Nitrate reduction in seedlings of carob (Ceratonia siliqua L.)

The distribution of nitrate reduction in young carob plants (Ceratonia siliqua L. cv. Mulata) was studied. Plants (three months old) were grown in aerated nutrient solutions under greenhouse conditions. Reduction of nitrate was estimated both in the leaves and in the roots by: (a) accumulation of nitrite in vitro, (b) accumulation of nitrite in vivo and (c) disappearance of nitrate in vivo. Disappearance of nitrate in vivo offered the closest estimation of the rate of nitrate reduction. Leaves consistently showed rates of nitrate reduction lower than those in roots, regardless of assay method. Lowering root temperature had no marked effect on the distribution of nitrate reductase activity in the plant; increasing nitrate concentration in the shoot, however, markedly increased leaf nitrate reduction. The relatively low level of nitrate reductase in leaves appears to be caused mainly by a limitation of nitrate transport from root to shoot. Nitrate reduction takes place in both the root and the shoot of young carob plants with the shoot accounting for approximately 20% of total nitrate reduction in the plant.