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Four birch taxa,  Betulaalleghaniensis, B. davurica, B. nigra  `Heritage', and  B. papyrifera , were grown in a greenhouse and exposed to water deficit stress for 35 days. Daily water was withheld from the plants until the combined weight of each container and the plant was reduced to 40% of the control. Supplemental water was supplied to each container to maintain 40% of the initial combined weight. Predawn water potential, gas exchange, and abscised leaf area were measured daily during the first week and at 3-day intervals after the first week. Predawn water potential dropped to lowest value before the containers were rehydrated and remained low throughout the treatment period. Net CO 2  assimilation (A), stomatal conductance (g s ) and evapotranspiration (Et) of all taxa decreased significantly during the first week of water deficits. Changes in A and Et varied among taxa after the first week of water deficits. Water use efficiency (WUE = A/Et) of four birch taxa decreased to significant lower values than control in the first week that water was withheld, and then increased to significantly higher values than control. Gas exchange data were fit into 2- or 3-segmented linear model. The type and shape of the models, and the joint of each segment gave an indication of how the gas exchange responded to the long-term water deficit stress. At the end of the water deficit treatment,  B. papyrifera  had abscised most of its leaves and maintained a high A, while  B. alleghaniensis  maintained more leaves on the tree, but had a relatively low A.

(451) Acclimation to Long-term Water Deficit Stress in Four Birch Taxa (Betula L.): Water Relations and Gas Exchange