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) were observed in C4 grasses
adapted to lower MAP, as shown recently for C3 species
(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). 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, gmax, SR and
Narea (Table 1).
Species adapted to conditions with
high CO2 demand, like high light and low water, have
been shown to exhibit greater SDada and SR which may
help decrease the effective leaf thickness and hence CO2diffusion pathlength thus increasing gm and supporting
higher Anet (Parkhurst, 1978; Mott & O’Leary, 1984;
Muir, 2018). We recently demonstrated that greater SDadaand 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 and Anet. C4grasses adapted to drier habitats also exhibited greater
gmax (Table 1), which could enhance the capacity of
C4 grasses to achieve higher Anet in the
shorter periods when water is available (Franks & Beerling, 2009).