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由于受直射光、散射光或受遮阴叶层分布以及一天中光、温和水汽压亏缺变化的剧烈影响,植物冠层的水分利用效率(WUE)较为复杂.通过测定叶片的碳同位素比率(δ13Cp),估测叶片新固定碳同位素的比率(δ13Cplant)和冠层水平的碳同位素甄别率(⊿canopy),计算华南地区鹤山丘陵地马占相思林冠层WUE.结果显示:从冬季至夏季,最高气温(Tamax)和水汽压亏缺(Dmax)明显增高;冬季早上和傍晚的δ13Cplant最高,春夏季傍晚有较低的δ13Cplant,秋季的δ13Cplant日变动较小,冬季冠层有较其他季节高的δ13Cplant和较低的⊿canopy.由春季至冬季,δ13Cplant趋向于增高,而⊿canopy则趋向于下降.夏季马占相思林冠层平均(±SD)WUE[(2.01±0.38)mmol mol-1]明显低于冬季和春季值[分别为(6.90±0.26)、(5.65±0.14)mmol mol-1],年均WUE为(4.36±2.32)mmol mol-1.环境变化引起叶片胞间和空气CO2浓度的变化,从而改变⊿canopy和δ13Cplant,造成不同季节冠层WUE的差别.冠层WUE与水汽压亏缺(D1,kPa)、气温(Ta,℃)和叶片水势(Ψ,MPa)成显著负相关.利用δ13Cplant计算的WUE与根据Wang和Leuning的Ball-Woodrow-Berry模式求算的WUE契合较好.结果表明,以新固定碳的稳定同位素比率方法是研究环境胁迫对冠层WUE限制的有效方法.
The water use efficiency (WUE) of plant canopy is more complicated due to the severe influence of the distribution of direct light, scattered light or shadowed leaf layer and the changes of light and moderate vapor pressure deficit in one day. By measuring the leaf carbon isotope ratio δ13Cp), the ratio of new fixed carbon isotope (δ13Cplant) and the carbon isotope discrimination (⊿canopy) of canopy height were calculated to calculate the WUE of canopy Acacia mangium forest in Heshan hilly area of South China.The results showed that from winter to summer, The highest temperature (Tamax) and water vapor pressure deficit (Dmax) were significantly increased. The highest δ13Cplant was highest in winter and early evening, the lower δ13Cplant in spring and summer, the lower δ13Cplant in autumn, and the higher winter canopy δ13Cplant and lower ⊿canopy. From spring to winter, δ13Cplant tended to increase, while ⊿canopy tended to decrease. The average (± SD) WUE [(2.01 ± 0.38) mmol mol -1] of summer (6.90 ± 0.26) and (5.65 ± 0.14) mmol mol-1 respectively in winter and spring, and the average annual WUE was (4.36 ± 2.32) mmol mol-1. The environmental changes caused the intercellular and air CO2 concentrations The changes, which change ⊿canopy and δ13Cplant, (WUE) of canopy in different seasons.The WUE of canopy was significantly negatively correlated with the water vapor pressure deficit (D1, kPa), temperature (Ta, ℃) and leaf water potential (Ψ, MPa) And the WUE calculated by Leuning’s Ball-Woodrow-Berry model is better.The results show that the new method of stable isotope ratio of carbon is an effective method to study the WUE limit of canopy under environmental stress.