C4 grasses adapted to low MAP show traits
associated with greater gm and photosynthetic C-gain
A maximum Anet for a given rate of transpirational
water-loss, through coordination of leaf-level photosynthetic and
hydraulic traits, will in part determine species WUE and fitness during
adaptation to drier growth habitats. A key question posed by earlier
studies is how species adapted to drier habitats maintain similar or
even higher Anet, at a given gsw, during
periods of active photosynthesis than species from more humid habitats
(Wright et al. , 2001; Reich et al. , 2003). A greater
Anet, at a given gsw, can be achieved by
increasing gm through selection for leaf mesophyll
traits like Smes and Sc (Ivanovaet al. , 2018a; Ivanova et al. , 2018b). In the current
study, greater values for mesophyll traits like Smes and
Sc (Table 1, Fig. 6) were observed in C4grasses adapted to lower MAP, as shown recently for C3species (Ivanova et al. , 2018a; Ivanova et al. , 2018b).
Smes and Sc have been demonstrated to be
important determinants of gm in C3 (Muiret al. , 2014; Peguero-Pina et al. , 2017) and
C4 species (Pathare et al. , 2020) . For
C4 species, Smes is a more accurate
determinant of gm then Sc as the first
site of CO2 fixation is in mesophyll cytosol and not the
mesophyll chloroplast (Barbour et al. , 2016; Pathare et
al. , 2020). If all else remains constant, then a greater
Smes increases the number of parallel pathways for
CO2 diffusion inside leaves leading to higher
gm under high light and low water availability
(Terashima et al. , 2001; Ivanova et al. , 2018a; Ivanovaet al. , 2018b). Indeed, along with greater Smes,
we also observed greater values for gm in the
C4 grasses adapted to low MAP (Table1, Fig. 6). Our
findings thus support the previous work on C3 plants
suggesting the importance of mesophyll traits for plants adapted to
drier habitats (Ivanova et al. , 2018b). Also, for the first time
we show that C4 grasses adapted to low MAP also exhibit
mesophyll traits that lead to greater gm -an important
trait that could help achieve greater Anet at a given
gsw (Flexas et al. , 2016; Cousins et al. ,
2020; Pathare et al. , 2020).
In addition to greater gm and Smes,
C4 grasses adapted to low MAP also showed greater
SDada, SR and Narea (Table 1, Fig. 6).
Greater SDada and SR mostly occur in species adapted to
conditions with high CO2 demand, like high light and low
water, where, they are proposed to decrease the effective leaf thickness
and hence CO2 diffusion pathlength thus increasing
gm and supporting higher Anet(Parkhurst, 1978; Mott & O’Leary, 1984; Muir, 2018). We recently
demonstrated that greater SDada and SR in
C4 grasses were associated with greater leaf thickness
and lead to greater gm and Anet as a
result of increase in Smes (Pathare et al. , 2020)
. The current study further supports the well-established positive link
of SDada and SR with habitat MAP (Mott & O’Leary, 1984;
Bucher et al. , 2017) and suggests that a greater
SDada and SR in drier habitats could be a strategy used
by C4 grasses to facilitate greater gm.