COMPARATIVE FIELD PERFORMANCE OF 3 DIFFERENT GAS-EXCHANGE SYSTEMS

We compared portable and continuously monitoring gas exchange systems under field conditions, using Protea glabra Thunb. as a test species. The aim was to determine if the same patterns of gas exchange and anci llary parameters could be obtained with rather different measurement s ystems, and whether the same interpretation and conclusions about envi ronmental control of gas exchange could be drawn. The following system s were compared: 1, a 'closed' portable IRGA manufactured by LI-Cor (L I-6200); 2, an 'open' portable porometer manufactured by Walz; and 3, a continuously monitoring minicuvette system with temperature control facility, also manufactured by Walt. All three systems yielded similar diurnal curves for CO2 uptake, although absolute flux values for the minicuvette system were lower than those obtained for the portable sys tems. This was likely due to stem respiration and self-shading of leav es on the shoot enclosed in the minicuvette. Differences in sampling t echnique between the two portable systems, primarily with regard to ch anges in leaf orientation, resulted in some differences in absolute va lues of gas fluxes and ancillary parameters such as leaf temperature a nd leaf to air vapour pressure difference. However, data from all thre e systems allowed similar interpretations to be made about the environ mental dependencies of gas exchange patterns. It appears that each sys tem has certain drawbacks associated with widely varying field conditi ons. A combination of portable and continuous monitoring techniques wo uld seem to be the most powerful approach to investigating the gas exc hange patterns of terrestrial plants in their natural environment.