A phylogenetic view of low-level CAM in Pelargonium (Geraniaceae)

Crassulacean acid metabolism (CAM) is common in several plant families and is often associated with succulence. Few studies have examined the occurrence of CAM from a phylogenetic perspective. The genus Pelargonium is promising for such a study because members are characterized by dramatic variation in growth form (including geophytes, shrubs, and stem succulents) and because growth form diversity is expressed to the greatest extent in a monophyletic group comprising 80% of Pelargonium species. This clade, predominantly from the winter rainfall region of southern Africa, likely proliferated in response to Miocene or Pliocene aridification. We present a survey for CAM across Pelargonium, emphasizing the winter rainfall clade. Dawn/dusk fluctuations in titratable acidity were examined in 41 species, with detailed measurements of carbon uptake and stomatal conductance under progressive water stress in four species. No species exhibited obligate CAM. When well-watered, most species exhibited stomatal conductances and acid fluctuations characteristic of C, photosynthesis, though some exhibited more pronounced increases in nocturnal acidity, suggesting CAM cycling. In four species examined during dry-down, water stress led to increased nighttime acid levels and decreased daytime stomatal conductance. Ultimately, stomata closed and external carbon uptake ceased, consistent with CAM idling. These results are discussed from the perspective of the evolution of CAM flexibility.