The kinetics of ammonium and nitrate uptake by young rice plants

Abstract

Summary An important process which affects the fate of fertilizer nitrogen (N) applied to a rice crop is crop N uptake. This uptake rate is controlled by many factors including the N-ion species and its concentration. In this study the relation between N concentration at the root surface and N uptake was characterized using Michaelis-Menten kinetics.The equation considers two parameters, Vmax and Kin, which are measures of the maximum rate of uptake and the affinity of the uptake sites for the nutrient, respectively.Uptake rates of intact rice plants growing in a continuously flowing nutrient solution system were fitted to the Michaelis-Menten model using a weighted regression analysis. For NHq-N the Km values for 4-and 9-week-old rice plants indicated a high affinity for the ammonium ions relative to concentrations reported for rice soils after fertilization. The Vmax values expressed on a unit-rootmass basis decreased with plant age, indicating a reduction in the average density of uptake sites on the root surface.The kinetics of NO3-N uptake was similar to that of NH4-N when NO3-N was the only N source. However, if NH4-N and NO3-N were present simultaneously in the solution the Vmax for the uptake of NO3-N was severely reduced, while the Km was affected very little. This inhibition appears to be noncompetitive.Fertilization of young rice plants leading to concentration of N at the root surface above approximately 900 pM will not increase crop uptake and may contribute to inefficient N recovery by the crop. The existence of NH4-N and NO3-N simultaneously at the root surface may also lead to inefficient N recovery because of reduced uptake of NO3-N.

Cloning and Functional Characterization of a Constitutively Expressed Nitrate Transporter Gene,OsNRT1, from Rice

“…In addition, in two wild rice species (Oryza perennis and Oryza officinalis), one hybridizing band was found in the RNA get blotting analysis using OsNRT1 as a probe, and the intensity of the band remained approximately constant before and after nitrate induction (data not shown). Several studies have shown that the nitrate uptake activities of rice are repressed when ammonium is also present in the nutrient medium (Youngdahl et al, 1982;Kronzucker et al, 1999). However, our data showed no repressed expression of OsNRT1 mRNA when ammonium was present in the assay solution (data not shown).…”

“…The one exception is CHL1 (AtNRT1), which has been shown to be a dual-affinity nitrate transporter (Wang and Crawford, 1998;Liu et al, 1999). Although higher plants comprise both low-and high-affinity nitrate transport systems (encoded by NRT1, NRT2, and other yet-to-be identified gene families), physiological studies of nitrate uptake in rice have focused primarily on the high-affinity uptake phase, using micromolar levels of nitrate (Sasakawa and Yamamoto, 1978;Youngdahl et al, 1982;Hasegawa and Ichii, 1994;Raman et al, 1995;Kronzucker et al, 1999). Few studies have investigated the kinetics of low-affinity nitrate uptake in rice.…”

Ammonium Uptake by Rice Roots (III. Electrophysiology)

“…3A). Both net flux and unidirectional influx of NH4+ in rice roots have been shown to respond to [NH4+], in a similar fashion (Youngdahl et al, 1982;Wang et al, 1991Wang et al, , 1993b (Wang et al, 1993b). Since our studies were undertaken with the same rice variety as employed for the 13NH4+ influx experiments, these differences may arise from the fact that there were differences in growth conditions for plants used for the two studies or that membrane depolarization reflects the net, rather than the unidirectional, effect of ion fluxes.…”

Comparative kinetic analysis of ammonium and nitrate acquisition by tropical lowland rice: implications for rice cultivation and yield potential

“…It is therefore not surprising that NH % + nutrition, as opposed to NO $ − nutrition, has received almost exclusive attention in rice (Bonner, 1946 ;Fried et al, 1965 ;Shen, 1969 ;Wang et al, 1993). However, some reports have indicated that rice does possess some capacity for root NO $ − absorption (Ismunadji & Dijkshoorn, 1971 ;Sasakawa & Yamamoto, 1978 ;Youngdahl et al, 1982 ;Raman et al, 1995) and for the reduction of NO $ − in leaves (Tang & Wu, 1957). Isolated nursery trials have shown that NO $ − pretreatment of rice seedlings can result in enhanced transplanting success (Yamasaki & Seino, 1965), and, perhaps most strikingly, several varieties of Indica rice have been shown to exhibit superior growth on NO $ − under certain conditions (Ta & Ohira, 1981 ;.…”