Photosynthesis and stomatal conductance of the New Zealand tree, Meryta sinclairii, grown under two watering regimes

We measured leaf gas exchange(photosynthesis, A, and stomatal conductance, g(s)) at different levels of quantum irradiance (Q) in the warm-temperate, New Zealand tree Meryta sinclairii, because it has the largest simple leave s of the New Zealand flora. Five-year-old, potted seedlings were watered or not watered in a rain-exclusion screenhouse for three weeks during summer (6-27 March 1998). Maximum photosynthetic rate (A(max)) was 7.6 mu mol m(-2 ) s(-1). Data for 18 March are presented, which show that photosynthesis sa turated at a low level of irradiance. When photosynthesis was 50% of A(max) , the quantum irradiance Q = 60 mu mol m(-2) s(-1). A linear relation betwe en photosynthesis and stomatal conductance explained 85% of the variation. Water limitation reduced leaf area growth by a factor of 3.