Investigating the function of leaf inclination in the sand dune herb Hydrocotyle bonariensis
The coastal sand dune habitat is a stressful habitat for plants due to the combination of unique abiotic factors (e.g., salt spray and sand movement) and high air temperatures, substrate temperatures, and incident sunlight, especially at midday, during the growing season. Hydrocotyle bonariensis, a clonal large-leaf herb common to the sand dune community of barrier islands, has been shown to increase leaf angle from horizontal over the growing season to a more vertical orientation in response to high incident sunlight. The aim of this study was to further investigate the function of leaf inclination in H. bonariensis by comparing leaf incident sunlight on adaxial and abaxial surfaces, leaf temperature, and leaf fluorescence (as a proxy for photosystem II efficiency) between control inclined leaves and experimental leaves held in horizontal orientation during a growing season in the sand dune community. Leaf temperatures for similar-sized leaf pairs were measured hourly for one week each month from June to August 2014 using thermocouples, and diurnal measurements of incident sunlight on both leaf surfaces and photosystem efficiency were made at the end of the week each month.
The ratio of incident sunlight on the adaxial to abaxial leaf surface was significantly greater in experimental horizontal leaves compared to control inclined leaves in all months measured. Incident sunlight on the adaxial leaf surface was 5 to 7 times greater than the abaxial surface in experimental horizontal leaves at midday, while incident sunlight on the adaxial surface was 2 to 3 times greater than the abaxial surface in control inclined leaves. Furthermore, diurnal leaf temperature was significantly greater in experimental horizontal leaves compared to control inclined leaves in all months measured. At midday, leaf temperatures were 5˚ to 10˚ higher than air temperatures in experimental horizontal leaves compared to 2˚ to 6˚ higher in control vertical leaves. Diurnal photosystem efficiency was lower in experimental horizontal leaves each month, with significantly lower photosystem efficiency in August. These results suggest that leaf inclination in H. bonariensis functions to reduce sunlight exposure, especially during midday, resulting in reduced leaf temperature and increased photosystem II efficiency. This combined reduction in leaf temperature and sunlight absorption likely facilitates diurnal photosynthetic carbon gain for H. bonariensis in the sand dune community.