Tjeerd Blacquière

Effects of N source on proton excretion, ionic balance and growth ofAlnus glutinosa (L.) Gaertner: comparison of N2 fixation with single and mixed sources of NO3 and NH4

SummaryNon-nodulatedalnus glutinosa plants were grown for 6 weeks in nutrient solutions using 3 combined-N treatments (NO3; NO3/NH4; and NH4) at a total N level of 4 meq.l−1, and growth was ccmpared with nodulated plants at zero N (N2 fixation). Of the combined-N sources, 100 per cent NH4 resulted in the highest dry matter yields when the solution pH was adjusted daily atc. 6. The dry matter yield was lowest with NO3.During the first 3 weeks, the yield of the N2-fixing plants was as high as that of the NH4 plants, but fell relatively behind during the second 3-week period. These effects could be attributed to higher initial N contents and higher shoot:root ratios, respectively, in the N2-fixing plants. Specific rates of N acquisition in the root were of a comparable order of magnitude for the combined-N and zero-N treatments.When NO3 was taken up, it was almost completely reduced in the roots. Regardless of N source there was a large excess of cations (C) relative to inorganic anions (A) in the plants, which was presumed to be balanced by an equivalent amount of organic anions (C-A). The relatively small differences in generation of organic anions for the various modes of N supply indicated the relative importance of the proton pump when NH4 or N2 was the N source. Proton or hydroxyl-ion effluxes, calculated on the basis of plant analyses, were generally in good agreement with measured excretion values. The acidity generation with N2 fixation amounted toc. 0.5 meq H+.mmol−1 Norg, which was distinctly higher than the range of 0.1–0.2 mentioned by Raven and Smith43 for dinitrogen-fixing plants.Without pH adjustment, specific rates of cation uptake and carboxylate generation were strongly depressed as the acidity increased, when NO3/NH4, NH4 and N2 were the N sources. Growth ofAlnus glutinosa appeared to be still normal at a pH ofc. 2.8. During the final 3 weeks, only the NH4 plants ceased growing at a pH of 2.6.

AMMONIUM AND NITRATE NUTRITION IN PLANTAGO-LANCEOLATA AND PLANTAGO-MAJOR L SSP MAJOR .1. ASPECTS OF GROWTH, CHEMICAL-COMPOSITION AND ROOT RESPIRATION

P. lanceolata andP. major were grown in culture solutions with nitrate or ammonium as the nitrogen source. Dry matter accumulation in the shoot was faster with nitrate than with ammonium, whilst that of the roots was not affected by the nitrogen source. As a consequence, the shoot-to-root ratio was lower with ammonium than with nitrate. InP. lanceolata, dry matter percentage of shoot and root tissue was lower with nitrate nutrition, suggesting better elongation growth than with ammonium. However, in shoot tissue ofP. major the opposite was found. The rate of root respiration declined with time, and this was almost completely due to a declining activity of the alternative path, which amounted to about 30–60% of total root respiration. Respiration via the cytochrome path was for a part of time slightly increased by ammonium, whereas the activity of the alternative path was strongly enhanced. The concentration of ethanol-soluble carbohydrates (SC) in the roots of both species was higher when nitrate was used, but no difference in the concentration of starch was found. When the plants were transferred from one nitrogen source to the other, many parameters, including the concentration of nitrate and chloride, and the shoot to root ratio, adjusted to the new situation in both species.P. lanceolata andP. major were grown in culture solutions with nitrate or ammonium as the nitrogen source. Dry matter accumulation in the shoot was faster with nitrate than with ammonium, whilst that of the roots was not affected by the nitrogen source. As a consequence, the shoot-to-root ratio was lower with ammonium than with nitrate. InP. lanceolata, dry matter percentage of shoot and root tissue was lower with nitrate nutrition, suggesting better elongation growth than with ammonium. However, in shoot tissue ofP. major the opposite was found. The rate of root respiration declined with time, and this was almost completely due to a declining activity of the alternative path, which amounted to about 30–60% of total root respiration. Respiration via the cytochrome path was for a part of time slightly increased by ammonium, whereas the activity of the alternative path was strongly enhanced. The concentration of ethanol-soluble carbohydrates (SC) in the roots of both species was higher when nitrate was used, but no difference in the concentration of starch was found. When the plants were transferred from one nitrogen source to the other, many parameters, including the concentration of nitrate and chloride, and the shoot to root ratio, adjusted to the new situation in both species.

