摘要 : Photosynthetic pathway characteristics were studied in nine species of <i>Heliotropium</i> (<i>sensu lato</i>, including <i>Euploca</i>), using assessments of leaf anatomy and ultrastructure, activities of PEP carboxylase and C<su... 展开 Photosynthetic pathway characteristics were studied in nine species of <i>Heliotropium</i> (<i>sensu lato</i>, including <i>Euploca</i>), using assessments of leaf anatomy and ultrastructure, activities of PEP carboxylase and C₄ acid decarboxylases, and immunolocalization of ribulose 1.5-bisphosphate carboxylase/oxygenase (Rubisco) and the P-subunit of glycine decarboxylase (GDC). <i>Heliotropium europaeum</i>, <i>Heliotropium calcicola</i> and <i>Heliotropium tenellum</i> are C₃ plants, while <i>Heliotropium texanum</i> and <i>Heliotropium polyphyllum</i> are C₄ species. <i>Heliotropium procumbens</i> and <i>Heliotropium karwinskyi</i> are functionally C₃, but exhibit 'proto-Kranz' anatomy where bundle sheath (BS) cells are enlarged and mitochondria primarily occur along the centripetal (inner) wall of the BS cells; GDC is present throughout the leaf. <i>Heliotropium convolvulaceum</i> and <i>Heliotropium greggii</i> are C₃-C₄ intermediates, with Kranz-like enlargement of the BS cells, localization of mitochondria along the inner BS wall and a loss of GDC in the mesophyll (M) tissue. These C₃-C₄ species of <i>Heliotropium</i> probably shuttle photorespiratory glycine from the M to the BS tissue for decarboxylation. <i>Heliotropium</i> represents an important new model for studying C₄ evolution. Where existing models such as <i>Flaveria</i> emphasize diversification of C₃-C₄ intermediates, <i>Heliotropium</i> has numerous C₃ species expressing proto-Kranz traits that could represent a critical initial phase in the evolutionary origin of C₄ photosynthesis. 收起