of the area of a canopy portion (LAØ), assessed by summing the areas of all the leaves, where each single-leaf area was assessed by regressing the leaf diameter (the maximum width perpendicular to the main rip) against the related leaf area calculated on the basis of a relation between the leaf diameter and the leaf area, previously assessed through an area meter on a 20-leaf sample; II) the point quadrat output (LApq); and III) the canopy leaf area index (LAI) obtained through LAI-2000 (Li-Cor) technology. The measurements were assessed on six cultivars in three replicate rows (8-12 plants per cultivar per row) in a vineyard trained to a vertical trellis system. Conclusion: When we correlated the three independent control parameters with each other, we obtained highly significant correlations between LApq and LAØ, but less significant correlations between these two and LAI-2000 outputs. Also, the correlations between ultrasonic outputsoutputs and LAØ and LApq were significant, with R2 ranging between 0.84 and 0.85. On the contrary, no significant correlation was found between ultrasonic outputs and LAI-2000 outputs. These results were obtained by averaging all the values belonging to each replicated cultivar (10.5 m along the row, i.e., twelve contiguous vines); on the contrary, when the analysis was done over a shorter distance (3.5 m, i.e., four contiguous vines), the reliability of the ultrasonic-based method decreased. Significance and impact of the study: These results point to the ultrasonic technology as a powerful tool to estimate large-scale leaf canopy area, with potential applications in precision farming. At the moment, however, the limitation of this approach is the requirement of reference values for leaf area (e.g., assessed by point quadrat) to obtain absolute and not only relative outputs. With this application we can quantify, in a few hours, the canopy of a whole vineyard, in order to analyze different vegetation zones or to follow canopy development.