Smith Thomas L, Eickmeier William G
Department of General Biology, Vanderbilt University, 37235, Nashville, TN, USA.
Oecologia. 1983 Feb;56(2-3):374-380. doi: 10.1007/BF00379715.
Carbon dioxide exchange pattern, carboxylating enzyme specific activity and titratable acidity were measureed for field collected individuals of Sedum pulchellum Michx., a winter annual of xeric cedar glade communities in central Tennessee. Evidence of a weak-CAM mode of photosynthesis was found on 4 of 27 sampling dates during a three year period. Three of these dates occurred during 1978, the driest study season, and nocturnal CO uptake contributed a maximum of 10% of the plants daily CO uptake. Weak CAM in the field was associated with elevated phosphoenol-pyruvate (PEP) carboxylase activity and, to a lesser degree, with decreased ribulose bisphosphate (RuBP) carboxylase activity. In contrast, carboxylating enzyme specific activities of the sympatric, obligate CAM species Opuntia compressa, a dominant perennial of the cedar glades, were highly stable.Weak CAM was also induced in S. pulchellum upon water stress in the laboratory. Again, weak CAM was associated with elevated PEP carboxylase and decreased RuBP carboxylase activities and represented a relatively minor CO uptake contribution. The potential ecological significance of this limited photosynthetic flexibility in S. puchellum is not obvious but it may play an important role in successful yearly reproduction in this annual species at a time when water stress is frequent.
对田纳西州中部干旱雪松林地群落的一年生冬季植物美丽景天（Sedum pulchellum Michx.）的野外采集个体，测定了二氧化碳交换模式、羧化酶比活性和可滴定酸度。在三年期间的27个采样日期中的4个，发现了弱景天酸代谢（CAM）光合作用模式的证据。其中三个日期出现在1978年，即最干旱的研究季节，夜间二氧化碳吸收量最多占植物每日二氧化碳吸收量的10%。野外的弱CAM与磷酸烯醇式丙酮酸（PEP）羧化酶活性升高有关，在较小程度上与核酮糖二磷酸（RuBP）羧化酶活性降低有关。相比之下，同域分布的专性CAM植物扁果仙人掌（Opuntia compressa）是雪松林地的优势多年生植物，其羧化酶比活性高度稳定。在实验室中，水分胁迫也会诱导美丽景天产生弱CAM。同样，弱CAM与PEP羧化酶活性升高和RuBP羧化酶活性降低有关，且二氧化碳吸收贡献相对较小。美丽景天这种有限的光合灵活性的潜在生态意义并不明显，但在水分胁迫频繁的时期，它可能在这种一年生植物的成功年度繁殖中发挥重要作用。