Ionic balance, proton efflux, nitrate reductase activity and growth ofHippophaë rhamnoides L. ssp.rhamnoides as influenced by combined-N nutrition or N2 fixation

Growth of 2-month-old nonnodulatedHippophaë rhamnoides seedlings supplied with combined N was compared with that of nodulated seedlings grown on zero N. Plant growth was significantly better with combined N than with N2 fixation and, although not statistically significant for individual harvests, tended to be highest in the presence of NH4+, a mixture of NH4+ and NO3− producing the highest yields. Growth was severely reduced when solely dependent on N2 fixation and, unlike the combined-N plants, shoot to root ratios had only slightly increased after an initial decrease. An apparently insufficient nodule mass (nodule weight ratio <5 per cent) during the greater part of the experimental period is suggested as the main cause of the growth reduction in N2-fixing plants.Thein vivo nitrate reductase activity (NRA) of NO3− dependent plants was almost entirely located in the roots. However, when grown with a combination of NO3− and NH4+, root NRA was decreased by approximately 85 per cent.H. rhamnoides demonstrated in the mixed supply a strong preference for uptake of N as NH4+, NO3− contributing only for approximately 20 per cent to the total N assimilation.Specific rates of N acquisition and ion uptake were generally highest in NO3−+NH4+ plants. The generation of organic anions per unit total plant dry weight was approximately 40 per cent less in the NH4+ plants than in the NO3− plants. Measured extrusions of H+ or OH− (HCO3−) were generally in good agreement with calculated values on the basis of plant composition, and the acidity generated with N2 fixation amounted to 0.45–0.55 meq H+. (mmol Norg)−1.Without acidity control and in the presence of NH4+, specific rates of ion uptake and carboxylate generation were strongly depressed and growth was reduced by 30–35 per cent. Growth of nonnodulatedH. rhamnoides plants ceased at the lower pH limit of 3.1–3.2 and deterioration set in; in the case of N2-fixing plants the nutrient solution pH stabilized at a value of 3.8–3.9 without any apparent adverse effects upon plant performance.The chemical composition of experimental and field-growing plants is being compared and some comments are made on the nitrogen supply characteristics of their natural sites.

Growth, ionic balance, proton excretion, and nitrate reductase activity in Alnus and Hippophaë supplied with different sources of nitrogen

SummaryPre-cultivated, nodulated and non-nodulated plants of black alder (Alnus glutinosa) and sea buckthorn (Hippophaë rhamnoides ssp.rhamnoides) were grown on different N sources, with and without acidity control. Dry matter yields were lowest when plants were supplied with only NO3− and were much greater when NH4+ was supplied either alone or in combination with NO3− as long as the external pH was controlled; the final yields of the N2-fixing plants were relatively low, especially withH. rhamnoides. Without acidity control, yields were greatly reduced in the presence of NH4+.Proton or hydroxyl-ion effluxes, calculated on the basis of plant analyses, agreed well with measured excretion values. Without pH adjustment, the total proton efflux into the external solution was greater inA. glutinosa than inH. rhamnoides.Both species, but particularlyA. glutinosa, displayed the highest nitrate reductase activity in the roots.

NITRATE REDUCTASE-ACTIVITY IN LEAVES AND ROOTS OF ALNUS-GLUTINOSA (L) GAERTNER

SummaryThein vivo nitrate reductase activity (NRA) was determined inAlnus glutinosa plants grown nonsymbiotically on ammonium, nitrate, a combination of both, or symbiotically with atmospheric nitrogen as the only nitrogen source. Root NRA was absent when ammonium or atmospheric nitrogen was the nitrogen source. With nitrate in the culture solution the roots showed a high NRA. However, the leaf NRA behaved quite differently: with negligible activities on all nitrogen sources except atmospheric nitrogen. The foliar NRA measured, however, is likely not due to the activity of the plant but of microbial origin. Methods commonly used to facilitate produced nitrite to leak out of the tissue, such as addition of propanol and cutting the plant material, did not increase the nitrite release from the leaves. A turbidity developed when testing the samples for nitrite which was positively correlated with the NRA. Populations of microorganisms in the phyllosphere did not differ between the nutritional treatments. Bacteria, able to grow on a low-nitrogen medium, were present on the leaves. Nitrifiers could not be detected. The bacteria on the leaves appear to produce nitrite when incubated with leaf material.

The response ofPlantago lanceolata L. andP. major L. spp.major to nitrate depletion

To study aspects of the ecology of grassland species, in a comparative experiment, plants ofP. lanceolata andP. major were grown in pots in a greenhouse, and subjected to a gradual nitrate depletion for several weeks. Control plants were weekly supplied with nitrate. Growth, leaf appearance and disappearance, concentrations of cations and inorganic anions, soluble and insoluble reduced nitrogen concentrations,in vivo nitrate reductase activity (NRA) and the concentration of non-structural carbohydrates in several parts of the plants were followed. Depletion of nitrate caused a reduction of shoot growth, both in biomass and number of leaves. Withering of leaves increased. Accumulation of root dry matter was little (P. lanceolata), or not (P. major) affected. The concentration of reduced nitrogen in all tissues also decreased, both that of the soluble and that of the insoluble fraction. As a result, nitrogen use efficiency (NUE, g dry matter produced per mmol N incorporated) increased by nitrate depletion. NRA was higher in the roots than in the leaves, and decreased with increasing nitrate depletion.In control plants, nitrate became also limiting. This resulted in decreasing nitrate concentrations in leaves and roots. In the leaves, the decrease in nitrate concentration was preceded by a decrease in NRA. The decrease of the nitrate concentration was parallelled by an increase in the concentration of soluble sugar.No major differences in the response towards nitrate depletion were observed between the two species.