Peter Craig I, Johnson Steven D
School of Biological and Conservation Sciences, University of KwaZulu-Natal, Private Bag X01, Scottsville, 3209, South Africa.
Ann Bot. 2006 Mar;97(3):345-55. doi: 10.1093/aob/mcj041. Epub 2005 Dec 22.
Pollination by insects that spend long periods visiting many flowers on a plant may impose a higher risk of facilitated self-pollination. Orchids and asclepiads are particularly at risk as their pollen is packaged as pollinia and so can be deposited on self-stigmas en masse. Many orchids and asclepiads have adaptations to limit self-deposition of pollinia, including gradual reconfiguration of pollinaria following removal. Here an unusual mechanism--anther cap retention--that appears to prevent self-pollination in the South African orchid Eulophia foliosa is examined.
Visits to inflorescences in the field were observed and pollinators collected. Visitation rates to transplanted inflorescences were compared between a site where putative pollinators were abundant and a site where they were rare. Anther cap retention times were determined for removed pollinaria and atmospheric vapour pressure deficit was recorded concurrently. Anther cap anatomy was examined using light microscopy.
Eulophia foliosa is pollinated almost exclusively by Cardiophorus obliquemaculatus (Elateridae) beetles, which remain on the deceptive inflorescences for on average 301 s (n = 18). The anther cap that covers the pollinarium is retained for an average of 512 s (n = 24) after pollinarium removal by beetles. In all populations measured, anther cap dimensions are greater than those of the stigmatic cavity, thus precluding the deposition of self-pollinia until after the anther cap has dropped. An anatomical investigation of this mechanism suggests that differential water loss from regions of the anther cap results in opening of the anther cap flaps. This is supported by observations that as atmospheric vapour pressure deficits increased, the duration of anther cap retention was reduced.
Flowers of E. foliosa are specialized for pollination by elaterid beetles. Retention of anther caps for a period exceeding average visit times by beetles to inflorescences appears to prevent facilitated self-pollination in E. foliosa effectively.
长时间访问一株植物上多朵花的昆虫授粉可能会带来更高的促进自花授粉风险。兰花和萝藦科植物尤其面临这种风险，因为它们的花粉被包装成花粉块，因此可能大量沉积在自花柱头上。许多兰花和萝藦科植物具有限制花粉块自沉积的适应性，包括在花粉块被移除后逐渐重新配置花粉块柄。本文研究了一种不同寻常的机制——花药帽保留，这种机制似乎能防止南非兰花大叶美冠兰自花授粉。
在野外观察对花序的访问情况并收集传粉者。比较了在假定传粉者丰富的地点和传粉者稀少的地点对移植花序的访问率。测定了移除花粉块后花药帽的保留时间，并同时记录大气蒸汽压亏缺。使用光学显微镜检查花药帽的解剖结构。
大叶美冠兰几乎完全由斜斑心蚤蝇（叩甲科）甲虫授粉（叩甲科），这些甲虫在具有欺骗性的花序上停留平均301秒（n = 18）。在甲虫移除花粉块后，覆盖花粉块柄的花药帽平均保留512秒（n = 24）。在所有测量的种群中，花药帽的尺寸大于柱头腔的尺寸，因此在花药帽掉落之前排除了自花粉块的沉积。对这种机制的解剖学研究表明，花药帽不同区域的水分流失差异导致花药帽瓣打开。大气蒸汽压亏缺增加时花药帽保留时间缩短的观察结果支持了这一点。
大叶美冠兰的花朵专门由叩甲科甲虫授粉。花药帽保留的时间超过甲虫对花序的平均访问时间，这似乎有效地防止了大叶美冠兰的促进自花授粉。
