Regulation of growth, water use efficiency and d13C by the nitrogen source in Casuarina equisetifolia Forst. & Forst.

Carbon isotope composition (d13C) was measured in a glasshouse experiment with N2-fixing and NO3 –- or NH4 +- fed Casuarina equisetifolia Forst. & Forst plants, both under well-watered and drought conditions. The abundance of 13C was higher (more positive d13C) for NH4 +- than for NO3 – -grown plants and was lowest for N2-fixing plants. NH4 +-fed plants had more leaf area and dry weight and higher water use efficiency (on a biomass basis) than N2- 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 N2 as the N source. The difference observed in d13C 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 d13C) to be explained by differences in plant composition and engagement of the various carboxylation reactions. The more positive d13C 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.Mr. D. González helped in growing and sampling plants. We are grateful to Professor J. A. Raven for his critical reading of the manuscript. This work was funded by the European Commission, STD 3 Programme (TS3*-CT91-0027) and at SCRI by the Scottish Office Agriculture, Environment and Fisheries Department.Peer reviewe