Coordination of traits associated with gmand Kleaf
Our findings on the coordination of traits associated with
photosynthetic C-gain and transpirational water-loss in
C4 grasses contrast some of those reported previously
for C3 species. For instance, C4 grasses
adapted to drier habitats exhibit traits associated with greater
gm and lower Kleaf (Table 1). Also,
gm, Anet and traits associated with
gm like SDada, SR and
Smes (Pathare et al ., 2020) scaled positively
with traits like IVD, leaf thickness and VED (Fig. 1) which are known to
be important determinants of Kleaf (Sack et al. ,
2013; Buckley et al. , 2015) These results suggest that
Kleaf may be negatively related to gmand hence Anet for the C4 grasses
belonging to habitats with diverse MAP. Indeed, Kleafestimated using anatomical traits scaled negatively with
gm and Anet. This finding contrasts the
previous reports of a positive relationship of Kleafwith gm and Anet observed in
C3 species (Sack & Holbrook, 2006; Flexas et
al. , 2013; Xiong et al. , 2017; Drake et al. , 2019) and
could be partly explained by the carbon concentrating mechanism of
C4 species that maintains high Anet at
relatively low gsw compared to C3species (Ocheltree et al. , 2016) and the relationship of
gm and Kleaf with leaf thickness.
Specifically, greater leaf thickness in C4 grasses from
drier habitats was associated with greater VED and lower total VLA (Fig.
1) which may imply a lower Kleaf and also an increase in
space available for photosynthetic tissue (Brodribb et al. , 2007;
McKown et al. , 2014; Zwieniecki & Boyce, 2014; Buckley et
al. , 2015). Also, in these C4 grasses, greater leaf
thickness was associated with a greater SDada, SR and
Smes implying a greater gm and
Anet (Muir, 2018). Consequently, we observed a negative
relationship of Kleaf with gm and hence
Anet in these C4 grasses. The negative
relationship of SDada with total VLA and
Kleaf (Fig. 1) also contrasts the previous reports for
C3 species (Drake et al ., 2019) and suggests
that, for the C4 grasses the presence of a greater
number of stomata may not be associated with greater investment in leaf
water transport tissue and hence Kleaf , though it is
associated with a greater gm and Anet(Pathare et al ., 2020).
Species adapted to drier habitats are known to employ a safer xylem
strategy, wherein, resistance to cavitation is achieved by maintaining
lower Kleaf, which however comes at the cost of
Anet thus leading to the safety versus efficiency
trade-off (Zimmermann, 1983; Meinzer et al. , 2010). However,
C4 grasses adapted to low MAP exhibited traits
associated with lower Kleaf, but greater
gm and Anet (Table 1, Fig. 1). Previous
studies have also observed a decoupling between Kleafand Anet for the C4 grasses (Kocacinar
& Sage, 2003; Ocheltree et al. , 2016). Our results along with
these previous findings suggest that maintaining a greater
Kleaf in order to achieve greater Anetmay not be a necessity for C4 grasses and that the
safety-versus efficiency trade-off may not apply to the
C4 grasses which can achieve greater gmand Anet in drier habitats whilst maintaining a lower
Kleaf.
The Kleaf values estimated here (10 to 27 mmol
m-2 s-1 MPa-1)
using anatomical traits, are within the range measured previously (5 to
30 mmol m-2 s-1MPa-1) for diverse C4 grasses (Liu &
Osborne, 2015; Liu et al. , 2019). Because Kleafwas not measured directly, but estimated using a semi-empirical model
based on diverse plant groups (Brodribb et al. , 2007; de Boeret al. , 2016), there are some uncertainties associated with using
this model for C4 grasses. For instance, the presence of
Kranz anatomy, presence or absence of bundle-sheath suberisation and the
relationship of stomatal density with VLA (Fig.1) can all influence the
estimates of Kleaf. There is a need to address these
uncertainties in future efforts for estimating Kleaf in
C4 grasses. Though we investigated the relationship of
Kleaf with gm and important mesophyll
and BS traits, studies suggest that these traits may correlate strongly
with the Kox component of Kleaf, then
Kx (Sack & Scoffoni, 2013; Buckley et al. ,
2015). Investigating the relationship of gm with
Kox, once a direct method for estimating
Kox is developed (Song & Barbour, 2016; Barbour, 2017;
Barbour et al. , 2017), will provide greater ability to scale up
from physiological processes to whole-leaf functions.