Leaf primordia initiation takes place at the flanks of SAM and then passes through common developmental stages. Very different final leaf shapes and sizes result from varying the timing and further patterning events within these developmental stages. Similar final leaf shapes may also result from very distinct early events. Begonia section Gireoudia is a recently radiated group of species with highly divergent leaf forms. I have used a classical genetic approach and candidate gene approach to explain the evolution of leaf form in this genus. These results suggest that convergent evolution of peltate leaves may be through changes at different loci. Key developmental regulators KNOX and ARP genes are reported to be involved in the evolution of leaf form in different species. I have shown that in at least one species ARP is linked to the evolution of peltate leaf form. In a second species there is no link between STM-like KNOX genes and leaf dissection. Estimates of the rate of evolution of ARP CDS showed that different domains of the genes are under different selection pressures. Myb domain2 of ARP genes is under positive selection and variable between two copies of ARP genes in Begonia. Results of complementation tests with Begonia ARP genes in Arabidopsis show that ARPs from Begonia are functionally equivalent to Arabidopsis AS1 genes and one of the two ARP genes in Begonia may be a dominant negative. Expression analysis based on insitu hybridization in compound, peltate and simple leaved Begonias is described. There is no variation in expression patterns between peltate, non peltate or compound leaved Begonia species for BARP1 and KNB1 genes.

Plants are exposed to various external stresses influencing physiology, anatomy, and morphology. Shape, geometry, and size of shoots and leaves are particularly affected. Among the latter, peltate leaves are not very common and so far, only few studies focused on their properties. In this case study, four Begonia species with different leaf shapes and petiole attachment points were analyzed regarding their leaf morphology, anatomy, and biomechanical properties. One to two plants per species were examined. In all four species, the petiole showed differently sized vascular bundles arranged in a peripheral ring and subepidermal collenchyma. These anatomical characteristics, low leaf dry mass, and low amount of lignified tissue in the petiole point toward turgor pressure as crucial for leaf stability. The petiole-lamina transition zone shows a different organization in leaves with a more central (peltate) and lateral petiole insertion. While in non-peltate leaves simple fiber branching is present, peltate leaves show a more complex reticulate fiber arrangement. Tensile and bending tests revealed similar structural Young’s moduli in all species for intercostal areas and venation, but differences in the petiole. The analysis of the leaves highlights the properties of petiole and the petiole-lamina transition zone that are needed to resist external stresses.