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 and SR,
thus supporting the first hypothesis. However, these traits have also
been associated with greater Kleaf in C3species (Brodribb & Holbrook, 2004; Xiong et al. , 2017; Drakeet 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, Fig. 6), which indicates
deeper vein placement and an increase in pathway for water movement
outside the xylem (Brodribb et 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, Fig. 6).
This may reduce the parallel water flow pathways outside xylem thus
decreasing Kox and hence Kleaf (Buckleyet al. , 2015). Together, these anatomical traits suggest that
Kleaf would be lower in C4 grasses
adapted to low MAP. 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, for C4 grasses the greater
photosynthetic C-gain in drier habitats may not be associated with
greater leaf-level water-loss.