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 versus 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 if 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 leaves and stems used C3 photosynthesis during the day, while curvature in the nocturnal response of A in all stems, coupled and 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 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 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.

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