SUMMARYThe tolerance of plants to toxic metals is frequently measured by comparing rates of root growth in culture solutions with and without the addition of the metal, but many variants of the technique are available. Toxicity is often reduced by the presence of other ions, particularly calcium. Control growth rates may be measured beforehand on the same roots or in parallel on a duplicate set. With a single toxic concentration tolerance is defined as the ratio of the two growth rates; with a range of concentrations the slope of the regression of growth on concentration may be used, but there are some complications with stimulation at low concentrations. Probit analysis may be valuable. There is good evidence that tolerance differences are largely of genetical origin.

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The abundance of transcripts of cab‐7 and cab‐3C, which code for the chlorophyll a/b binding proteins of the light‐harvesting complexes I and II, respectively, and the abundance of transcripts of Rca, which encodes Rubisco activase, were reduced in tomato plants exposed to high CO2 for up to 9d, whereas the abundance of mRNA from psa A–psa B and psb A, which encode the proteins of the core complex of PSI and the D1 protein of PSII, respectively, and the abundance of glycolate oxidase, which is involved in photorespiration, were not affected. However, the abundance of the transcript for the B subunit of ADP‐glucose pyrophosphorylase was increased after 1 d at elevated CO2. The chlorophyll a/b ratio decreased significantly over 9 d of exposure to elevated CO2. The responses of the nuclear genes to high CO2 were enhanced when leaves were detached so as to deprive them of any major sink. The responses of these transcripts to high CO2 were mimicked when sucrose or glucose was supplied to the leaf tissue, whereas acetate or sorbitol had no effect. Carbohydrate analyses of leaves grown in high CO2 or supplied with sucrose revealed that major increases occurred in the amount of glucose and fructose. Based on these and other published data, a molecular model involving the repression or activation of the transcription of nuclear genes coding for chloroplast proteins by photosynthetic end‐products is proposed to account for photosynthetic acclimation to high CO2 in tomato plants and other species.

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Summary Seedlings of Betula spp. were grown in conjunction with isolates of Amanita muscaria Hooker and Paxillus involutus Fr. under aseptic conditions. Mycorrhizas were established in sterilized peat and vermiculite to which nutrient media had been added. Zinc was supplied at various levels and growth of the host was assessed after eight weeks. The mycorrhizas increased the tolerance to Zn of both tolerant and non‐tolerant Betula. This was coupled with a reduction in the translocation of zinc to the shoots of Betula, but an accumulation of zinc in the mycorrhizas.

SUMMARYSamples of six species of grass from a salinized pasture were grown in culture solution with varying concentrations of sodium chloride added. The rates of root growth showed a high correlation with the conductivity of water extracts of the soils of origin. The results can be interpreted as evidence for strong selection for the appropriate level of response to salt at each point on the salinity gradient. Salt tolerance was found to he highly heritable in Festuea rubra. INTRODUCTIONMuch work has been done on the adaptation of local populations of grass species to toxic soils (Antonovics, Bradshaw and Tumer, 1971;Davies and Snaydon, 1973). This has usually involved one species at a time and the material has often been collected from sites situated far apart and differing in many factors in addition to the one being studied. Davies and Snaydon's work on the Park Grass at Rothamsted was distinguished by its use of a single site to which a variety of treatments had been applied in close proximity, but they too were studying a single species. The discovery of a site at Upton Warren, near Droitwich, where brine pumping had led to subsidence and flooding in what had previously been permanent pasture afforded the opportunity to study the selective effect of salt on a whole community. The hollows have gradually filled with saline water and for the past 20 or 30 years the water level and presumably the salinity have fluctuated irregularly with evaporation and rainfall. The surrounding vegetation now shows the ecological effects of this regime.Many of the original species have heen completely eliminated from the vicinity of the pools. On the other hand new species such as Puccinellia distans* and Spergularia marina have invaded the area. This paper is concerned with the varying response to salt found in those grass species which have been able to adapt to the new conditions. MATERIAL AND METHODSA 10-m grid was laid down over an area of about 100-m X 200-m and where possible a plant of each of the following species was collected from near each intersection, together with a soil sample: Agropyron repens; Agrostis stolonifera; Festuea nt bra; Hordeum seealinum; Lolium perenne:Puccinellia distans.

Aseptic cultures of Paxillus involutus Fr. and clones of Betula pendula Roth, and B. puhescens Ehrh. were used in experiments, involving X-ray microanalysis and split-plate culture, to investigate the mechanism of ectomycorrhizal amelioration of zinc toxicity to Betula. Results imply that as the fungal mycelium colonizes fresh soil, zinc is adsorbed to the surface of hyphae, thereby lowering the concentration of zinc in the soil solution surrounding roots. In consequence, less zinc is taken up, and growth is better than in the non-mycorrhizal condition. The results also show that the metal may be adsorbed to electronegative sites in the hyphal cell walls and extra-hyphal, polysaccharide slime. The possible dual role of this slime in iungn^/Betula compatibility and the amelioration mechanism is discussed.

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