Do Casuarina cunninghamiana seedlings dependent on symbiotic N-2 fixation have higher phosphorus requirements than those supplied with adequate fertilizer nitrogen?

P requirements of Casuarina cunninghamiana seedlings inoculated with Frankia and reliant on symbiotic N-2 fixation were compared with those of uninoculated seedlings grown with 4 levels of fertilizer N in a solution culture system. Growth responses to increasing P supply depended on the N treatment that had been imposed. At the two lowest levels of N supply (0 mu M and 100 mu M) growth was relatively poor and there was no response to increasing P supply above 10 mu M and 50 mu M P respectively. In contrast, inoculated seedlings and those with higher levels of combined N (500 mu M and 1000 mu M) had significantly greater shoot dry weights (except at 0.1 mu M P), and they responded to increasing P supply to between 50 and 100 mu M P. At each level of P supply, the dry weights of seedlings in these 3 N treatments were similar. Nitrogen concentrations in shoots of seedlings provided with fertilizer N decreased with increasing P supply to 50 mu M and then remained unchanged. In contrast, N concentrations in shoots of inoculated seedlings increased with increasing P supply to 25 mu M P. At the 2 highest levels of P supply, N concentrations in shoots of inoculated plants were significantly higher than those in seedlings provided with fertilizer N. In all N treatments, P concentrations in shoots increased with increasing P supply; concentrations were similar in inoculated seedlings and those with the 2 highest levels of fertilizer N across all levels of P supply. Alleviation of P deficiency in inoculated seedlings increased nodule number, nodule dry weight, N-2 fixation g(-1) nodule dry weight (nodule 'efficiency'), P concentration in nodules, proportion of total seedling biomass allocated to nodules and average nodule size. However, all these parameters reached their maximum values at levels of P supply at, or below, those required for maximum host-plant growth (50 to 100 mu M P). The results indicate that the P requirement for host plant growth per se is similar to, or higher than, that required for symbiotic N-2 fixation processes.