PORTABLE THROUGH-FLOW CUVETTE SYSTEM FOR MEASURING WHOLE-CANOPY GAS-EXCHANGE OF APPLE-TREES IN THE FIELD

A monitoring and control system for sequentially measuring whole-tree- canopy gas exchange of foul apple (Malus domestica Borkh.) trees in th e field is described. A portable, highly transparent, open-top whole-c anopy cuvette was developed for complete enclosure of the above-ground portion of the tree. The flux of whole-canopy CO2 and H2O vapor was e stimated from differential CO2 concentration and H2O-vapor partial pre ssure between ambient/reference air entering the cuvette and analysis air leaving the cuvette, as measured by infrared gas analysis, The bul k air-flow rate through the chamber was measured with a Pitot static t ube inserted into the air-supply duct and connected to a differential pressure transducer. Performance of the whole-canopy cuvette system wa s tested for its suitability for gas-exchange measurements under field conditions. The air flow through the whole-canopy cuvette was 22000 L .min(-1) (approximate to 5.5 air exchanges/min) during the day, provid ing adequate air mixing within the cuvette, and 4000 L.min(-1) (approx imate to 1 air exchange/min) during the night. Daily average leaf temp eratures within the cuvette were 2-3 degrees C higher than to those on trees outside the cuvette. Photosynthetic photon flux transmitted thr ough the chamber malls was at least 92% of the incident ambient radiat ion. Moreover, the whole-canopy cuvette was evaluated without tree enc losure to determine the degree of ''noise'' in differential CO2 concen tration and H2O-vapor partial pressure and was found to be acceptable with Delta CO2 +/- 0.3 mu mol.mol(-1) and Delta H2O +/- 5 Pa. Whole-ca nopy carbon gas exchange and transpiration of four cropping 'Braeburn' /M.26 apple trees followed closely incident radiation over the course of a day.