It is shown that in the majority of higher plant halophytes examined proline is the major component of the amino acid pool in plants collected from the field. In Triglochin maritima L. free proline can represent 10-20% of the shoot dry weight. Under non-saline conditions proline levels are low and increase as the salinity is raised. Comparisons of inland and coastal populations of Ameria maritima Willd. suggest that the capacity to accumulate proline is correlated with salt tolerance. It is suggested that proline functions as a source of solute for intracellular osmotic adjustments under saline conditions.

SUMMARYAtmospheric nitrate deposition varies across Northern Britain, and has increased approximately fourfold since the last century in the Manchester Region. Ombrotrophic mires depend on an atmospheric supply of elements, and some components of nitrogen deposition at two mire sites in northern Britain were investigated. The sites differed in proximity to major pollution sources. Growth of Sphagnum cuspidatum HofTm. in bog pools was less in the polluted site and this was associated with a marked increase in tissue nitrogen concentration. Field experiments in which S. cuspidatum plants from unpolluted sites were exposed solely to atmospheric deposition in artificial bog pools showed that much of this large increase in nitrogen concentration could have resulted from atmospheric deposition alone. Concentrations of nitrate and ammonium within the range observed in bulk deposition at the polluted site reduced the growth of 5. cuspidatum in a laboratory experiment. The results are discussed in relation to a general increase in atmospheric nitrogen deposition, and its potential importance to plant growth.

SUMMARYSphagnum cuspidatum Ehrh. ex. HofFm. was removed from a relatively remote moorland site at the Migneint, N. Wales and from Holme Moss, S. Pennines -a site that has been subjected to atmospheric pollution deposition for a period of at least the last 200 yr.When exposed to elevated ammonium (NHj"") concentrations {0-1 and 10 mM) under laboratory conditions for a period of 30 d, S. cuspidatum from N. Wales showed a marked reduction in growth, whereas in the S. Pennine population, growth was stimulated above that of the untreated control tissue at both 0-1 and 1-0 mM NH,~. The largest growth stimulation, however, was seen at 0-1 mM.The effects of increased NH^' in the growth medium on tissue total chlorophyll concentration after 30 d exposure were similar in nnoss from both study sites. There -was a small linear decline in chlorophyll concentration with increasing ammonium concentration.Moss from both sites was exposed to 0-1 mM NH^"" for a period of 20 d. There was a marked difference in the response of the different moss populations as indicated by changes in the concentrations of the individual ammo acids; notably a dramatic transient increase in glutamine (three-fold), arginine (19-fold), and asparagine (fourfold) in the moss from N. Wales. In contrast, in the S. Pennine moss, changes in tissue amino-N concentration were very mucb smaller. Possible mechanisms to account for the intraspecific differences in response of the moss from the two stud>' sites ro increased ammonmm concentrations are discussed.

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SUMMARYFrost hardiness in Calluna vulgaris (L.) Hull, which had received ammonium nitrate applications in the field for 30 months, was assessed using scores of visible injury and measurements of the rate of total electrolyte leakage from excised shoots following controlled freezing treatments in the laboratory. There was good overall correlation between the two methods (Spearman correlation coefficient 0-77), but leakage measurements were more sensitive than injury scores to the efFects of nitrogen. Visible injury was not significantly altered by nitrogen supply (Kruskal-Wallis non-parametric test). Ion leakage was analyzed in different ways, using either calculations of the first-order rate coefficients or expressions of relative conductivity. These analyses produced similar results with respect to the effect of frost and nitrogen. Shoots of nitrogen-fertilized (40, 80 and 120 kg ha"^ yr'^) C. vulgaris sampled in October 1991 showed significantly {P < 0-05) less leakage after overnight frosts of -15 and -20 °C than did the water-treated controls. In October the temperature which killed 50% of the shoots (LT,,,,), derived from the leakage data, was raised by at least 4 °C by the highest nitrogen treatments compared with the control plants. Frost treatments to pot-grown C. vulgaris in November produced similar visible injury to attached and excised shoots from the same plants, both being significantly less damaged by a -18 °C frost after a 7-month exposure to an NaNOg mist solution (1-0 mM, pH 4-5) than were water-misted controls. Ammonium-mist treatments also marginally reduced frost injury, but the effects were not statistically significant. These results demonstrate that frost hardiness of C. vulgaris in the field can be assessed rapidly and accurately in the laboratory by analysis of electrolyte leakage from excised shoots.

SUMMARYNitrate utilization by calcicole and calcifuge species has heen measured using a field assay for nitrate reductase. Certain calcifuge species, notably members of the Ericaceae, showed no detectable enzyme activity in the field and very low values in the laboratory even after nitrate feeding. It is concluded that these species have a restricted ability to utilize nitrate. Other calcifuge species and all the species from calcareous soils had detectable nitrate reductase activities in the field and responded to nitrate addition by large increases in enzyme activities. Some of these species were used to compare nitrate utilization in a number of acidic and calcareous soils. Nitrate was utilized in all the soils, but utilization was minimal in ombrogenous peat. A comparison of closely adjacent acidic and calcareous soils showed that nitrate is an important source of inorganic combined nitrogen in at least some acidic soils.

SUMMARYThe availability and utilization of nitrate by salt-marsh plants has been studied. All the species examined are able to assimilate nitrate, which appears to be the most important form of nitrogen available on the marsh. Speeies, from the strand line and lower marsh have higher nitrogen contents and nitrate reductase levels, than those from the upper marsh. Competition between species for nitrate is more important in the upper than lower marsh, because of low nitrate supply. Species such as Armeria maritima, Glaux maritima, Limonium vulgare and Triglochin maritima are able to compete more effectively for nitrate in the upper marsh than Puccinellia maritima, Salicornia europaea, Suaeda tnarititna, Spergularia media and Spartinax anglica. The results indicate there is an increase in nitrogen supply down the marsh, which may be associated with the input of nitrate during tidal innundation.

SUMMARYThe effects of the availability of inorganic nitrogen on in vivo activity of nitrate reductase in Sphagnum capillifolium (Ehrh.) Hedw. and Sphagnum fuscum (Schimp.) Klinggr. were investigated in both field and laboratory experiments. Nitrate reductase activity per unit mass was highest in the capitulum and declined down the stem. Nitrate reductase activity was substrateinducible, maximum and total activities induced being correlated with the natural logarithm of nitrate concentration in the range 0 to 10 mM. Repeated treatment with 05 mM nitrate did not give rise to repeated similar patterns of nitrate reductase activity, induction decreasing with later successive applications. An initial ammonium treatment in a field experiment did not affect nitrate-induced enzyme activity. However, repeated applications of ammonium caused marked changes in the patterns of induction and a general repression of total nitrate reduction. This repression may result from both an effect of ammonium on nitrate uptake and a direct effect of the accumulation of ammonium or ammonium assimilation products on the enzyme itself. Repression by repeated nitrate supply may also operate in a similar way. The effects of ammonium and nitrate supply on the induction of nitrate reductase were shown to interact.The initial rate of induction of nitrate reductase activity increased with temperature, but prolonged nitrate supply resulted in highest enzyme levels at lowest temperatures. This effect may be due to greater rates of activity giving rise to greater repression by the products of nitrate assimilation at the higher temperatures. The results are discussed in relation to the use of nitrate reductase activity as a monitor of atmospheric nitrate deposition in ombrotrophic bryophyte species.