CAM photosynthesis in Bulnesia retama (Zygophyllaceae), a non-succulent desert shrub from South America.

Background and Aims Bulnesia retama is a drought-deciduous, xerophytic shrub from arid landscapes of South America. In a survey of carbon isotope ratios (δ¹³C) in specimens from the field, B. retama exhibited less negative values, indicative of CAM or C₄ photosynthesis. Here, we investigate whether B. retama is a C₄ or CAM plant. Methods Gas-exchange responses to intercellular CO₂, diurnal gas-exchange profiles, δ¹³C and dawn vs. afternoon titratable acidity were measured on leaves and stems of watered and droughted B. retama plants. Leaf and stem cross-sections were imaged to determine whether the tissues exhibited succulent CAM or C₄ Kranz anatomy. Key Results Field-collected stems and fruits of B. retama exhibited δ¹³C between −16 and −19 ‰. Plants grown in a glasshouse from field-collected seeds had leaf δ¹³C values near −31 ‰ and stem δ¹³C values near −28 ‰. The CO₂ response of photosynthesis showed that leaves and stems used C₃ photosynthesis during the day, while curvature in the nocturnal response of net CO₂ assimilation rate (A) in all stems, coupled with slightly positive rates of A at night, indicated modest CAM function. C₄ photosynthesis was absent. Succulence was absent in all tissues, although stems exhibited tight packing of the cortical chlorenchyma in a CAM-like manner. Tissue titratable acidity increased at night in droughted stems. Conclusions Bulnesia retama is a weak to modest C₃ + CAM plant. This is the first report of CAM in the Zygophyllaceae and the first showing that non-succulent, xerophytic shrubs use CAM. CAM alone in B. retama was too limited to explain less negative δ¹³C in field-collected plants, but combined with effects of low stomatal and mesophyll conductance it could raise δ¹³C to observed values between −16 and −19 ‰. Modest CAM activity, particularly during severe drought, could enable B. retama to persist in arid habitats of South America. [ABSTRACT FROM AUTHOR]