Determining Transpiration from Pinus patula Shoots—A Comparative Evaluation of the Cut-Shoot Method and Two Null-Balance Diffusion Porometers

Abstract

The cut-shoot method and two types of small-chambered null-balance diffusion porometer were evaluated for suitability in studies of stomatal conductance and transpiration in Pinus patula. In the course of the cut-shoot evaluation, excised shoots were found to transpire at a constant rate for up to four minutes following excision before the effects of the cut became apparent. An experiment involving seedlings in sealed pots showed the four-minute post-excision transpiration rate to be the same as before excision. The porometers were each used to measure leaf conductance of 10 fascicle groups on five shoots selected from different seven-year-old trees. Good correlation was found between transpiraton rates derived from the two porometers and the cut-shoot method. Variation in porometer resistance readings within and between shoots was high. It was concluded that the cut-shoot method is more suitable for use in studies of whole tree canopies where destructive sampling of shoots is possible. The use of small-chambered porometers in such cases is limited by the number of readings required to obtain a reliable estimate of transpiration. Their use is more appropriate in studies involving a limited number of fascicles or shoots, where adequate sampling can be achieved.