Water relations traits of C(4) grasses depend on phylogenetic lineage, photosynthetic pathway, and habitat water availability

The repeated evolution of C(4) photosynthesis in independent lineages has resulted in distinct biogeographical distributions in different phylogenetic lineages and the variants of C(4) photosynthesis. However, most previous studies have only considered C(3)/C(4) differences without considering phylogeny, C(4) subtype, or habitat characteristics. We hypothesized that independent lineages of C(4) grasses have structural and physiological traits that adapt them to environments with differing water availability. We measured 40 traits of 33 species from two major C(4) grass lineages in a common glasshouse environment. Chloridoideae species were shorter, with narrower and longer leaves, smaller but denser stomata, and faster curling leaves than Panicoideae species, but overall differences in leaf hydraulic and gas exchange traits between the two lineages were weak. Chloridoideae species had two different ways to reach higher drought resistance potential than Panicoideae; NAD-ME species used water saving, whereas PCK species used osmotic adjustment. These patterns could be explained by the interactions of lineage×C(4) subtype and lineage×habitat water availability in affected traits. Specifically, phylogeny tended to have a stronger influence on structural traits, and C(4) subtype had more important effects on physiological traits. Although hydraulic traits did not differ consistently between lineages, they showed strong covariation and relationships with leaf structure. Thus, phylogenetic lineage, photosynthetic pathway, and adaptation to habitat water availability act together to influence the leaf water relations traits of C(4) grasses. This work expands our understanding of ecophysiology in major C(4) grass lineages, with implications for explaining their regional and global distributions in relation to climate.