以黑毛组的黑毛石斛(Dendrobium williamsonii)和长距石斛(D. longicornu)为材料, 对其光合生理特性进行了系统研究。2种石斛叶片的解剖结构为异面叶, 气孔仅分布在下表面, 具气孔盖, 叶脉维管束鞘不含叶绿体, 无花环型结构。2种石斛的光强、CO2浓度和温度的响应研究表明, 它们的光补偿点(LCP)和光饱和点(LSP)分别为5–10 μmol·m−2·s−1和800–900μmol·m−2·s−1,表观量子效率为0.02; CO2补偿点和CO2饱和点分别为80–90 μmol·mol−1和800 μmol·mol−1, 羧化效率在0.015–0.021之间; 光合作用的最适温度为28–30°C。光合特性的研究表明, 2种石斛的净光合速率(Pn)日变化均呈双峰型曲线, 首峰出现在12:00, 最大光合速率为5 μmolCO2·m−2·s−1, 次峰出现在15:00, 夜间不吸收CO2。2种石斛叶片的叶绿素a/叶绿素b比值在2.51–2.66之间。酶活性的测定结果表明, 2种石斛的PEPCase活性极低, 但RuBPCase和GO酶活性较高。以上的研究结果均表明, 黑毛石斛和长距石斛光合作用碳同化途径属C3植物类型, 具有半阴生植物的特点。
We examined the photosynthetic physiology of Dendrobium williamsonii and D. longicornu. Cross-sections of the leaves of the 2 Dendrobium species showed that they were bifacial. The abaxial epidermis showed a large amount of stomata, covered by the waxy stomata; vascular bundles showed no chloroplast or Kranz structures. Photosynthesis of the 2 species responded to light intensity, CO2 concentration and temperature. Light compensation points were 5–10 μmol·m−2·s−1, light saturation points were 800–900 μmol·m−2·s−1; apparent quantum yields were ~0.02; CO2 compensation points and saturation points were 80–90 μmol·mol−1 and 800 μmol·mol−1, respectively; and carboxylation efficiencies were 0.015–0.021. The optimal temperature for photosynthesis was 26–30°C. The diurnal variation in net photosynthetic rates (Pn) were all bimodal curves, with the first peaks at ~5 μmol CO2·m−2·s−1 and emerging at between 11:00–12:00, and the second peaks emerging at about 15:00; no CO2 absorption occurred at night. Additionally, the chlorophyll a/b (Chl a/b) ratios of leaves were 2.51–2.66. Measurement of key enzymes in the photosynthetic pathway indicated relatively high RuBPCase and GO activities but low PEPCase activities. D. williamsonii and D. longicornu are typical semi-shade C3 plants.
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