Effects of water and nitrogen supply on water use efficiency and carbon isotope discrimination in edible canna (Canna edulis Ker-Gawler)

Transpirational water use efficiency (WUE) is affected by nutrient and water supply, but relatively little is known about the response of Andean root and tuber crops, such as edible canna (Canna edulis Ker-Gawler). Two Canna edulis genotypes were studied in semi-controlled greenhouse experiments at the International Potato Centre (CIP) near Quito, Ecuador. Under conditions of low water supply, shoot dry matter (DM). leaf area and specific leaf nitrogen (SUN) all decreased, while WUE was higher and carbon isotope discrimination (Delta) decreased. The large water-storing hypodermal cells characteristic of this plant's leaf morphology shrank, concertina-like, the leaves became thinner, and the specific leaf area increased significantly (from 26 m(-2) kg(-1) to 43 m(-2) kg(-1)). in a second experiment, plants were grown with three different levels of nitrogen supply: from low to high N supply, shoot DM increased significantly (from 16 to 37 g), along with leaf area and SLN. At the lowest level of nitrogen supply (N0), WUE was significantly lower and Delta increased. As expected, the linear correlation between Delta and WUE was negative, but variation in Delta could only explain 49% of the variation in WUE. If, in addition to Delta, measured root fraction (RF) and estimated values of leaf-to-air vapour pressure deficit (Deltae) and respiratory and unproductive carbon loss were used to calculate WUE, the correlation between measured and calculated WUE was substantially improved, except for the water stress treatment. it is considered that, for root and tuber crops, RF and Deltae are the major variables when Delta of single leaves is to be used for up-scaling to plant WUE.