Abstract
Shade plants of ombrophilous forests are subjected to light-limiting conditions and need to invest in architectural structures associated with leaf symmetry to increase light capture. This study investigated the leaf architecture of six Araucaria forest tree species with distinct symmetry: Cupania vernalis, Casearia sylvestris, Schinus terebinthifolius, Piper gaudichaudianum, Roupala brasiliensis and Cedrela fissilis . We hypothesized that symmetry, associated with other traits, minimizes self-shading. Asymmetry index, petiole length, total leaf area, leaf angle, internode length and stem diameter were measured. The asymmetry index did not indicate a clear distinction between asymmetric and symmetric leaves. Leaves classified as asymmetric had higher values for the asymmetry index in the median and basal regions of the leaf, while symmetrical leaves had higher values in the apical region. The results also indicated an adjustment among structural leaf traits that facilitated a three-dimensional organization that produced an advantageous arrangement for light capture, which seems to be a response to selective pressure by the heterogeneous light conditions of the ombrophilous forest understory.
Highlights
Small amounts of diffuse radiation punctuated by sun flecks of short duration (Valladares & Niinemets, 2008) results in less than 2% of active photosynthetic radiation reaching the understory stratum of ombrophilous forests
The asymmetry was greater in the median and basal regions of the leaves; for P. gaudichaudianum the asymmetry was greater in the basal region of the leaf (Table 1) and for R. brasiliensis it was greater in the median region of the leaf (Table 1)
The highest asymmetry indexes were for P. gaudichaudianum and R. brasiliensis, with the highest values being at the leaf base
Summary
Small amounts of diffuse radiation punctuated by sun flecks of short duration (Valladares & Niinemets, 2008) results in less than 2% of active photosynthetic radiation reaching the understory stratum of ombrophilous forests. Other traits associated with lamina expansion have been related to the process of light capture such as leaf angle, petiole length and specific leaf mass, as well as leaf shape and phyllotaxis (Valladares & Brites, 2004; Boeger, Garcia, & Soffiatti, 2009). The organization of these traits defines leaf architecture and optimizes carbon gain by individual leaves, contributing to the total performance of the plant (Ackerly, 1999). The combination of these traits can influence the degree of self-shading and minimizing leaf overlap, which in turn can cause an increase in photosynthetic rates (Givnish, 1984), especially in ombrophilous forests
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