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) 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.