Tree Physiology 8, l-9 0 1991 Heron Publishing-Victoria, Canada Response of gas exchangeto water stress in seedlings of woody angiosperms BING-RUI NI and STEPHEN G. PALLARDY School of Natural Resources, University of Missouri-Columbia, Columbia, MO 65211, USA Received April 25, 1990 Summary Responses of net photosynthesis (A), leaf conductance to water vapor (gwV) and instantaneous water use efficiency (WUE) to decreasing leaf and soil water potentials (Yt, Y,) were studied in three-month-old white oak (Quercus alba L.), post oak (Q. stellara Wangenh.), sugar maple (Acer saccharum Marsh.), and black walnut (Juglans nigra L.) seedlings. Quercus seedlings had the highest A and gwVwhen plants were well watered. As the soil was allowed to dry, both A and g,, decreased; however, trace amounts of A were observed at a ‘I’1 as low as -2.9 MPa in Q. stellata and -2.6 MPa in Q. alba and A. saccharum. Photosynthesis was not measurable at ‘3-5 lower than -2.2 MPa in J. nigra and water stress-induced leaflet senescence was observed in this species. Within each species, g wVshowed a similar relationship to soil and leaf Y, but the response to Yt was shifted to more negative values by 1.2 to 1.6 MPa. As Y’, declined below -1 MPa, the difference between soil and leaf Y diminished because of the suppression of transpiration. There was no indication that Y’, had a mom direct influence on gwv than did Yt. Water use efficiency showed an initial increase as the soil dried, followed by a decline under severe water stress. Water use efficiency was highest in J. nigra, intermediate in Quercus species and lowest in A. saccharum. Them was an evident relationship between gas exchange characteristics and natural distribution in these species, with the more xeric species showing higher A and g wy under both well-watered and water- stressed conditions. There was no trend toward increased efficiency of water use in the more xeric species. Introduction As a consequence of competition and differential capacity for drought tolerance, tree species are often segregated along moisture gradients. The structural and physiolog- ical adaptations associated with drought tolerance of plants are numerous and diverse (Pallardy 1981), and they have seldom received detailed study in woody species that vary in natural distribution. Although many attributes associated with drought tolerance appear to be more important for plant survival than for high growth rates (Turner 1986), maintenance of the capacity for gas exchange under drought condi- tions may favor both survival and productivity. Chabot and Bunce (1979), for example, noted a close association between actual lower altitudinal limits of montane arborescent species in Arizona and those predicted from a leaf carbon balance model. Many studies have shown that community leaf area and productivity are closely related to the amount of water available for transpiration on a site (e.g., Grier and Running 1977, Gholz 1982). Hence on xeric sites high efficiency of water use might be of substantial adaptive advantage. However, the information available on compar- ative water use efficiency of xeric and mesic species is equivocal. High water use by guest on January 22, 2015 http://treephys.oxfordjournals.org/ Downloaded from