Fox John F
Department of Biology and Wildlife, University of Alaska Fairbanks, 99775-0180, Fairbanks, Alaska, USA.
Institute of Arctic Biology, University of Alaska Fairbanks, 99775-0180, Fairbanks, Alaska, USA.
Oecologia. 1992 May;90(2):283-287. doi: 10.1007/BF00317187.
Pollen limitation of seed set differs from resource limitation in its implications for the evolution of floral traits. Willow flowers attract insects, but also abundantly produce wind-dispersed pollen. I demonstrated pollen limitation in single branches bearing 2-4 inflorescences (catkins) in a field experiment with five species by artificially increasing or decreasing the pollen load. Because the responses by single branches might be explained by diversion of resources to better-pollinated branches within a plant, a second experiment with one species tested both pollen limitation of whole plants and the autonomy of catkins. Seed set of single willow catkins is unaffected by experimental alterations of seed set in other catkins on the same plant. Hand-pollination of single catkins and of whole plants increased seed set to the same degree, suggesting there is little or no competition for resources between catkins only 5-10 cm apart. Thus, seed set in willows appears to be pollen limited, favoring insect pollination and the evolution of entomophilous traits. The data support previous views that willows have a dual pollination system utilizing wind and insects.
种子结实的花粉限制在对花部性状进化的影响方面与资源限制有所不同。柳树花既吸引昆虫，也大量产生靠风力传播的花粉。在一项针对五个物种的田间实验中，我通过人工增加或减少花粉量，证明了带有2 - 4个花序（柔荑花序）的单个枝条存在花粉限制。由于单个枝条的反应可能是植物内部资源向授粉更好的枝条转移所致，因此针对一个物种进行的第二项实验测试了整株植物的花粉限制以及柔荑花序的自主性。同一植株上其他柔荑花序的结实情况发生实验性改变，并不会影响单个柳树柔荑花序的结实率。对单个柔荑花序和整株植物进行人工授粉，结实率提高的程度相同，这表明相距仅5 - 10厘米的柔荑花序之间几乎不存在资源竞争。因此，柳树的结实似乎受花粉限制，这有利于昆虫授粉以及虫媒性状的进化。这些数据支持了之前的观点，即柳树具有利用风和昆虫的双重授粉系统。
