© 1991 Heron Publishing—Victoria, Canada
Response of gas exchange to 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 (Ψl, Ψs) were studied in three-month-old white oak (Quercus alba L.), post oak (Q. stellata Wangenh.), sugar maple (Acer saccharum Marsh.), and black walnut (Juglans nigra L.) seedlings. Quercus seedlings had the highest A and gwv when plants were well watered. As the soil was allowed to dry, both A and gwv decreased; however, trace amounts of A were observed at a Ψl as low as –2.9 MPa in Q. stellata and –2.6 MPa in Q. alba and A. saccharum. Photosynthesis was not measurable at Ψl lower than –2.2 MPa in J. nigra and water stress-induced leaflet senescence was observed in this species. Within each species, gwv showed a similar relationship to soil and leaf Ψ, but the response to Ψl was shifted to more negative values by 1.2 to 1.6 MPa. As Ψs declined below –1 MPa, the difference between soil and leaf Ψ diminished because of the suppression of transpiration. There
was no indication that Ψs had a more direct influence on gwv than did Ψl. 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. There was an evident relationship between gas exchange characteristics and natural distribution in these species, with the
more xeric species showing higher A and gwv under both well-watered and water-stressed conditions. There was no trend toward increased efficiency of water use in the
more xeric species.
