Extent of Solar Tracking Differs between Two Co-occurring Congeneric Geophytes That Differ in Leaf Shape

Abstract

Premise of research. By adjusting leaf angles to orient toward or away from the sun, solar tracking modifies direct light interception, which consequently modifies leaf temperature. Leaf temperature is also influenced by leaf shape, due to its effect on heat transfer across boundary layers. Two co-occurring, closely related geophytic species of Pelargonium that differ dramatically in leaf shape were observed to exhibit differences in diurnal leaf movements. We asked whether leaf movements were consistent with solar tracking, whether they differed between the two species, and whether leaf shape and movement interacted to influence leaf temperature.Methodology. We measured solar tracking, leaf temperatures, stomatal conductance, and light interception across a diurnal time course in midwinter in the field at De Hoop Nature Reserve in South Africa.Pivotal results. Highly dissected leaves of P. triste moved to track the sun more closely than shallowly lobed leaves of P. lobatum. In contrast to predictions based on leaf energy budget calculations, average diurnal temperatures for both leaf shapes remained close to ambient, with the only significant deviation occurring early in the morning in P. triste. The best model predicting the extent of leaf temperature difference from ambient included leaf shape, leaf movement, and the proportion of the leaf surface receiving direct light, and, notably, the interaction between leaf shape and movement.Conclusions. Leaf three-dimensionality interacts with movement to dictate direct light absorption, and all are critical to fully understanding leaf thermal profiles of nonflat leaves under field conditions.