Contrasting Leaf Shapes Vary in Extent of Solar Tracking: Revisiting Darwin\u27s Service to the Plant

Solar tracking affects leaf temperature through the amount of direct light intercepting the lamina. However, leaf shape also affects leaf temperature due to heat transfer across boundary layers, and no studies have addressed the effect of leaf shape on solar tracking and leaf temperatures in the field. We compare solar tracking, leaf temperatures, photosynthetic rates and leaf longevity in two co-occurring species with contrasting leaf morphologies: Pelargonium lobatum has a large, shallowly lobed leaf and P. triste has a highly dissected leaf composed of many small leaflets. Pelargonium triste tracked the sun more closely than P. lobatum, although P. lobatum intercepted more direct light at midday despite less solar tracking. Leaf temperatures for the two species were more similar than predicted based on leaf energy budgets. Photosynthetic rates declined more in P. lobatum than P. triste at the end of the growing season and P. triste leaves were estimated to live 20% longer. Leaf three-dimensionality has thermal effects beyond that predicted from a traditional energy budget equation. By moving, P. triste leaves heat up quickly in the morning yet experience little direct light at midday from self-shading leaflets. Pelargonium lobatum leaves warm more slowly but reach the same maximum temperature as P. triste. We hypothesize that the one-month longer life span of P. triste leaves is in part due to the avoidance of lethal leaf temperatures as summer approaches. The combination of leaf shape and solar tracking may be directly linked to differences in leaf longevity in these exposed geophytes through their modulation of direct light absorption and the downstream effects on leaf thermodynamics and photosynthetic rate. Leaf shape may predict leaf function better than many commonly measured functional traits, and the full extent of Darwin’s “service to the plant” results from the synergistic effects of leaf movement and morphology