Morphological and anatomical variations in stem-related photosynthetic structures of the Fabaceae

Leaves are the photosynthetic organs of most plants. However some species have other photosynthetic structures as well as leaves or have lost the ability to produce green leaves and have evolved alternative organs for carbon fixation. The Fabaceae is a speciose family that has species with probably the widest range of photosynthetic structures in land plants. We examined five pea species where leaves are often ephemeral or have been entirely reduced to nonphotosynthetic bracts to understand the ways in which alternative structures have been formed. The evolutionary incentive to use stems for photosynthesis is likely related to light and seasonal water availability. Cytisus scoparius sheds leaves under dry conditions and photosynthesis is then restricted to ridged stems where the ridges are derived from highly modified stipular tissues. In Genista sagittalis the annual shoots do have leaves but the stems form extensions derived from leaf bases that laterally expand to form continuous leaf-like wings down the stem increasing photosynthetic area. Anatomically Cytisus and Genista are relatively mesomorphic and avoid water stress by losing leaves (Cytisus) or having annual stems (Genista). The other three species grow in a mediterranean-type climate with pronounced summer drought. Leaf blades are reduced to brown bracts but the leaf bases extend between the stem nodes to form photosynthetic ridged stems or cladodes. Jacksonia alata is a diminutive species with limited photosynthetic area which may contribute to its subordinate position in its community. Leptosema aphyllum and L. tomentosum have much broader cladodes and are larger shrubs. These three species have stems and cladodes with a dense anatomical structure and abundant sclerenchyma that allow the species to take advantage of the winter-spring rainfall but enable their survival through the hot and arid summer. The five species emphasise that different structures can be modified to achieve a similar outcome.