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.