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

Abstract

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.