In habitats with low water availability, a fundamental challenge for
plants will be to maximize photosynthetic C-gain whilst minimizing
transpirational water-loss. This tradeoff between C-gain and water-loss
can in part be achieved through the coordination of leaf-level
photosynthetic and hydraulic traits. To test the relationship of
photosynthetic C-gain and transpirational water-loss we grew under
common growth conditions 18 C4 grasses adapted to
habitats with different mean annual precipitation (MAP) and measured
leaf-level structural and anatomical traits associated with mesophyll
conductance (gm) and leaf hydraulic conductance
(Kleaf). The C4 grasses adapted to lower
MAP showed greater mesophyll surface area exposed to intercellular air
spaces (Smes) and adaxial stomatal density
(SDada) which supported greater gm.
These grasses also showed greater leaf thickness and vein-to-epidermis
distance which may lead to lower Kleaf. Collectively,
these leaf traits associated with gm and
Kleaf scaled positively with photosynthetic rates
(Anet) and leaf-level water-use efficiency (WUE) with
low MAP adapted grasses exhibiting greater Anet and WUE.
In summary, we identify a suite of leaf-level traits that appear
important for adaptation of C4 grasses to habitats with
low MAP and may be useful to identify C4 species showing
greater Anet and WUE in drier conditions.