中文摘要：

利用PMS压力室压取叶片液汁并借助稳定碳同位素质谱仪测定碳同位素比率，分析了马占相思（Acaciamangium）叶片液汁光合产物的甄别率（△）和估测了水分利用效率。结果表明：阴天马占相思林冠层日平均空气CO2的碳同位素比率（δa）为一7．57‰±1．41‰，晴天则为-8．54‰±0．67‰；阴天叶片液汁的光合产物碳同位素比率（δp）日变化呈鞍型，而晴天则从早上至黄昏逐步降低；晴天δp与叶片／空气水汽压亏缺（D）明显呈负相关，阴天的δp变化相对较小；δp随叶片水势（ψ）降低而降低，显示水分胁迫引起δp降低；叶片液汁的D和经气体交换法获得的细胞胞间（Pi）和外界（Pa）CO2分压之比呈正相关，测定结果与理论上碳同位素相关扩散和生化分馏相一致。分析结果显示，田间马占相思空气CO2经气孔扩散的稳定同位素效应α=4．6‰，有关Pi的净固定的稳定碳同位素效应b=28．2‰认为液汁碳同位素甄别率是外围空气CO2进入光合产物的净甄别率，由叶片液汁△估测的水分利用效率与气体交换法所得结果相一致（R^2=0．86，P〈0．001）。本文所采用的叶片液汁光合产物测定即时△以及计算水分利用效率的方法，可减少田间条件下环境因素明显变化对水分和碳同位素的影响，该方法有助于进一步开展由植株至冠层扩展的碳和水分平衡的生理生态研究。

英文摘要：

By using PMS pressure chamber and isotope mass spectrometer （MAT-252）, the leaf juice of Acacia mangium was obtained, and the carbon isotope discrimination （△） 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 （δa, 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 （δp） was of saddle type in cloudy days, but dropped down from morning to late afternoon in sunny days. A strong negative correlation between δp and leaf/ air water vapor deficit （D） was observed in sunny days, but a much smaller change in δp was found in cloudy days. The δp also decreased with decreasing leaf water potential （ψ） , reflecting that water stress could cause the decrease of δp. The carbon isotope discrimination of the leaf juice was positively correlated with the ratio between intercellular （Pi） and atmospheric （Pa） 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 Pi 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 （R^2 = 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 stud