Carbon-Isotope Ratios and Photosynthetic Pathways in the Neotropical Family Rapateaceae

Abstract: The Rapateaceae is a small, mainly Neotropical family of terrestrial or occasionally epiphytic herbs that grow on mesic, nutrient-poor sites. Some recent studies suggest that the Rapateaceae may be closely related to the Bromeliaceae, one of the major families containing CAM plants. To investigate the photosynthetic pathway in Rapateaceae, the plant carbon-isotope ratio (δ¹³C) was determined for samples from dried herbarium specimens for 85 of the approximately 100 species in the family. The δ¹³C values ranged from - 37.7 to - 19.8 ‰. Most Rapateaceae showed δ¹³C values typical of C₃ plants. However, six species ( Kunhardtia rhodantha Maguire, Marahuacaea schomburgkii (Maguire) Maguire, Saxofridericia compressa Maguire, Stegolepis grandis Maguire, St. guianensis Klotzsch ex Körn. and St. squarrosa Maguire) showed δ¹³C values less negative than - 23 ‰, i.e., at the higher end of the range for C₃ plants and at the lower end of the distribution for plants exhibiting CAM. The δ¹³C values became significantly less negative with increasing altitude (regression analysis indicating a change from about - 30.7 ‰ at sea level to - 22.5 ‰ at 2500 m). Although other environmental factors and the type of tissue analysed may also influence δ¹³C values, these results suggest that some Rapateaceae may be capable of performing CAM. Further studies, including measurements of diel gas exchange patterns and leaf organic-acid fluctuations, would be needed to demonstrate CAM in Rapateaceae unequivocally, but living material of many of these enigmatic plants is difficult to obtain.