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.