First Finding of Succulence and C4/CAM-Cycling Photosynthesis in a Grass: Ecophysiology of Spinifex littoreus in Coastal Regions of Taiwan

Abstract

Spinifex littoreus (Brum. f.) Merr. is a halophytic grass that is distributed on coastal sand dunes in Taiwan (and throughout Southeast Asia). This study is the first report of leaf succulence in a grass, with 64 % of the leaf cross-sectional area occupied by water-storage parenchyma (hydrenchyma). Leaf water content, saturated water content and mesophyll succulence indices were also similar to other reported succulent plants. In addition to the previous report of C4 photosynthesis, the current study found diel acid fluctuation and nocturnal stomatal closure, which indicates CAM photosynthesis (in a CAM-cycling form), thus indicating that S. littoreus is a grass with C4/CAM-cycling photosynthesis. This finding is the first report of any grass with CAM photosynthesis of any kind. The presence of CAM acid fluctuation was found to be associated with exposed areas and at the edges of plant colonies. Also, the presence of CAM acid fluctuation in some populations was not consistent across years, and was not correlated with temperature or precipitation differences. Photosynthetic rates among populations varied, which might either be the result of different environmental conditions, or genotypic variability among populations throughout Taiwan. This study comprises the first report of a succulent grass, and the first report of C4/CAM-cycling photosynthetic pathway in monocots. This plant may have an important prospect as a model organism for studying the regulation and evolutionary history of C4 and CAM photosynthesis, as well as the potential value for experimental breeding programs, improving drought and salt tolerance in cereal crops.