Huber Heidrun, Stuefer Josef F
Department of Plant Ecology and Evolutionary Biology, Utrecht University, P.O. Box 800.84, NL-3508 TB Utrecht, The Netherlands fax:+31-30-2518366; e-mail:
Oecologia. 1997 May;110(4):478-486. doi: 10.1007/s004420050183.
Shade-induced changes in the branching pattern of clonal plants can lead to conspicuous modifications of their growth form and architecture. It has been hypothesized that reduced branching in shade may be an adaptive trait, enabling clonal plants to escape from unfavourable patches in a heterogeneous environment by allocating resources preferentially to the growth of the main axis (i.e. linear expansion), rather than to local proliferation by branching. However, such an adaptionist interpretation may be unjustified if (1) branching frequency is a function of the ontogenetic stage of plants, and if (2) shading slows down the ontogenetic development of plants, thereby delaying branch formation. In this case, architectural differences between sun- and shade-grown individuals, harvested at the same chronological age, may not represent a functional response to changes in light conditions, but may be a by-product of effects of shade on the rate of plant development. To distinguish between these two alternatives, individuals of the stoloniferous herb Potentilla reptans were subjected to three experimental light conditions: a control treatment providing full daylight, and two shade treatments: neutral shade (13% of ambient PPFD; no changes in light spectral composition) and simulated canopy shade (13% PPFD and a reduced red:far-red ratio). Plant development was followed throughout the experiment by daily monitoring primary stolon growth as well as branch and leaf initiation. Biomass and clonal offspring production were measured when plants were harvested. At the end of the experiment shaded plants had produced significantly fewer branches than clones grown in full daylight. In all three treatments, however, initiation of secondary stolons occurred at the same developmental stage of individual ramets. Shading significantly slowed down the ontogenetic development of plants and this resulted in the observed differences in branching patterns between sun- and shade-grown individuals, when compared at the same chronological age. These results hence provide evidence that shade-induced changes in the branching pattern of clonal plants can be due to purely allometric effects. Implications for interpreting architectural changes in terms of functional shade-avoidance responses are discussed.
遮荫引起的克隆植物分枝模式变化会导致其生长形态和结构发生显著改变。有假说认为，在遮荫条件下分枝减少可能是一种适应性特征，使克隆植物能够通过将资源优先分配给主轴生长（即线性扩展），而非通过分枝进行局部增殖，从而逃离异质环境中的不利斑块。然而，如果（1）分枝频率是植物个体发育阶段的函数，并且（2）遮荫减缓了植物的个体发育进程，进而延迟分枝形成，那么这种适应性解释可能不合理。在这种情况下，在相同时间年龄收获的阳生和阴生个体之间的结构差异，可能并不代表对光照条件变化的功能性响应，而可能是遮荫对植物发育速率影响的副产品。为了区分这两种情况，对匍匐草本委陵菜的个体施加三种实验光照条件：一种是提供全日照的对照处理，以及两种遮荫处理：中性遮荫（环境光合光子通量密度的13%；光光谱组成无变化）和模拟树冠遮荫（13%光合光子通量密度且降低了红：远红比率）。在整个实验过程中，通过每日监测初生匍匐茎生长以及分枝和叶片起始情况来跟踪植物发育。收获植物时测量生物量和克隆后代产量。在实验结束时，遮荫植物产生的分枝明显少于在全日照下生长的克隆体。然而，在所有三种处理中，次生匍匐茎的起始都发生在各个分株的相同发育阶段。遮荫显著减缓了植物的个体发育进程，这导致在相同时间年龄比较时，阳生和阴生个体之间观察到分枝模式的差异。因此，这些结果提供了证据，表明遮荫引起的克隆植物分枝模式变化可能纯粹是由于异速生长效应。文中还讨论了从功能性避荫响应角度解释结构变化的意义。
