Leaf heliotropism in Styrax camporum Pohl from the Brazilian cerrado: distinct gas exchange and leaf structure, but similar leaf temperature and water relations

Abstract

Styrax camporum is a common shrub species from the cerrado regions of São Paulo State, Brazil. Although its leaves do not have a morphologically delimited pulvinus, the apical leaves are oriented parallel to the sun (paraheliotropic leaves), most notably around noon. Horizontal leaves at the base of shoots are oriented perpendicular to the sun's rays (diaheliotropic leaves). The ecophysiological significance of this is not fully understood. We investigated how paraheliotropism influences daily CO2 assimilation (A) and other gas exchange rates, water relations, leaf temperature (Tl), and how these relate to leaf structure as assessed by anatomical leaf descriptions. Paraheliotropic leaves had greater A and stomatal conductance (g s), which led to greater transpiration rates (E) during most of the day. However, no categorical data proved that these responses were responsible for the lower Tl for vertical leaves, which only occurred at 1100 h and 1600 h. There was no indication that Tl influenced A, E, or g s. However, diaheliotropic leaves did not assimilate less carbon because of a smaller stomatal aperture. Both leaf types had satisfactory stomatal apertures and the same g s range, resulting in similar intercellular CO2 levels. Paraheliotropic leaves had higher A rates under increased irradiance, suggesting an avoidance of excessive irradiance. Both leaf types had compact spongy parenchyma, representing an increased photosynthetic capacity per unit leaf area. Paraheliotropism in S. camporum does not seem to be related to leaf temperature control, even in the stressful Brazilian cerrado habitat.