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Portulacaceae is one of 19 families of terrestrial plants in which species having C4 photosynthesis have been found. Representative species from major clades of the genus Portulaca were studied to characterize the forms of photosynthesis structurally and biochemically. The species P. amilis, P. grandiflora, P. molokiniensis, P. oleracea, P. pilosa, and P. umbraticola belong to the subgenus Portulaca and are C4 plants based on leaf carbon isotope values, Kranz anatomy, and expression of key C4 enzymes. Portulaca umbraticola, clade Umbraticola, is NADP-malic enzyme (NADP-ME)-type C4 species, while P. oleracea and P. molokiniensis in clade Oleracea are NAD-ME-type C4 species, all having different forms of Atriplicoid-type leaf anatomy. In clade Pilosa, P. amilis, P. grandiflora, and P. pilosa are NADP-ME-type C4 species. They have Pilosoid-type anatomy in which Kranz tissues enclose peripheral vascular bundles with water storage in the centre of the leaf. Portulaca cf. bicolor, which belongs to subgenus Portulacella, is an NADP-ME C4 species with Portulacelloid-type anatomy; it has well-developed Kranz chlorenchyma surrounding lateral veins distributed in one plane under the adaxial epidermis with water storage cells underneath. Portulaca cryptopetala (clade Oleracea), an endemic species from central South America, was identified as a C3–C4 based on its intermediate CO2 compensation point and selective localization of glycine decarboxylase of the photorespiratory pathway in mitochondria of bundle sheath cells. The C4 Portulaca species which were examined also have cotyledons with Kranz-type anatomy, while the stems of all species have C3-type photosynthetic cells. The results indicate that multiple structural and biochemical forms of C4 photosynthesis evolved in genus Portulaca.

Although the C(3) pathway is inferred as the ancestral state in Cactineae, some CAM activity has been reported in the literature in almost every family of the suborder, leaving open the possibility that CAM may have one origin in the group. Incongruence among loci could be due to internal short branches, which possibly represent rapid radiations in response to increasing aridity in the Miocene.

The C4 pathway evolved ca. 23 Myr in the Portulacaceae. The number of times that the pathway evolved in the family is uncertain. The diversity of leaf anatomical types and C4 biochemical variants suggest multiple independent origins of C4 photosynthesis. Evidence for a switch from C4 to C3-C4 intermediacy supports the hypothesis that intermediates represent a distinct successful strategy.

No part of this publication may be reproduced, stored, transmitted or disseminated, in any form, or by any means, without prior written permission from the publisher, to whom all requests to reproduce copyright material should be directed in writing.This authorization does not extend to any other kind of copying, by any means, in any form, and for any purpose other than private research use. AbstractThe seed morphology of 234 species distributed among 16 genera of the tribe Miconieae (Melastomataceae) was examined and documented with the use of scanning electron microscopy (SEM). Based on our observations and previously published investigations, we propose a set of 37 morphological characters for describing size, general shape, raphal zone, appendage, testa characters, and individual cell features of Miconieae seeds. Using these characters, we assembled detailed descriptions of all species in our sampling and constructed a data matrix for preliminary phenetic analysis (UPGMA algorithm; results to be presented elsewhere). In general, seed characters were easily identified and coded, although some of them presented difficulties, mostly due to the compression effects induced by fruit walls and neighboring seeds (e.g., shape). In contrast with other studies, we avoided the use of morphotypes, which have been shown to underutilize phenotypic variation. Our phenetic analysis does not agree with the traditional generic circumscription of the tribe, the sectional classification within Miconia, or with molecular phylogenetic analyses. This raises questions about the systematic and phylogenetic value of seed morphological characters in the tribe Miconieae. ResumenCon el uso del microscopio electrónico de barrido se examinó y documentó la morfología de las semillas de 234 especies, distribuidas en 16 géneros, de la tribu Miconieae (Melastomataceae). Con base en nuestras observaciones y apoyados en estudios previos, proponemos el uso de 37 caracteres morfológicos para describir el tamaño, la forma general, la zona rafal, el apéndice, los caracteres de la testa y las características de las células individuales de las semillas de Miconieae. Tales caracteres fueron utilizados para crear descripciones detalladas de todas las especies incluidas en nuestro muestreo y construimos una matriz de datos para efectuar un análisis fenético preliminar (algoritmo UPGMA). En términos generales, los caracteres de las semillas fueron identificados y codificados fácilmente, aunque algunos de ellos presentaron dificultades debido principalmente a los efectos de compresión inducidos por las paredes del fruto y las semillas adyacentes (p. ej., forma de la semilla). A diferencia de otros estudios, evitamos el uso de tipos morfológicos, ya que se ha demostrado que éstos subestiman la variación fenotípica. Nuestro análisis fenético no coincide con la circunscripción genérica tradicional de la tribu, ni con la clasificación seccional de Miconia, ni con los análisis filogenéticos moleculares. Estos resultados ponen en tela de juicio el valor siste...

Portulacaceae shows great diversity in C4 photosynthetic phenotypes: all species in clade Cryptopetala are C3-C4 intermediates, while clade Pilosa has a unique anatomical form of Kranz with diversity in C4 biochemistry.

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