First report of C₄/CAM-cycling photosynthetic pathway in a succulent grass, Spinifex littoreus (Brum. f.) Merr., in coastal regions of Taiwan

Leaf succulence is considered an adaptation to drought and salt stress, and it is often correlated with CAM photosynthesis. Although grasses often grow in arid and salty environments, leaf succulence (in the form of hydrenchyma) has only previously been reported in one C₃ grass. Leaf succulence is reported here for the first time in the Southeast Asian C₄ grass Spinifex littoreus, according to anatomical, water content, saturated water content, and mesophyll succulence indices. Because succulence and CAM are highly correlated, diel leaf acidity and photosynthetic parameters were measured to determine whether CAM occurs in S. littoreus in different coastal regions of Taiwan. In situ diel acidity fluctuations and nighttime stomatal closure indicate CAM-cycling in this grass. This rare form of intermediacy (C₄/CAM-cycling) has only previously been reported in Portulaca. The results of this study comprise the first report of any grass with CAM photosynthesis of any kind. Spinifex littoreus may provide an important model, in addition to Portulaca, for studying the regulation and evolutionary history of C₄ and CAM photosynthesis, and the finding of CAM biochemistry in a grass might also prove to be of special value for experimental breeding programs for improving drought and salt tolerance in cereal crops.