C4 grasses adapted to low MAP show leaf anatomical traits associated with lower Kleaf and transpirational water-loss
Our results demonstrate that C4 grasses adapted to low MAP show greater gm and associated traits like Smes, Sc, SDada, SR and gmax, thus supporting the first hypothesis. However, these traits have also been associated with greater Kleaf in C3 species (Brodribb & Holbrook, 2004; Xiong et al. , 2017; Drake et al. , 2019), which could imply higher water costs in drier habitats. However, C4 grasses adapted to low MAP also showed greater leaf thickness and VED (Table 1), which indicates deeper vein placement and an increase in pathway for water movement outside the xylem (Brodribbet al. , 2007; McKown et al. , 2014; Buckley et al. , 2015). Additionally, in these C4 grasses, an increase in leaf thickness was correlated with an increase in IVD i.e. a decrease in total VLA (Table 1). This may reduce the parallel water flow pathways outside xylem thus decreasing Kox and hence Kleaf (Buckley et al. , 2015). Together, these anatomical traits suggest that Kleaf would be lower in C4 grasses adapted to low MAP. Indeed, Kleaf, estimated using leaf thickness and VED, tended to be lower in C4 grasses adapted to low MAP (Table 1). Furthermore, species adapted to low MAP also showed lower BSias and higher BSCW-traits that may lower Kox and hence Kleaf (Griffithset al. , 2013; Buckley et al. , 2015; Scoffoni et al. , 2017). In summary, though C4 grasses adapted to habitats with relatively low MAP exhibit traits associated with greater gm and photosynthetic C-gain, they also possess traits associated with lower Kleaf and water-loss. This supports our second hypothesis as well as the previous expectation of selection for traits associated with lower Kleaf in C4 species during adaptation to drier conditions (Zhouet al. , 2018). Thus, C4 grasses from drier habitats can achieve greater photosynthetic C-gain at a given water-loss compared to C4 grasses from relatively wet habitats.