Intraspecific variation in leaf morphology and photosynthetic traits in Boninia grisea Planchon (Rutaceae) endemic to the Bonin Islands, Japan

Boninia grisea is a woody species comprising two varieties that have adapted to habitats with different wind conditions. Boninia grisea var. grisea is a tree occurring in forests on mountain slopes, whereas B. grisea var. crassifolia is a shrub occurring at sites exposed to strong wind at forest edges and on mountain plateaus. The former has somewhat larger and thinner leaves than the latter. Despite the differences in habitat wind conditions and leaf morphology, plants of both taxa are found in a wide range of light environments from sunny to shady. We measured leaf morphological and photosynthetic traits for B. grisea plants of both taxa growing in the field and attempted to clarify the differences between them in terms of leaf morphological and photosynthetic acclimation to sun and shade. Boninia grisea var. crassifolia had a thicker leaf upper epidermis and a thicker spongy mesophyll layer with thicker cell walls than B. grisea var. grisea, suggesting that the former had mechanically tough leaves. Under sunny conditions, the stomatal conductance and area-based photosynthetic capacities of the two B. grisea taxa were comparable. Under shady conditions, B. grisea var. crassifolia had thicker leaves with a higher photosynthetic light saturation, area-based nitrogen content and stomatal conductance than B. grisea var. grisea, suggesting that the former was less acclimated to deep shade than the latter.