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.