Comparison of four methods for measuring osmotic potential of tree leaves

Osmotic potential (pi) of tree leaves is usually determined by pressure-volume (PV) analysis, which is too time consuming for large-scale assessments such as ecological surveys and genetic screening. We investigated methods for rapid measurement of pi in sclerophyllous species. The osmotic potential of expressed sap (ES), freeze-thaw leaf discs and hot water extracts - measured using a variety of techniques - were compared with PV analysis using sclerophyllous species; Eucalyptus globulus Labill., E camaldulensis Dehnh., E. cladocalyx F. Muell. and non-sclerophyllous species; Populus nigra L and Alnus jorullensis H.B.K. All methods were sensitive to changes in pi and produced results that were unequal but well correlated (r(2) 0.82-0.90). The proportion of leaf water extracted in ES was reduced by low water content and sclerophylly without affecting method accuracy. PV analysis provided a poor correction for apoplastic dilution of ES. Results of ES osmometry and PV analysis were comparable, and osmometry of ES is proposed as a suitable method for large-scale investigations of leaf pi.