Effects of fruit size on seed germination in Psittacanthus mayanus (Loranthaceae)

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Background and Aims: Mistletoe seeds germinate readily after being removed from the exocarp by seed dispersers. While the consequences of directed bird dispersal from the perspective of mistletoe-bird mutualistic interactions are well-studied, the effects of fruit size variation on seed germination processes have received limited attention. Here we investigated the different phases of seed germination of manually attached seeds of a Psittacanthus mistletoe, from seed attachment to early haustorial formation, to better understand the germination process of the seed unit (embryo and cotyledons). Methods: We collected fully developed ripe fruits from Psittacanthus mayanus (Loranthaceae) mistletoe plants growing on Lysiloma divaricatum (Fabaceae) host trees, measured (length and width of the fruit and length of cupular pedicels), weighed, and then used them in a germination experiment. For this experiment, we manually removed the exocarp from the ripe fruits and placed the squashed seeds on wooden rectangle sticks, securing them with their own viscin. Seeds were monitored daily for 100 days under common environmental conditions. Key results: Psittacanthus mayanus seeds showed a high germination rate, completing the process in just under two weeks on average. Germination was asynchronous among seeds, with considerable variation in the time each seed took to germinate. Among the morphological characteristics evaluated, only fruit width significantly affected the germination process. Specifically, fruit width influenced seed germination probability, the time they took to germinate, and the number of cotyledons they developed. Seeds from wider fruits showed higher germination probability but developed fewer cotyledons. Conclusions: Our findings suggest that fruit width could be an important trait influencing the reproductive success of this mistletoe species.