In some deciduous trees, the leaves on the exterior of the tree canopy differ from those inside the tree canopy. The exterior leaves are referred to as “sun leaves,” while the interior leaves are “shade leaves.” These leaves have differences in shape, internal anatomy, and chemistry that translate into specialized abilities to use different kinds of solar radiation effectively.
Sun leaves are typically smaller, more elongate, and thicker than shade leaves, with more layers of chlorophyll-containing tissues and more extensive internal vascular systems. It is thought that sun leaves are better adapted to capture and use direct solar radiation (when it isn’t too intense to cause heat and other stress-related damage). Their elongate shape is also correlated with higher levels of solar radiation reaching the inner canopy where the shade leaves are located. Shade leaves appear to effectively use diffuse solar radiation, which reaches the inner canopy after having been scattered by other objects, like the outer sun leaves, in the path of direct light. Shade leaves can also be found on the exterior canopy on the side that faces away from the prevailing sun.
It appears that exterior sun leaf characteristics can change with environmental conditions (they show plasticity), and that their shape in particular can influence the internal canopy environment that shade leaves experience. Plasticity in sun leaves seems to help stabilize inner canopy conditions, buffering it from abiotic stresses. Genetic variation and tree size also affect leaf characteristics, but it seems that whole-tree photosynthesis can be optimized by having sun and shade leaves respond differently to the environment. Sun and shade leaves occur in other species, too. In oaks, external leaves have narrower lobes while the lobes of shade leaves are broader.
“1. Canopy plasticity, the expression of different leaf phenotypes within the crown of an individual tree has complex functional and evolutionary implications that remain to be thoroughly assessed. We hypothesized that it can lead to disparity in how leaves in different positions of the canopy change with allometric growth and population genetic structure.
2. Leaf phenotypes of the inner and outer canopy were estimated using eight morphological and physiological characters…With these data, we investigated the extent to which leaf phenotypes change with plant size, genetic processes and in = response to environmental conditions inside and outside the canopy.
3. The size of trees measured in the field was clearly associated with the phenotype of sun [leaves] but not to that of shade leaves. The phenotype of sun leaves depended on both direct and diffuse light, while that of shade leaves was found to correlate only with diffuse radiation. Additionally, light availability inside the canopy was conditioned by the shape of external leaves, and increasing elongation of sun leaves led to higher radiation in the inner canopy.
4. The field phenotypes of both inner and outer canopy leaves were correlated with genetic variation among populations. Conversely, in the common garden, the different genotypes expressed a homogeneous sun phenotype, while phenotypic differences among populations remained apparent in shade leaves.
5. We conclude that, in agreement with our working hypothesis, canopy plasticity is both cause and consequence of the environment experienced by the plant and might lead to the differential expression of genetic polymorphisms among leaves. Furthermore, we propose that it can contribute to buffer abiotic stress and to the partition of light use within the tree crown.” (de Casas et al. 2011:802)
“The diverging phenotypes of modules in different positions of the canopy might thus maximize overall performance, with sun leaves most active when conditions for light capture are optimal (i.e. during spring, and early morning and late afternoon in summer without drought (Diaz-Espejo, Nicolas & Fernandez 2007) and shade leaves ensuring a stable photosynthetic performance throughout the year.” (de Casas et al. 2011:810)