作 者 ：王文杰,祖元刚,王慧梅
期 刊 ：生态学报 2007年 27卷 4期 页码：1583~1595
关键词：树枝(干)；叶绿素；类胡萝卜素；光合途径；树皮光学特性；
Keywords:stem (branches), chlorophyll, carotenoids, photosynthetic pathway, bark optical feature,
摘 要 ：尽管多数林木非同化器官树干、树枝内都存在绿色组织（Chlorenchyma），但是其生态学意义很少为人注意。综合前人研究结果，得出以下结论：（1）树枝的光合速率（暗呼吸与饱和光照下呼吸之差）的在0~10μmol m-2s-1之间，而大部分的结果认为光合速率在0.5~3.0μmol m-2s-1之间。而且多数研究认为其是对自身呼吸的再固定，而不是对外界大气CO2的吸收，对再固定率（光合/呼吸比值）分析表明80%以上的研究结果认为树干光合作用能够固定40%~100%呼吸所释放的CO2。但其对个体乃至林分整体碳平衡的影响报道的较少。（2）不仅非同化器官光合作用直接影响其呼吸作用，冠层叶片光合产物对非同化器官的气体代谢也产生显著影响，但新形成光合产物和储藏碳水化合物是否存在功能上的差异不同学者意见不一。（3）尽管叶绿素含量多在80~450 mg m-2之间，低于相应叶片叶绿素含量340~620 mg m-2，但其叶绿素a/b平均值（2.5）显著低于叶片的平均值（3.6）（p<0.0001），说明更加适应于阴生环境。（4）有关类胡萝卜素在非同化器官的功能，比较传统的观点认为其单位质量或者单位面积含量远低于叶片，是一种适应于低光照环境的表现，但最近研究表明其单位叶绿素含量远高于叶片，可能是在高浓度CO2条件下、叶绿体片层结构酸性化导致光合过程中光化学淬灭能力低下，需要以叶黄素循环为主导能量淬灭过程保证其功能正常。（5）在树皮光学特性方面，大部分光照被吸收，而有少部分被反射和透射。在透光率方面，70%左右的结果认为树皮的透光率在5%~15%之间，而超过85%的结果认为透光率在0％~20%之间。而且光照可以在木质化树干导管、纤维及管胞有效进行轴向导光，树干内部的光以红外和远红外光为主，其它低波长光的透射能力远低于红外光。（6）尽管已有研究表明某些草本植物茎具有C4特征，但对于多数、特别是木本植物未见报道，特别是对于茎内高浓度CO2、高红/蓝光比、低氧特征对光合机构的影响尚需要深入研究。
Abstract:Many, if not most, trees contain greenish, photosynthetically active chlorenchyma tissue below the outer periderm or rhytidome of branches and even stems. However, not so many people have realized their ecological significance and photosynthetic function. Here, by reviewing previous studies and our own research work, some general conclusions were got as following:
（1）Branch（stem）photosynthesis (difference between dark respiration rate and respiration at saturation light regime) was 0~10μmol m-2s-1, while most of the data were ranged from 0.5 μmol m-2s-1 to 3.0 μmol m-2s-1. Although net photosynthetic uptake of CO2 is rarely found, stem internal re-fixation of CO2 in young branches and stems may compensate for the potential respiratory carbon loss. Over 80% of the referenced data shows that this photosynthetic uptake could refix 40%~100% of the respiratory CO2 loss. However, a few studies have scaled up from instantaneous studies to tree or forest level.
（2) Not only the photosynthetic carbon-fixation itself, but also photosynthates from canopy leaves strongly affect the respiratory CO2 loss from non-photosynthetic organs. However, there are still discrepant opinions on the functional difference of newly-formed photosynthates and stored photosynthates.
（3) Chlorophyll content in barks is generally 80~450 mg m-2，which is lower than those in corresponding leaves (340-620mg m-2). Moreover, the ratio between chlorophyll a and chlorophyll b, chl. a/b for barks is 2.5 in average, which is significantly lower than that of leaves, 3.6 in average (p<0.0001), indicating shade-environmental acclimation of barks.
（4) Traditional view on the function of carotenoids for non-photosynthetic organs is its low light acclimation since the total content at bases of area and dry mass in barks are much lower than that in leaves. However, recent studies have shown that total carotenoids and its componential compounds at a base of chlorophyll fresh mass are much higher than that corresponding leaves. This has proved to be related to the acidified chloroplast stroma by extremely high CO2 concentration in barks. It is suggested that high internal CO2 concentrations in barks impede photosynthesis possibly through acidification of protoplasm and impairment of the pH-dependent high energy state quenching followed by reduction in the efficiency of heat dissipation.
（5) Optics for the barks of stems and branches are characterized as that most of light being absorbed and only part of light being reflected and transmitted. About 70% of the data show that transmission is ranged from 5% to 15%, while over 85% show that the transmission is ranged from 0% to 20%. Moreover, vessels, fibres (both xylem and phloem fibres) and tracheids in woody plants can also conduct light efficiently along the axial direction, via their lumina (vessels) or cell walls (fibres and tracheids). Only the spectral region of far-red and near infra-red light is efficiently conducted and transmitted in branches and stems.
（6) Although some study on herbal plants showing a C4 feature, many other species, particularly woody branches and stems is unknown. The influences of special microenvironment (extremely high CO2 concentration, increased red to blue photon ratio, and hypoxia) on the photosynthetic machinery need further studies.