Growth and nitrate assimilation characteristics of tomato [Lycopersicon esculentum] cultivars grown under varying nitrogen levels

Growth and nitrogen content were different among nine tomato cultivars grown under three nitrogen levels (50, 100, 150 mg-N/L). Applied nitrogen efficiency to growth was highest in 'Odoriko', and lowest in 'June Pink'. It was suggested that the difference in tomato growth was influenced not only by the difference of nitrogen uptake but also by the difference of nitrogen efficiency ratio (dry weight per nitrogen content). A positive correlation between the tomato growth and the content of assimilated nitrogen was observed. Therefore, it was suggested that the ability of nitrogen assimilation was different among the cultivars, and that the difference in ability of nitrogen assimilation influenced the difference in the nitrogen efficiency ratio and growth. We compared 'Odoriko' and 'June Pink' for nitrate (NO3sup(-)) reduction, which is the most important step in nitrogen assimilation. It was shown that there were differences of nitrate reductase (NR) activity and rate of nitrate assimilation between the two cultivars. We analyzed the kinetic properties of NO3sup(-) influx on high/low-affinity transport systems (HATS and LATS), and on the responses of net uptake, influx and efflux to external NO3sup(-) of pretreatments on LATS to clarify the NO3sup(-) uptake characteristics of the two cultivars. The Vmax value on HATS and the slope of influx on LATS in 'Odoriko' were higher than in 'June Pink'. These results suggested that 'Odoriko' had a greater ability of NO3sup(-) influx. The rates of decline on net uptake were different between the two cultivars in the experiment for analysis of the effects of pretreatment to NO3sup(-) net uptake on LATS. The rate of net uptake declined remarkably on 'June Pink' more than on 'Odoriko', especially in the 4 mM NO3sup(-) treatment. In order to clarify the mechanism for the deference in the effect on net uptake, we analyzed the effects of pretreatment to influx and efflux. The decline of influx on LATS was not shown in either cultivar. On the other hand, the efflux rates of 'June Pink' were higher than of 'Odoriko' in each of the treatments, and the differences in the efflux rate were extended with increased NO3sup(-) concentration in the pretreatments. These results indicated that the higher efficiency of NO3sup(-)-N utilization on 'Odoriko' was due to the lower efflux rate and the higher net uptake with high external NO3sup(-) levels. It was presumed that higher NR activity and ability of nitrate reduction on 'Odoriko' caused the lower efflux on 'Odoriko', and the extending of the difference on efflux at higher external NO3sup(-) levels.