Abstract

C4 grasses are grouped into three biochemical subtypes following the major C4 acid decarboxylation pathway (NAD-ME, NADP-ME and PCK). The C4 subtypes have characteristic leaf anatomy, biochemistry and physiology, as well as distinct geographic distribution. NAD-ME and NADP-ME grasses predominate at the lower and higher end of rainfall gradients, respectively. This study investigates the drought response of NAD-ME and NADP-ME C4 grasses belonging to three taxonomic groups (main Chloroid assemblage, Paniceae and Andropogoneae). Eighteen C4 grasses were grown from seeds in potted soil in a glasshouse. Established seedlings were either watered daily or exposed to two successive drying cycles, after which they were harvested. Under well-watered conditions, average plant dry mass was greatest for the Paniceae (mostly NADP-ME), followed by the Chloroid (NAD-ME) then the Andropogoneae (NADP-ME), but water use efficiency (WUE; dry matter gain per unit water transpired) was similar for the three C4 taxa. Drought inhibited dry matter accumulation similarly among the C4 groups, but WUE was enhanced to a significantly greater extent in the NAD-ME than the NADP-ME species. There was no difference among the C4 groups in leaf water relations at which shoot elongation ceased. Among the NAD-ME species only, a trade-off was found between growth sensitivity to drought and plant size. Of the parameters measured in this study, only WUE was differentially affected by drought in the two C4 subtypes. This response may be an adaptive factor contributing to the greater abundance of NAD-ME, relative to NADP-ME, grasses in low rainfall areas.