Biological significance of leaf shape
Leaves are photosynthetic organs; thus, the shapes and sizes of leaves are very important factors influencing the success of plants. To absorb sufficient light energy, leaves must be as wide as possible. At the same time, to facilitate gas exchange (CO2, O2 and H2O), leaves must be as flat and thin as possible. However, if leaves are too wide and too thin, they will quickly become desiccated. Thus, leaf area and thickness are restricted by the availability of water in main.
In addition to the above-mentioned basic environmental factors, other factors can affect the optimum shape and size of leaves. For example, plants that are adapted to riparian habitats which are frequently flooded typically develop narrow, thick leaves. Plants with such morphological adaptations are categorized as rheophytes (van Steenis 1981). Typical rheophytes show two types of adaptations to two opposite types of environmental stress. Leaves of rheophytes are narrower than closely related species and are thus able to resist the strong flow of water. On the other hand, leaves of rheophytes are thicker than those of their ancestral species in order to tolerate desiccation during exposure to high levels of sunshine in fair weather.
Although shape of leaves is genetically fixed in a particular type in most rheophytes, leaf shape in a species is not necessarily fixed as one type by evolutionary traits. Many species have acquired plasticity of leaf shape allowing responses to environmental conditions. When plants are exposed to weak light, most show a shade-avoidance syndrome, e.g., an elongation of the petiole and a decrease (sometimes increase) of the leaf blade area. The shade-avoidance syndrome is a typical example of leaf shape plasticity. In a model plant, Arabidopsis thaliana (L.) Heynh. (Arabidopsis, hereafter), leaf-blade expansion is inhibited and, at the same time, petiole elongation is enhanced when the shade- avoidance syndrome is triggered (Nagatani et al., 1991; Reed et al., 1993; Tsukaya et al., 2002; Kozuka et ak., 2005). This change in leaf shape provides for a better leaf position. As the efficiency of photosynthesis is low unless leaves can absorb sufficient light, it is reasonable that leaves exhibiting shade-avoidance syndromes remain smaller by restricting the proliferation of leaf cells (Tsukaya et al., 2002).