The use of unsaturated salt solutions to generate conifer needle water-release curves
Reversible changes in cell wall water content, elasticity, and osmotic adjustment are all important drought tolerance mechanisms. Pressure chambers have been used extensively to generate water-release curves from which these basic water relations parameters are derived. Major limitations of this technique include the considerable time required to generate water-release curves and the difficulty in avoiding damage to tissue during the process. A simple and practical technique is descried that overcomes these limitations and allows the routine and rapid generation of water-release curves. Needles were removed from hydrated black spruce (Piceamariana (Mill.) B.S.P.) shoots, weighed on a precision balance, and suspended above unsaturated salt solutions of known molality in sealed test tubes held at 5 °C. Eighteen solutions were used with water potentials ranging from −0.09 to −4.67 MPa. Once needles had equilibrated with air above the salt solution (8–30 h depending on needle age), they were weighed and oven-dried. Water-release curves were obtained by plotting paired values of needle relative water content and solution water potential. Typically, 35 curves were generated over 3 days compared with 7 curves generated using a pressure chamber over the same period. There was excellent agreement between methods in estimates of elastic modulus, osmotic potential, and apoplastic water content.