Nitrogen Fixation by Purshia Tridentata: Some Ecological Aspects and Root Nodule Anatomy
This study examines several aspects of nitrogen fixation by Purshia tridentata (Pursh) D.C., a rosaceous shrub widespread in the central Oregon pumice region, especially as an understory species in Pinus ponderosa and Pinus contorta forests. Anatomical studies of root nodules under the light microscope revealed strong similarities to other non-legume nodules. The endophyte, apparently an actinomycete, was visible as a mass of hyphae with peripheral spherical vesicles in swollen infected cells in the central cortex of the nodules. Seasonal variations in morphology and anatomy are described. Acetylene reduction was used to assay nodule activity in both field and greenhouse plants. The maximum rates were observed at 20°C., although summer soil temperatures were frequently around 15°C., at which a much lower rate was observed. Acetylene reduction by excised nodules was linear for 5 hours and then slowly declined until finally ceasing after 19 hours. Nodule activity was found to decline in water stressed plants, essentially ceasing in plants with xylem pressure potentials below -25 bars. Field studies at 5 different sites revealed that nodule activity began in mid-May or early June when soil temperature at 20 cm. increased to above 10°C. Activity began later and remained lower until July20 in plants located under Pinus contorta probably because of the cooler temperatures at this site. Nodule activity at all sites was maximum in June and July. In late July, nodule activity declined sharply, corresponding with moisture stress readings in the -25 bar range. Daily acetylene reduction rates declined sharply each night; this decline was even more severe late in the season. Only 46% of Purshia plants were found to be nodulated. Several possible explanations for this low percent are discussed, but the primary reason appears to be low soil temperature and unfavorable moisture conditions. Previous speculations that Purshia may contribute significant amounts of nitrogen to the ecosystems in which it occurs are disputed using estimates based on seasonal acetylene reduction rates and a determination of nodule biomass/ha. The estimated nitrogen accretion rate was only 0.057 kg N/ha yr.
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