The effect of drought on plant water use efficiency of nine NAD-ME and nine NADP-ME Australian C4 grasses

We investigated the response to drought of nine NAD-malic enzyme (NAD-ME) and nine NADP-malic enzyme (NADP-ME) C 4 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 C 4 grasses, and ranged between 6.0 and 8.7 g dry mass kg -1 H 2O. 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 C 4 grasses. Leaf dry matter carbon (δ 13C) and oxygen (δ 18O) isotope compositions were significantly different between the two C 4 subtypes. Leaf δ 13C averaged -13.3 and -12.2"°, and leaf δ 18O averaged 26.0 and 26.9"° in well-watered NAD-ME and NADP-ME grasses, respectively. Drought significantly reduced leaf δ 13C in most C 4 grasses by an average 0.5"°. Leaf δ 18O was not significantly affected by drought, indicating that leaf δ 18O does not reflect drought-induced changes in leaf transpiration of C 4 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 C 4 grasses under drought was primarily related to control of water loss relative to carbon gain at the leaf, rather than the plant, level.