Nitrate uptake patterns by nitrogen‐depleted wheat seedlings (Triticum vulgare var. Atlas 66) exhibited an initial lag phase which was lessened, but not completely overcome, by presence of solution Ca. Maintenance of a more rapid subsequent rate of nitrate uptake, which developed after the initial lag phase, depended upon presence of solution K. The rapid phase was enhanced when both Ca and K were present and was curtailed by shoot excision and by Ca deficiency. Sizeably larger amounts of nitrate were recovered in the shoots after 24 hours in KNO3 as compared to Ca(NO3)2, and with the former salt, smaller proportions of the absorbed nitrate were reduced. The data suggest a beneficial influence of Ca both on initial nitrate uptake by root tissue and on transport to shoots while a continual K supply was apparently beneficial in nitrate transport.
Ammonium markedly inhibited nitrate absorption by nitrogenstarved wheat seedlings but did not decrease the proportion of absorbed nitrate that was reduced. Seedlings high in nitrate (absorbed prior to the experimental periods) reduced similar amounts of this nitrate regardless of whether or not ammonium was present and being absorbed during the period of measurement. Ammonium or products of ammonium assimilation did not interfere with the induction, stability, or activity of nitrate reductase. Consequently, the hypothesis that ammonium depresses nitrate uptake indirectly by inhibiting nitrate reduction is rejected, and it is suggested that the ammonium effect is directly on the nitrateuptake process.
Supplying nitrate to N‐depleted wheat seedlings (Triticum vulgare, var. Atlas 66) stimulated the uptake of both divalent (Sr, Mn, and Mg) and monovalent (Cs, Na, K) cations. Rapid nitrate uptake, and the lower ambient acidity resulting from it, appeared to be partially responsible for the increase in cation uptake. Nevertheless, pretreatments with nitrate exerted some stimulation in Sr and Cs uptake, indicating an additional action of nitrate after it had entered the tissues. Pretreatment with nitrate, as well as its presence during the Sr uptake period, increased transport of Sr to the shoots more than total uptake. Transport to shoots of previously absorbed Sr was enhanced by subsequent nitrate treatments, the amount transported being significantly greater with nitrate salts of Ca, Mg, and Na than with K or NH4.
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