Does the same morphology mean the same physiology? Morphophysiological adjustments of Philodendron hederaceum (Jacq.) Schott, an isomorphic aroid, to ground-canopy transition

Abstract

Lianescent aroids grow horizontally while on the ground (shaded, humid habitats), but switch to vertical growth when they reach a host (exposed, desiccant habitats). There are three morphotypes with respect to leaf adjustment to canopy conditions: isomorphic, allomorphic, and heteromorphic. Allomorphs and heteromorphs exhibit marked leaf modifications (e.g., leaf area) that have a strategic effect on the plant’s carbon balance, since they increase its light foraging capacity in the canopy. Isomorphs do not show external leaf morphology modifications. Therefore, we hypothesized that leaf anatomy and physiology change instead, allowing their light foraging capacity to improve while they grow towards the canopy. We evaluated leaf area, lamina-specific mass, mesophyll anatomy, venation density and thickness, stomatal density, chlorophyll concentration and fluorescence in leaves of the isomorph Philodendron hederaceum, grown to three heights (terrestrial, 1.5 and 3 m). Stomatal density, chlorophyll a content, and electron transport rate were significantly higher in the canopy leaves (60, 54, and 50%, respectively). Thus, P. hederaceum exhibits leaf adjustments that enhance its light foraging capacity. However, these changes are relatively less marked than those exhibited by sympatric heteromorphs and allomorphs. Although the canopy leaves of P. hederaceum exhibited a limited improvement in light foraging capacity, the species’ construction and maintenance costs are expected to be lower than those of its sympatric heteromorphs and allomorphs. Future studies that consider leaf lifespan will add another perspective for understanding the costs and benefits of these growth habits in aroid vines.