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

This article investigates the photosynthetic pathway of *Bulnesia retama*, a drought-deciduous, xerophytic shrub native to arid regions of South America, to determine whether it employs C4 or crassulacean acid metabolism (CAM) photosynthesis. Through analyses of carbon isotope ratios (δ¹³C), gas exchange, tissue anatomy, and titratable acidity under varying water conditions, the study finds that *B. retama* primarily uses C3 photosynthesis during the day but exhibits weak to modest CAM activity at night, especially under drought stress, with no evidence of C4 photosynthesis. The plant lacks succulent anatomy typical of strong CAM species, and the observed δ¹³C values in field specimens (−16 to −19 ‰) are explained by a combination of modest CAM function and low stomatal and mesophyll conductance rather than CAM alone. This represents the first documented occurrence of CAM in the Zygophyllaceae family and highlights CAM as an additional drought-adaptive strategy in non-succulent, xerophytic shrubs occupying arid South American habitats.