Regulation of growth, water use efficiency and δ13C by the nitrogen source in Casuarina equisetifolia Forst. & Forst.

Carbon isotope composition (delta(13)C) was measured in a glasshouse experiment,vith N-2-fixing and NO3- or NH4+-fed Casuarina equisetifolia Forst. & Forst plants, both under well-watered and drought conditions. The abundance of C-13 was higher (more positive delta(13)C) for NH4+- than for NO3--grown plants and was lowest for N-2-fixing plants. NH4+-fed plants had more leaf area and dry weight and higher water use efficiency (on a biomass basis) than N-2- and NO3--grown plants and had lower water consumption than plants supplied with NO3- either with high or low water supply. Specific leaf areas and leaf area ratios were higher with NH4+ than with NO3- or N-2 as the N source. The difference observed in delta(13)C between plants grown with different N sources was higher than that predicted by theory and was not in the right direction (NH4+-grown plants with a more negative delta(13)C) to be explained by differences in plant composition and engagement of the various carboxylation reactions. The more positive delta(13)C in NH4+- than in NO3--grown plants is probably due to a decreased ratio of stomatal to carboxylation conductances, which accounts for the lower water cost of C assimilation in NH4+-grown plants.