Nitrate and Ammonium Uptake by Rooted Cuttings of Sweet Potato1

Abstract

AbstractNitrogen uptake by rooted cuttings of sweet potato [Ipomoea batatas (L.) Lam.] has not been studied extensively although “slips” (rooted sprouts) are the normal plant part used in commercial propagation. Experiments with rooted cuttings of sweet potato, cultivar ‘Jewell’, were conducted in solution culture to investigate the effects of ambient acidity and the presence and concentration effects of ammonium and nitrate on rates of nitrate and ammonium uptake. Plant stem cuttings were rooted in Hoagland's solution (plus or minus nitrate) then transferred to 980 ml containers of experimental solutions. In experiment I rates of N uptake from 0.1 mM NH4NO3 were measured at pH of 4.5, 5.5, and 6.5. In experiment II rates of N uptake were measured from 0.1 mM solutions with and without the presence of the ammonium in the case of nitrate uptake and vice versa. In experiment III nitrate uptake was measured from 0.1 mM nitrate without ammonium and in the presence of 0.1 mM and 1 mM ammonium. Solutions were sampled periodically and analyzed for nitrate, ammonium, and K. Nitrate uptake rate was decreased by increasing ambient pH while ammonium uptake rate was increased. Presence of ambient ammonium at equimolar concentration decreased rate of nitrate uptake ,∼30% at pH 5.5. However, a tenfold increase in ammonium concentration did not further reduce rate of nitrate uptake although rate of ammonium uptake was doubled. Presence of equimolar nitrate had no effect on rate of ammonium uptake at 0.1 mM ammonium. Efflux of K to initially K‐free solutions was increased by ambient ammonium, but a transition from K efflux to influx occurred without a consistent change in rates of ammonium or nitrate uptake. A suggestion is made that two components of nitrate uptake by sweet potato roots exist, one of which is assumed to be associated with nitrate reductase and inhibited by ammonium.