Xj. Min et al., A comparative kinetic analysis of nitrate and ammonium influx in two early-successional tree species of temperate and boreal forest ecosystems, PL CELL ENV, 23(3), 2000, pp. 321-328
Root NO3- and NH4+ influx systems of two early-successional species of temp
erate (trembling aspen: Populus tremuloides Michx.) and boreal (lodgepole p
ine: Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) forest ecosyste
ms were characterized. NO3- and NH4+ influxes were biphasic, consisting of
saturable high-affinity (HATS) and constitutive non-saturable low-affinity
transport systems (LATS) that were evident at low and relatively high N con
centrations, respectively. NO3- influx via HATS was inducible (IHATS); nitr
ate pre-treatment resulted in 8-10-fold increases in the V-max for influx i
n both species. By contrast, HATS for NH4+ were entirely constitutive. In b
oth species, V-max values for NH4+ influx were higher than those for NO3- u
ptake; the differences were larger in pine (6-fold) than aspen (1.8-fold).
In aspen, the K-m for NH4+ influx by HATS was approximately 3-fold higher t
han for IHATS NO3- influx, while in pine the K-m for IHATS NO3- influx was
approximately 3-fold higher than for NH4+ influx. The aspen IHATS for NO3-
influx appeared to be more efficient than that of pine (V-max values for as
pen being approximately 10-fold higher and K-m values being approximately 1
3-fold lower than for pine). By contrast, only small differences in values
for the NH4+ HATS were evident between the two species. The kinetic paramet
ers observed here probably result from adaptations to the N availabilities
in their respective natural habitats; these may contribute to the distribut
ion and niche separation of these species.