Set-up, calibration and testing of a custom-built system for measuring whole-canopy transpiration in grapevine

Upgrading of a whole-vine gas exchange system able to take automated measurements of canopy water loss under close-to-ambient conditions is reported. Vine canopies are enclosed in clear, inflated polyethylene chambers fed with ambient air at a flow speed suitable to limit chamber overheating within 2-3°C. Water loss is calculated from the air flow and the inlet-outlet water vapor differential computed from dry and wet bulb temperatures. Calibration of canopy water loss estimated by the system at various flow rates vs. actual gravimetric water loss recorded for a vine canopy of about 15 m of leaf area resulted in a close linear relationship (R2 = 0.93), although the air flow had to be adjusted above 40 L/s to assure a temperature increase inside the chamber no higher than 3.1°C. Data examples the system can deliver are also reported with emphasis on the variations in the diurnal trend of whole-canopy transpiration (WCE) as affected by different canopy shapes and orientations. The system was sensitive enough to detect a temporary decrease of WCE in the mid-day hours when measurements were taken on NS-oriented vertical hedgerow canopies as compared to canopies either EW-oriented or arranged to resemble the spherical shape of the goblet trellis. Its simplicity, ease of set up, transportability and low cost make the system a valuable tool to assess water loss at the grapevine canopy level. Automation also allows different treatments to be compared in the long run and the flexible plastic enclosure enables measurements to be run on canopies widely differing in shape and size.