Department of Biological Sciences, Florida International University, Miami, FL 33199, USA.
Ann Bot. 2013 Jun;111(6):1277-83. doi: 10.1093/aob/mct063. Epub 2013 Apr 21.
The oldest group of plants in which nectar secretions have been observed are the Polypodiopsida (ferns sensu lato). Nectaries have been reported in a dozen extant genera. The function of these nectaries has been investigated in several fern species, and in some circumstances has been demonstrated to have an antiherbivore role, attracting and maintaining biotic defence (ants and/or other predatory arthropods). This study documents foliar nectaries in Pleopeltis crassinervata, a widespread Central American epiphyte growing on a variety of trees in cloud forest areas of Veracruz, Mexico. This is a new record for this genus and species.
As previous experimental work on epiphytic species of Polypodium has demonstrated a protective role of ants for developing fronds, we conducted similar experiments (using nylon nail polish to cover nectaries rather than excluding ants with bands of sticky resin as in earlier work). The fronds of Pl. crassinervata developed over 6 weeks, at which time damage was assessed. The experiment was simultaneously conducted on a sympatric species lacking nectaries, Polypodium furfuraceum. Herbivore placement experiments were conducted with large and small caterpillars on both of these ferns.
Fronds with nectaries covered suffered greater damage from herbivores over the course of their development, compared with fronds that had uncovered nectaries functioning normally. The parallel experiment on Po. furfuraceum showed no difference between manipulated and control fronds. Six species of ants (Brachymyrmex minutus, Crematogaster formosa, Paratrechina longicornis, Solenopsis geminata, S. picea and Wasmannia auropunctata) were observed visiting nectaries of Pl. crassinervata; most were effective in removing herbivore larvae placed on the fronds.
The long evolutionary history of ferns may explain why some previous studies of fern nectaries have shown little or no benefit to ferns from nectary visitors, as any coevolved herbivores are those resistant to ant defence. The results suggest that ants protect Pl. crassinervata fronds against herbivory. The presence of nectaries, and the relationship with ants, may contribute to this fern's widespread occurrence and persistence in the face of disturbance, though many other factors also play a role. Ant defence may be more likely to benefit a widespread species of disturbed habitats that encounters a wide range of non-adapted herbivores.
最早被观察到分泌花蜜的植物群体是 Polypodiopsida（广义的蕨类植物）。现已在十几个现存属中报道了蜜腺。对几种蕨类植物的这些蜜腺的功能进行了研究，在某些情况下，已证明其具有抗草食性作用，吸引并维持生物防御（蚂蚁和/或其他捕食性节肢动物）。本研究记录了 Pleopeltis crassinervata 中的叶状蜜腺，Pleopeltis crassinervata 是一种广泛分布于中美洲的附生植物，生长在墨西哥韦拉克鲁斯云雾林地区的各种树上。这是该属和种的新记录。
正如先前对 Polypodium 附生种的实验工作所证明的那样，蚂蚁对发育中的叶片具有保护作用，因此我们进行了类似的实验（使用尼龙指甲指甲油覆盖蜜腺，而不是用粘性树脂带排除蚂蚁，如早期工作）。 Pl. crassinervata 的叶片在 6 周内发育，此时评估了损伤情况。同时在缺乏蜜腺的同域种 Polypodium furfuraceum 上进行了相同的实验。在这两种蕨类植物上，用大型和小型毛毛虫进行了草食者放置实验。
与正常运作的未遮盖的蜜腺相比，有蜜腺覆盖的叶片在其发育过程中遭受了更大的草食者伤害。对 Po. furfuraceum 的平行实验表明，处理过的和对照叶片之间没有差异。观察到六种蚂蚁（Brachymyrmex minutus、Crematogaster formosa、Paratrechina longicornis、Solenopsis geminata、S. picea 和 Wasmannia auropunctata）访问 Pl. crassinervata 的蜜腺；大多数蚂蚁有效地去除了放在叶片上的草食性幼虫。
蕨类植物的悠久进化历史可以解释为什么一些先前对蕨类植物蜜腺的研究表明，蜜腺访客对蕨类植物几乎没有或没有益处，因为任何共同进化的草食动物都是对蚂蚁防御有抵抗力的。结果表明，蚂蚁保护 Pl. crassinervata 的叶片免受草食性侵害。蜜腺的存在以及与蚂蚁的关系可能有助于这种蕨类植物在面临干扰时广泛存在和持续存在，尽管许多其他因素也发挥了作用。蚂蚁防御可能更有利于在受干扰的栖息地中广泛存在并遇到广泛的非适应草食者的物种受益。
