Environmental regulation of carbon isotope composition and crassulacean acid metabolism in three plant communities along a water availability gradient

Expression of crassulacean acid metabolism (CAM) is characterized by extreme variability within and between taxa and its sensitivity to environmental variation. In this study, we determined seasonal fluctuations in CAM photosynthesis with measurements of nocturnal tissue acidification and carbon isotopic composition (δ13C) of bulk tissue and extracted sugars in three plant communities along a precipitation gradient (500, 700, and 1,000 mm year−1) on the Yucatan Peninsula. We also related the degree of CAM to light habitat and relative abundance of species in the three sites. For all species, the greatest tissue acid accumulation occurred during the rainy season. In the 500 mm site, tissue acidification was greater for the species growing at 30% of daily total photon flux density (PFD) than species growing at 80% PFD. Whereas in the two wetter sites, the species growing at 80% total PFD had greater tissue acidification. All species had values of bulk tissue δ13C less negative than −20‰, indicating strong CAM activity. The bulk tissue δ13C values in plants from the 500 mm site were 2‰ less negative than in plants from the wetter sites, and the only species growing in the three communities, Acanthocereus tetragonus (Cactaceae), showed a significant negative relationship between both bulk tissue and sugar δ13C values and annual rainfall, consistent with greater CO2 assimilation through the CAM pathway with decreasing water availability. Overall, variation in the use of CAM photosynthesis was related to water and light availability and CAM appeared to be more ecologically important in the tropical dry forests than in the coastal dune

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 (δ 13C) was determined for samples from dried herbarium specimens for 85 of the approximately 100 species in the family. The δ 13C values ranged from -37.7 to -19.8 ‰. Most Rapateaceae showed δ 13C values typical of C 3 plants. However, six species (Kunhardtia rhodantha Maguire, Marahuacaea schomburgkii (Maguire) Maguire, Soxofridericia compressa Maguire, Stegolepis grandis Maguire, St. guianensis Klotzsch ex Körn and St. squarrosa Maguire) showed δ 13C values less negative than -23 ‰, 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 δ 13C 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 δ 13C 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

Supermatrices, supertrees and serendipitous scaffolding: Inferring a well-resolved, genus-level phylogeny of Styphelioidea(Ericaceae) despite missing data

For the predominantly southern hemisphere plant group Styphelioideae (Ericaceae) published sequence datasets of five markers are now available for all except one of the 38 recognised genera. However, several markers are highly incomplete therefore missing data is problematic for producing a genus level phylogeny. We explore the relative utility of supertree and supermatrix approaches for addressing this challenge, and examined the effects of missing data on tree topology and resolution. Although the super-tree approach returned a more conservative hypothesis, overall, both supermatrix and supertree analyses concurred in the topologies they returned. Using multiple genes and a dataset of variably complete taxa we found improved support for the monophyly and position of the tribes and genus level relationships. However, there was mixed support for the Richeeae tribe appearing one node basal to the Cosmelieae tribe or vice versa. It is probable that this will only be resolved through further sequencing. Our study supports previous findings that the amount of data is more critical than the completeness of the dataset in estimating well-resolved trees. Our results suggest that a ‘‘serendipitous’’ scaffolding approach that includes a mixture of well and poorly sequenced taxa can lead to robust phylogenetic hypotheses

Carbon isotope composition of canopy leaves in a tropical forest in Panama throughout a seasonal cycle

The seasonal variation in δ13C values was measured in leaves from 17 upper canopy, five mid-canopy and in four gap tree species, as well as in five epiphyte and five vine species, in a seasonally dry lowland tropical forest at Parque Natural Metropolitano near Panama City, Republic of Panama. No seasonal variation was detected in the δ13C values of mature exposed leaves from either the upper or mid-canopy. However, canopy position did influence the δ13C value. The mean isotopic composition of leaves from the mid-canopy was more negative than that of the upper canopy throughout the year. The δ13C value was also influenced by leaf development, with juvenile leaves on average 1.5‰ less negative than mature leaves. The five epiphyte species exhibited 13C values that were typical of crassulacean acid metabolism (CAM). Codonanthe uleana, with isotopic values of .19.9 to .22.1‰, is only the second species in the Gesneriaceae reported to express CAM, whereas values between .14.6 and .22.0‰ indicate that Peperomia macrostachya can exhibit different degrees of CAM. The isotopic composition of exposed mature leaves from the vines showed little interspecific variation and was similar to the upper-canopy leaves of the trees