Nitrogen metabolism in tomato seedlings

Tomato seedlings absorbed increasing amounts of nitrate-N. The total uptake was doubled as the concentration of nitrate was quadruplicated. NO₃?N absorption seemed to be accompanied by efflux of OH^? ions which shift the pH of the media to the alkaline side. A minor fraction of the absorbed nitrate accumulated in the tissues while the major part was assimilated into peptides and proteins. The dry matter gain was by the end of experiment relatively higher than the control samples raised on nitrogen-free nutrient solution. Nitrate assimilation seemed to involve its reduction down to ammonia level. Since neither nitrite nor ammonia was recovered in the tissue-medium system, it was postulated that the rate of reduction was slower than the rate of product assimilation. The first step in nitrate reduction (nitrate→nitrate) appeared to be limiting while further reduction steps occurred rapidly and accompanied by simultaneous assimilation of ammonia. The enzyme responsible for the first step of nitrate reduction,i.e., nitrate reductase, was extracted from tomato shoots and roots. The activity in root extract amounted to about 30% of that of the shoot. This may suggest the localization of nitrate reduction in the leaves and realizes the relation between nitrate metabolism and photosynthesis.