Carbon-isotope ratios and photosynthetic pathways in the neotropical family Rapateaceae

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 (delta (13)C) was determined for samples from dried herbarium specimens for 85 of the approximately 100 species in the family. The delta (13)C values ranged from - 37.7 to - 19.8 parts per thousand. Most Rapateaceae showed delta (13)C values typical of C(3) plants. However, six species (Kunhardtia rhodantha Maguire, Marahuacaea schomburgkii (Maguire) Maguire, Saxofridericia compressa Maguire, Stegolepis grandis Maguire, St. guianensis Klotzsch ex Korn. and St. squarrosa Maguire) showed delta (13)C values less negative than - 23 parts per thousand, i.e., at the higher end of the range for C(3) plants and at the lower end of the distribution for plants exhibiting CAM. The delta (13)C 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 delta (13)C 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.