Abstract: By using PMS pressure chamber and isotope mass spectrometer (MAT-252), the leaf juice of Acacia mangium was obtained, and the carbon isotope discrimination (Δ) representing most of the recently fixed carbon in the juice was determined. At the same time, the wateruse efficiency of A. mangium was estimated. The results indicated that the carbon isotope ratio in the air of forest canopy (δ_a, 10 m high above ground) was averagely -7.57‰±1.41‰ in cloudy days, and -8.54‰±0.67‰ in sunny days. The diurnal change of the carbon isotope ratio in photosynthetic products of leaf juice (δ_p) was of saddle type in cloudy days, but dropped down from morning to late afternoon in sunny days. A strong negative correlation between δ_p and leaf/air water vapor deficit (D) was observed in sunny days, but a much smaller change in δ_p was found in cloudy days. The δ_p also decreased with decreasing leaf water potential (Ψ), reflecting that water stress could cause the decrease of δ_p. The carbon isotope discrimination of the leaf juice was positively correlated with the ratio between intercellular (P_i) and atmospheric (P_a) partial pressure of CO₂. For A. mangium, the isotope effect in diffusion of air CO₂ via stomata was denoted by a=4.6‰, and that in net C₃ diffusion with respect to P_i was indicated by b=28.2‰. The results were in reasonable accord with the theoretically diffusional and biochemical fractionation of carbon isotope. It was defined that the carbon isotope discrimination of photosynthetic products in A. mangium leaf juice was in proportion to that from photosynthetic products in dry material. It fit well in water-use efficiency estimated by the carbon isotope discrimination in leaf juice, and by gas exchange method (R₂=0.86, P<0.001). The application of leaf juice to measure the instant carbon isotope discrimination would reduce the effects of fluctuating environmental factors during the synthesis of dry matters, and improve the eco-physiological studies on carbon and water balance when scaling from the plant to canopy in the fields.

Key words: Moina mongolica, Brachionus plicatilis, Interspecific interaction