Portable through-flow cuvette system for measuring whole-canopy gas exchange of apple trees in the field

Abstract

A monitoring and control system for sequentially measuring whole-tree-canopy gas exchange of four apple (Malus domestica Borkh.) trees in the field is described. A portable, highly transparent, open-top whole-canopy cuvette was developed for complete enclosure of the above-ground portion of the tree. The flux of whole-canopy CO2 and H2O vapor was estimated from differential CO2 concentration and H2O-vapor partial pressure between ambient/reference air entering the cuvette and analysis air leaving the cuvette, as measured by infrared gas analysis. The bulk air-flow rate through the chamber was measured with a Pitot static tube inserted into the air-supply duct and connected to a differential pressure transducer. Performance of the whole-canopy cuvette system was tested for its suitability for gas-exchange measurements under field conditions. The air flow through the whole-canopy cuvette was 22000 L·min-1 (≃5.5 air exchanges/min) during the day, providing adequate air mixing within the cuvette, and 4000 L·min-1 (≃1 air exchange/min) during the night. Daily average leaf temperatures within the cuvette were 2-3 °C higher than to those on trees outside the cuvette. Photosynthetic photon flux transmitted through the chamber walls was at least 92% of the incident ambient radiation. Moreover, the whole-canopy cuvette was evaluated without tree enclosure to determine the degree of 'noise' in differential CO2 concentration and H2O-vapor partial pressure and was found to be acceptable with ΔCO2 ± 0.3 μmol·mol-1 and ΔH2O ± 5 Pa. Whole-canopy carbon gas exchange and transpiration of four cropping 'Braeburn'/M.26 apple trees followed closely incident radiation over the course of a day.