The issue of canopy efficiency in the grapevine: assessment and approaches for its improvement.

This paper reviews methods available for assessing the efficiency of a grapevine canopy which, due to the flexible canes characterizing this species, can be trained to a multitude of geometrical forms. "Efficiency" is defined here as the ability to share high light interception with effective light distribution within the canopy while reaching concurrently adequate dry matter partitioning to clusters and renewal wood. A traditional approach to the canopy efficiency issue is the calculation of well known vine balance indices (i.e. yield-to-pruning weight or leaf area-to-yield ratios) which are indeed useful to highlight deviations from "equilibrium" but are also static (usually calculated at harvest) and often poorly correlated with "source" potential and grape quality parameters. Total amount and "quality" of intercepted light obtained through canopy laser scanning approaches are promising parameters in terms of correlation with actual whole-canopy photosynthesis (at least for canopy of low-to-moderate vigour) and assessment of training systems as related to light exposure of different organs depending upon their age and position. Yet, these methods are quite time consuming and hardly applicable on a large scale. Over the last decade an increasing popularity has been gained by the treeenclosure method consisting in measuring the gas exchange of entire canopies enclosed in "balloons" made of polyethylene or other plastic material. This method overcomes the inherent limitations of single-leaf measurements and provides valuable information as related to the incidence of factors such as shading, ageing, leaf healthiness at the whole canopy level as well as effects due to different summer pruning operations. Weaknesses are that no commercial versions of these systems are still available and that maximum care is needed to assure that the micro-environment inside the chambers is reproducing as close as possible the outside environment. The last frontier in assessment of grapevine canopy efficiency is represented by modelling. Many models have been proposed for the grapevine in the past but the great majority of them is quite complex in terms of algorithms and data input needed. More recently, a user-friendly model based on STELLA software for daily and seasonal estimates of carbon supply has been validated for the grapevine versus either direct measurements of CO2 exchange and destructive estimates of dry matter partitioning. Overall, regardless of methodologies available to assess canopy efficiency in the grapevine, we feel that approaches for its improvement in the future should consider more carefully the role of growing habit in determining canopy microclimate, regulation of shoot vigour and berry size, and the impact of summer pruning operations.