Regulation of NO3− Influx in Barley

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Abstract
Short-term (10 min) measurements of plasmalemma NO3- influx (.vphi.oc) into roots of intact barley plants were obtained using 13NO3-. In plants grown for 4 days at various NO3- levels (0.1, 0.2, 0.5 mM), .vphi.oc was independent of the level of NO3- pretreatment. Pretreatment with Cl- had no effect upon plasmalemma 13NO3- influx. Plants grown in the complete absence of 13NO3- (in CaSO4 solutions) subsequently revealed influx values which were > 50% lower than for plants grown in NO3-. Based upon the documented effects of NO3- or Cl- pretreatments on net uptake of NO3-, these observations suggest that negative feedback from vacuolar NO3- and/or Cl- acts at the tonoplast but not at the plasmalemma. When included in the influx medium, 0.5 mM Cl- was without effect upon 13NO3- influx, but NH4+ caused .apprx. 50% reduction of influx at this concentration.