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- Orchids are distributed around the world, however, the factors shaping their specific distribution and habitat preferences are largely unknown. Moreover, many orchids are at risk of becoming threatened as landscapes change, sometimes declining without apparent reason. One important factor affecting plant distribution is nutrient levels in the environment. Nitrates can inhibit not only orchid growth and persistence, but also seed germination.
- We used in vitro axenic cultures to exactly determine the germination sensitivity of seven orchid species to nitrates and correlated this with soil properties of the natural sites and with the species’ habitat preferences.
- We found high variation in response to nitrate between species. Orchids from oligotrophic habitats were highly sensitive, while orchids from more eutrophic habitats were almost insensitive. Sensitivity to nitrate was also associated with soil parameters that indicated a higher nitrification rate.
- Our results indicate that nitrate can affect orchid distribution via direct inhibition of seed germination. Nitrate levels in soils are increasing rapidly due to intensification of agricultural processes and concurrent soil pollution, and we propose this increase could cause a decline in some orchid species.
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We assessed the extent to which plants can acquire amino acids when supplied as single N-sources or when plants have access to a mixture of amino- and inorganic N sources. Because the uptake of different N-sources is temperature-dependent, the effects of temperature on amino-N uptake were also tested. Lolium perenne (perennial rye-grass) was grown hydroponically at 11 °C or 21 °C. Uptake of N was determined using 15N tracers at the growth temperature from solutions containing either nitrate, ammonium or glycine as single N sources and from a mixture containing all three N-forms. Estimates of the relative importance of amino acids such as glycine to the total N budget of plants will have been underestimated in studies where uptake was determined in single source solutions compared with those from solutions containing a mixture of N-forms. The proportion of total N acquired from the mixed N source as ammonium increased as temperature was reduced. Regarding the uptake and initial metabolism of glycine, uptake was probably the rate limiting step at 11 °C whilst it was the metabolism of glycine to serine at 21 °C. Although 15N incorporation into the plant amino-N pool was generally in proportion to the abundance of individual amino acids, its incorporation into the glycine pool was sometimes significantly less than predicted. 相似文献
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Concentration and flux of solutes from snow and forest floor during snowmelt in the West-Central Adirondack region of New York 总被引:7,自引:7,他引:0
Decreases in pH and increases in the concentration of Al and NO
3
–
have been observed in surface waters draining acid-sensitive regions in the northeastern U.S. during spring snowmelt. To assess the source of this acidity, we evaluated solute concentrations in snowpack, and in meltwater collected from snow and forest floor lysimeters in the west-central Adirondack Mountains of New York during the spring snowmelt period, 29 March through 15 April 1984.During the initial phase of snowmelt, ions were preferentially leached from the snowpack resulting in elevated concentrations in snowmelt water (e.g. H+ = 140 eq.l–1; NO
4
2–
= 123 eq.l–1; SO
3
–
= 160 eq.l–1). Solute concentrations decreased dramatically within a few days of the initial melt (< 50 eq.l–1). The concentrations of SO
4
2–
and NO
3
–
in snowpack and snowmelt water were similar, whereas NO–
3 in the forest floor leachate was at least two times the concentration of SO
4
2–
.Study results suggest that the forest floor was a sink for snowmelt inputs of alkalinity, and a net source of H+, NO
3
–
, dissolved organic carbon, K+ and Al inputs to the mineral soil. The forest floor was relatively conservative with respect to snowmelt inputs of Ca2+, SO
4
2–
and Cl–. These results indicate that mineralization of N, followed by nitrification in the forest floor may be an important process contributing to elevated concentrations of H+ and NO
3
–
in streams during the snowmelt period. 相似文献
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The nitrate reductase (NR, EC 1.6.6.1) activity in root nodules formed by hydrogenase positive (Hup+ ) and hydrogenase negative (Hup− ) Rhizobium leguminosarum strains was examined in symbioses with the pea cultivar Alaska ( Pisum sativum L.), Rates of activity were determined by the in vivo assay in nodules from plants that were only N2 -dependent or grown in the presence of 2 m M KNO3 . The rates varied widely among strains, regardless of the Hup phenotype of the R. leguminosarum strain used for inoculation, but the overall results indicated that nodules formed by Hup− strains accumulated more nitrite in the incubation medium than did those with Hup− phenotypes. Total plant dry weight and reduced nitrogen content of pea plants grown in the presence of 2 m M KNO3 and inoculated with single Hup+ and Hup− R. leguminosarum strains were statistically different among some strains. These observations suggest that the possible advantages derived from the presence of the Hup system on whole plant growth may be counteracted by the higher rates of NR activity in the Hup− strains in the R. leguminosarum -pea symbiosis. 相似文献
10.
Published data on, and hypotheses regarding the effect of NO− 3 on functioning of legume root nodules are reviewed. It is concluded that a short-term reversible effect of NO− 3 may act via an increased resistance to O2 diffusion in nodules; this is coupled to decreased bacteroid respiration. For longer exposures to NO− 3 nodule activity is irreversibly lost, but how this relates to carbohydrate deprivation or NO-2 accumulation is unclear. Complicating factors include denitrification reactions and the interaction of NO− 2 with leghaemoglobin. 相似文献