Insights into mistletoe seed germination: A study of hemiparasitic Psittacanthus Mart. (Santalales: Loranthaceae) mistletoes

Abstract

Most Psittacanthus (Loranthaceae) mistletoes depend on frugivorous birds for seed dispersal, and the behaviour of seed dispersers strongly influences their spatial distribution. However, the timing of seed germination stages remains poorly studied. In this study, ripe fruits from nine Psittacanthus species, inhabiting contrasting habitats and host tree species, were collected for seed germination experiments, in which ripe fruits had their exocarp manually removed, and squashed seeds were placed and glued with their viscin on wooden rectangle sticks and daily monitored for 150 d under common environmental conditions. The germination process from seed attachment involved stages such as drying of the viscin, seed coat breaking, cotyledon expansion, and production of sticky latex-like exudates. Notably, the breakdown of the seed coat and cotyledon number varied among species. Breakdown of the seed coat did not occur in some species at least until the day the experiment ended (P. sonorae and P. auriculatus) and cotyledon number ranged from two to eight, with highest cotyledon count to date recorded for P. schiedeanus. Germination indices differed among species, with P. schiedeanus exhibiting higher values for germination rate and synchrony. A strong phylogenetic signal was found in fruit length, fruit width and cotyledon number. After accounting for phylogeny, cotyledon number was negatively associated with mean germination time and germination uncertainty, and positively associated with mean germination rate and germination speed coefficient. The other germination indices were not significantly associated with fruit or seed morphological traits or altitude. Interspecific variation in cotyledon number, seed coat characteristics, and in the timing and duration of seed germination stages highlights distinct processes, likely influenced by environmental differences. These findings highlight species-specific germination processes, the impact of cotyledon number on germination speed, and the potential evolutionary significance of specific morphological traits among Psittacanthus species.