Pulvinus or Not Pulvinus, That is the Question: Anatomical Features of the Petiole in the Citrus Family (rutaceae, Sapindales)

Several Rutaceae species have petioles with a swollen apical and/or a basal region and a thinner middle portion. These swollen petiolar regions are morphologically similar to pulvini but are recognized by an array of other nomenclatures. We hypothesized that the swollen regions in Rutaceae petioles present anatomical features in keeping with pulvini. We accessed the anatomical features of the petioles in 11 species of Rutaceae belonging to four subfamilies, comparing the swollen and middle petiolar regions. Samples were cross-sectioned using razor blade and stained with Astra Blue and Safranin. Other samples were embedded in methacrylate resin; cross and longitudinal sections were obtained using a rotating microtome and stained with toluidine Blue. Semi-permanent and permanent slides were analyzed using a light microscope. Cortex is significantly wider than in the middle petiolar portion in the swollen petiolar regions. Collenchyma and parenchyma are present in the cortex in all the petiolar regions; sclerenchyma cells are more commonly observed in the middle petiolar region. Secondary growth occurs in all the petiolar regions, but it is more well established in the middle portion of petioles. Septate fibers with non-lignified walls and gelatinous fibers occur externally to the phloem in the swollen, but not in the middle petiolar regions—such petiolar region exhibits only perivascular fibers with completely lignified walls. Articulations were observed only in Citrus × limonia. Several anatomical features observed in the swollen petiolar regions in Rutaceae are similar to those described to pulvini in other Angiosperms and can be associated with lower rigidity of this region. However, considering the lack of records on the occurrence of leaf movement in this family so far, we suggest the use of “pulvinus-like thickenings” in Rutaceae petioles, and the terms “distal pulvinus-like thickening” and “proximal pulvinus-like thickenings” for the thickened regions near the apex and base of the leaf, respectively. Ultrastructural and leaf movement studies in Rutaceae are needed to clarify the nature and of the thickened petiole regions in the family.