Cardinal Temperatures and Thermal Times for Spore Germination and Their Relationship with Geographical Distribution in Staghorn Fern Species

Staghorn ferns (Platycerium), ornamental ferns found across pantropical regions, reproduce primarily via spores in natural habitats. The geographical distribution of Platycerium species with varying climates could be linked to phylogeny. Temperature is crucial for spore germination, influencing the ecological niches of gametophytes and sporophytes. This study examined the thermal requirements for spore germination and the geographical distribution of 11 Platycerium species. Fresh spores were sown on a thermal gradient plate (10 to 37.5°C). Regression analysis determined the base (Tb), optimum (To), and maximum (Tm) temperatures for germination, as well as the thermal time (θ50) needed for 50% of spores to germinate. The relationship between θ50 and climate parameters in the species' natural habitats was also analyzed. Platycerium superbum and P. bifurcatum had Tb values of 9.5– 9.8°C, while other species ranged between 10.3–12.3°C. Platycerium madagascariense had the lowest To (27.3°C) and Tm (33.2°C), while others had To values of 28.8–30.6°C and Tm values of 34.9–36.9°C. Species from xeric environments had lower θ50 values (50.6–58.4°Cd), while those from humid regions had higher θ50 values (64.5–86.4°Cd). The cardinal temperatures of spore germination in staghorn fern species are related to their geographical distribution but are not consistent with their phylogenetic relationship. Species from regions with wet and dry seasons have lower θ50 values, promoting spore germination and gametophyte growth during rainy seasons.