The effect of drought on plant water use efficiency of nine NAD–ME and nine NADP–ME Australian C 4 grasses

We investigated the response to drought of nine NAD-malic enzyme (NAD-ME) and nine NADP-malic enzyme (NADP-ME) C4 grasses. Species were grown from seeds in potted soil in a glasshouse. Seedlings were either watered regularly or exposed to two successive drying cycles of 8-10 d each, after which plants were harvested. Under well-watered conditions, average water use efficiency (WUE; dry mass gain per unit water transpired) was similar for NAD-ME and NADP-ME C4 grasses, and ranged between 6.0 and 8.7 g dry mass kg⁻¹ H2O. Drought enhanced WUE of most species, but to a significantly greater extent in NAD-ME (1.20-fold) than NADP-ME (1.11-fold) grasses. Inhibition of dry matter accumulation (average of 12%) and shoot elongation under drought was similar among the C4 grasses. Leaf dry matter carbon (δ¹³C) and oxygen (δ¹⁸O) isotope compositions were significantly different between the two C4 subtypes. Leaf δ¹³C averaged -13.3 and -12.2, and leaf δ¹⁸O averaged 26.0 and 26.9 in well-watered NAD-ME and NADP-ME grasses, respectively. Drought significantly reduced leaf δ¹³C in most C4 grasses by an average 0.5. Leaf δ¹⁸O was not significantly affected by drought, indicating that leaf δ¹⁸O does not reflect drought-induced changes in leaf transpiration of C4 grasses. In the experiment reported here, NAD-ME grasses increased their WUE under drought to a greater extent than their NADP-ME counterparts. Increased WUE of the C4 grasses under drought was primarily related to control of water loss relative to carbon gain at the leaf, rather than the plant, level.