Comparative Kinetics and Reciprocal Inhibition of Nitrate and Nitrite Uptake in Roots of Uninduced and Induced Barley (Hordeum vulgare L.) Seedlings

Abstract

Nitrate and N02-transport by roots of 8-day-old uninduced and induced intact barley (Hordeum vulgare L. var CM 72) seedlings were compared to kinetic patterns, reciprocal inhibition of the transport systems, and the effect of the inhibitor, p-hydroxymercuribenzoate. Net uptake of N03-and N02-was measured by following the depletion of the ions from the uptake solutions. The roots of uninduced seedlings possessed a low concentration, saturable, low Km. possibly a constitutive uptake system, and a linear system for both N03-and N02-. The low Km system followed Michaelis-Menten kinetics and approached saturation between 40 and 100 micromolar, whereas the linear system was detected between 100 and 500 micromolar. In roots of induced seedlings, rates for both N03-and N02-uptake followed Michaelis-Menten kinetics and approached saturation at about 200 micromolar. In induced roots, two kinetically identifiable transport systems were resolved for each anion. At the lower substrate concentrations, less than 10 micromolar, the apparent low Kms of N03-and N02-uptake were 7 and 9 micromolar, respectively, and were similar to those of the low Km system in uninduced roots. At substrate concentrations between 10 and 200 micromolar, the apparent high Km values of N03-uptake ranged from 34 to 36 micromolar and of N02-uptake ranged from 41 to 49 micromolar. A linear system was also found in induced seedlings at concentrations above 500 micromolar. Double reciprocal plots indicated that N03-and N02-inhibited the uptake of each other competitively in both uninduced and induced seedlings; however, Ki values showed that N03-was a more effective inhibitor than N02-. Nitrate and N02-transport by both the low and high Km systems were greatly inhibited by phydroxymercuribenzoate, whereas the linear system was only slightly inhibited.

The effects of light on induction, time courses, and kinetic patterns of net nitrate uptake in barley

“…This model conformed best with the measured data with regard to the obtained standard errors (data not shown) and the fitting of curves to the means of uptake rates (Fig. 3) Aslam et al 1992), but lower than those found in maize (24 mmol m -3 , Hole et al 1990), in wheat (27 mmol m -3 , Goyal & Huffaker 1986b) and in Arabidopsis (40 mmol m -3 , Doddema & Telkamp 1979) or in excised barley roots in earlier reports (110 mmol m -3 , Rao & Rains 1976a). Similar agreements were found for the 13 N-NO 3 -influx (Lee & Drew 1986: 14 mmol m -3 ;Siddiqi et al 1990: between 30 and 79 mmol m -3 depending on the N status of the plants).…”

“…The HATS is described as a saturable system, whereas the nature of the LATS is still not clear. Doddema & Telkamp (1979) and Goyal & Huffaker (1986b) found a second saturable system, but Siddiqi et al (1990) and Aslam, Travis & Huffaker (1992) observed a linear non-saturable component under higher concentrations of nitrate. Aslam et al (1992) even found two saturable, possibly constitutive systems in the low concentration range and a linear system at more than 500 mmol m -3 nitrate.…”

“…Doddema & Telkamp (1979) and Goyal & Huffaker (1986b) found a second saturable system, but Siddiqi et al (1990) and Aslam, Travis & Huffaker (1992) observed a linear non-saturable component under higher concentrations of nitrate. Aslam et al (1992) even found two saturable, possibly constitutive systems in the low concentration range and a linear system at more than 500 mmol m -3 nitrate. Despite some evidence that the LATS is of a passive nature Siddiqi et al 1991), it appears more likely that HATS and LATS both proceed thermodynamically uphill (King, Siddiqi & Glass 1992).…”

“…The negative constant term (V c ) was introduced in the linear system in order to describe a linear component of uptake, which is initiated only at higher concentrations Aslam et al 1992). Standard errors and confidence limits (Student's t P = 0·95) are denoted in the text by ± or by horizontal and vertical error bars in figures.…”

“…Further, the effect of light on the induction of net nitrate uptake by roots of intact barley plants was examined. Uptake was measured by monitoring the depletion of nitrate in the nutrient solution (see also Aslam et al 1992;Goyal & Huffaker 1986a,b;Behl, Tischner & Raschke 1988;Doddema & Telkamp 1979;Rao & Rains 1976a;Pearson & Steer 1977;Mäck & Tischner 1990) using an automatically computer-controlled HPLC system. For the calculation of kinetic parameters the data were fitted to different equations using a non-linear curve-fitting computer program.…”