Department of Botany, University of Wisconsin, Madison, Wisconsin 53706 USA;
Am J Bot. 2004 Feb;91(2):228-46. doi: 10.3732/ajb.91.2.228.
Six endemic genera/sections of lobeliads (Campanulaceae) occupy nearly the full range of light regimes on moist sites in the Hawaiian Islands, from open alpine bogs and seacliffs to densely shaded rainforest interiors. To determine whether this clade has undergone a corresponding adaptive radiation in photosynthetic adaptations, we studied the natural light habitats and physiological characteristics of 11 species representing each sublineage. Across species in the field, average photon flux density (PFD) varies from 2.3 to 30.0 mol · m(-2) · d(-1), and maximum assimilation rate (A(max)) ranges from 0.17 to 0.35 μmol CO(2) · g(-1) · s(-1). Across species, A(max), dark respiration rate (R), Michaelis-Menten constant (k), light compensation point, specific leaf area (SLA), maximum carboxylation rate (V(cmax)), maximum rate of electron transport (J(max)), photosynthesis at saturating CO(2) (A(satCO(2))), and carboxylation efficiency (α) all increase significantly and in tightly coupled fashion with PFD, in accord with classical economic theory. Area-based rates have a higher degree of physiological integration with each other and tighter coupling to PFD than the corresponding mass-based rates, despite the energetic importance of the latter. Area-based rates frequently show adaptive cross-over: high-light species outperform low-light species at high PFD and vice versa at low PFD. A(max)-mass has little relationship to leaf mass per unit area (LMA), leaf N content, or leaf lifespan individually, but a multiple regression explains 96% of the variance in A(max)-mass across species in terms of SLA, leaf N content, and average PFD. Instantaneous leaf compensation points range from 0.1 to 1.2% full sunlight, far lower than the ecological (whole-plant) compensation points (ECPs) of 1.1 to 29.0% sunlight calculated based on photosynthetic parameters, leaf longevity, and allocation to leaf vs. nonleaf tissue. The ECPs are much closer to the lower limits of PFD actually experienced by lobeliads, suggesting they may play an important role in restricting species distributions. Taken together, these data provide evidence for an adaptive radiation in photosynthetic traits that is strongly correlated with-and indeed may help determine-the light regime that each species inhabits.
六个特有属/节的半边莲科植物（Campanulaceae）占据了夏威夷群岛潮湿生境中光照条件的全范围，从开阔的高山沼泽和海崖到茂密的热带雨林内部。为了确定这个分支是否在光合作用适应方面发生了相应的适应性辐射，我们研究了代表每个亚谱系的 11 个物种的自然光照生境和生理特征。在野外的物种中，平均光量子通量密度（PFD）从 2.3 到 30.0 mol·m-2·d-1 不等，最大同化率（A(max)）范围从 0.17 到 0.35 μmol CO2·g-1·s-1。在物种间，A(max)、暗呼吸率（R）、米氏常数（k）、光补偿点、比叶面积（SLA）、最大羧化率（V(cmax)）、最大电子传递速率（J(max)）、饱和 CO2 下的光合作用（A(satCO2)）和羧化效率（α）都随着 PFD 的增加而显著增加，并且以紧密耦合的方式增加，这与经典的经济理论一致。基于面积的速率与 PFD 的耦合程度比基于质量的速率更高，而且彼此之间的生理整合度也更高，尽管后者在能量上更为重要。基于面积的速率经常表现出适应性交叉：高光物种在高光 PFD 下的表现优于低光物种，反之亦然，而在低 PFD 下则相反。A(max)-质量与单位叶面积的叶质量（LMA）、叶氮含量或叶寿命之间的关系不大，但多元回归可以解释 96%的物种间 A(max)-质量的变化，这与 SLA、叶氮含量和平均 PFD 有关。瞬时叶片补偿点范围从 0.1%到 1.2%的全日照，远低于根据光合作用参数、叶片寿命和叶片与非叶片组织分配计算得出的生态（整株植物）补偿点（ECP），范围从 1.1%到 29.0%的日照。ECP 更接近半边莲实际经历的 PFD 的下限，这表明它们可能在限制物种分布方面发挥重要作用。总的来说，这些数据为光合作用特征的适应性辐射提供了证据，这种辐射与物种栖息的光照条件密切相关，甚至可能有助于确定这种光照条件。
