Photosynthetic activity increases with leaf size and intercellular spaces in an allomorphic lianescent aroid...

This study aimed to investigate leaf anatomy, as well as photosynthetic gas exchange, that underlie the improvement in light foraging capacity, which appears to occur in aroid vines seeking light exposure. Three levels of plant height (soil level, 3 m and 6 m) were categorised for the aroid vine Rhodospatha oblongata Poepp. to represent the transition from ground to canopy. Compared with shaded leaves, leaves exposed to high light conditions were thicker, presenting a larger, spongy parenchyma characterised by a larger transversal area of intercellular spaces. In addition to the increase in maximum CO2 assimilation (Amax) and thicker and larger leaf lamina, we found an increased light saturation point, light compensation point and water use efficiency at 500 µmol PPFD. Nitrogen content per leaf dry mass remained constant across habitats, but Amax/N was 1.5-times greater in the canopy position than in the leaves at soil level, suggesting that CO2 gain did not rely on an N-related biochemical apparatus. The lower δ13C discrimination observed at high canopy leaves corroborated the higher photosynthesis. Altogether, these results suggest that the large and exposed aroid leaves maintained carbon gain coupled with light gain through investing in a more efficient proportion of intercellular spaces and photosynthetic cell surface, which likely allowed a less pronounced CO2 gradient in substomatal-intercellular space.