State Key Laboratory of Vegetation and Environmental Change, Institute of Botany, Chinese Academy of Sciences, No. 20 Nanxincun, Xiangshan, Beijing 100093, China.
Ann Bot. 2013 Jul;112(1):85-93. doi: 10.1093/aob/mct096. Epub 2013 May 3.
Plants have evolved a variety of seed dispersal mechanisms to overcome lack of mobility. Many species embed seeds in fleshy pulp to elicit endozoochory, i.e. disseminating seed through the animal gut. In contrast to well-studied fleshy fruited plants, dry-fruited plants may exploit this dispersal mutualism by producing fleshy appendages as a nutritional reward to entice animals to swallow their diaspores, but this has been little studied. In this study, it is hypothesized that these accessory fruits represent co-adaptations facilitating the syndrome of mammalian endozoochorous dispersal.
Field observations (focal tree watches, faecal surveys and fruiting phenology) with experimental manipulations (examination of seed germination and feeding trials) were conducted over 2 years in a native population of the raisin tree, Hovenia dulcis, which produces enlarged, twisted brown peduncles with external black seeds, in central China.
Birds were not observed to swallow seeds or carry infructescences away during 190 h of focal tree watches. However, H. dulcis seeds were detected in 247 faecal samples, representative of two herbivore and four carnivore mammalian species. Feeding trials revealed that peduncles attracted mammals to consume the entire infructescence, thereby facilitating effective seed dispersal. The germination rate of egested seeds proved higher than that of unconsumed seeds. It was also noted that this mutualism was most vulnerable in degraded forest.
Hovenia dulcis peduncle sets are confirmed to adapt primarily to mammalian endozoochory, a mutualistic association similar in function to fleshy pulp or foliage. This demonstrates that plant organ systems can be adapted to unique mutualisms that utilize animal dispersal agents. Such an ecological role has until now been attributed only to bird epizoochory. Future studies should consider more widely the putative role of peduncle sets and mammalian endozoochory as a dispersal mechanism, particularly for those plants that possess relatively large accessory fruits.
植物已经进化出多种种子传播机制，以克服自身缺乏移动能力的弱点。许多植物将种子嵌入肉质果肉中，引发动物介导的种子传播（endozoochory），即通过动物肠道传播种子。与研究较为充分的肉质果实植物不同，干果植物可能会通过产生肉质附属物来利用这种传播互惠关系，作为一种营养奖励，诱使动物吞下它们的传播体，但这方面的研究还很少。在这项研究中，我们假设这些附属果实是促进哺乳动物介导的种子传播这一共生现象的共同适应特征。
在中国中部的一个本地葡萄干树（Hovenia dulcis）种群中，我们进行了为期两年的实地观察（焦点树观察、粪便调查和果实物候学）和实验操作（种子萌发实验和饲养实验）。葡萄干树会产生增大的扭曲的棕色花梗，外部有黑色的种子。
在 190 小时的焦点树观察中，没有观察到鸟类吞下种子或将果实带走。然而，在 247 份粪便样本中检测到了 H. dulcis 种子，这些样本代表了两种食草动物和四种食肉类哺乳动物物种。饲养实验表明，花梗吸引了哺乳动物来消耗整个果实，从而促进了有效的种子传播。排出的种子的萌发率明显高于未被消耗的种子。我们还注意到，这种共生关系在退化的森林中最为脆弱。
Hovenia dulcis 的花梗集被证实主要适应于哺乳动物介导的种子传播，这是一种与肉质果肉或叶片功能相似的互利共生关系。这表明植物器官系统可以适应独特的利用动物传播媒介的互利共生关系。到目前为止，这种生态作用仅归因于鸟类的体外传播。未来的研究应该更广泛地考虑花梗集和哺乳动物介导的种子传播作为一种传播机制的可能作用，特别是对于那些具有相对较大附属果实的植物。
