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Ericaceous plants grow preferentially on soils with a low pH, and generally perform poorly in areas with high pH calcareous soils. The reasons for the calcifuge behaviour of species of one genus, Erica, was studied in a calcareous rendzina collected from the South Downs, with a view to identifying physiological characteristics which could be used to select for improved lime-tolerance in the Ericaceae. Varieties of E. carnea and E. x darleyensis were shown to be relatively resistant to chalk-soil compared with varieties of E. vagans. In soil-based screening experiments, E. vagans 'Lyonesse' and 'Mrs D. F. Maxwell' rapidly developed severe foliar chlorosis, or lime-induced chlorosis, and their growth in soil was depressed by up to 7-fold compared with plants in an acid (pH 4) peat compost. E. carnea and E. x darleyensis varieties were resistant to chlorosis and remained green in chalk soil. The addition of nutrients (NPK and trace elements) increased the growth of the lime-resistant varieties in both peat and chalk soil. In nutrient solution, the source of nitrogen (nitrate or ammonium ions) had little effect on the growth of either lime-resistant or lime-sensitive varieties, although some varieties performed better when a component of the N was in the form of N03'. In an Fe-free nutrient solution, 1 mM NaHCO3 markedly reduced the root and shoot growth of both lime-tolerant and sensitive varieties. The clear differences between lime-resistant and lime-sensitive varieties found in soil experiments were not apparent with NaHCO3 in solution culture indicating that low concentrations of bicarbonate in nutrient solution may not be an appropriate method of screening for improved lime-tolerance in Erica. Bicarbonate and high pH in nutrient solution depressed the uptake of the radioisotopes 59Fe and 86Rb in both lime-resistant and lime-sensitive cultivars. Iron deficiency, induced by withholding Fe in nutrient solution, resulted in an elevated rate of root Fe(III)EDTA (ethylenediaminetetraacetic acid) reduction compared with Fesufficient plants. The maximum induced Fe(III)EDTA reduction rate (Vn,. ) of limeresistant cultivars (0.96-0.107 pmol. g"' FWT. h'') was lower than that of lime-sensitive varieties (0.125-0.404 tmol. g' FWT. h-1), suggesting that roots of lime-sensitive varieties have a greater capacity for Fe absorption than lime-tolerant varieties. Spraying or watering with Fe chelate (FeEDDHA - ethylenediamine di(ohydroxyphenylacetic acid)) remedied chlorosis in the lime-sensitive varieties and increased leaf chlorophyll by up to six fold and whole plant biomass by up to two fold. Chlorosis was associated with a reduced concentration of o-phenanthroline-extractable ('active') Fe, and a high concentration of total Fe, compared with green tissue. It is concluded that in chalk soil, lime-induced chlorosis, caused by the immobilisation of Fe in the shoot and a reduction in 'physiologically active' Fe in the leaf tissue, characterises lime-sensitive heather varieties. Resistance to chalk soil in Erica appears not to be related to root physiology. Rather the ability to transport and distribute Fe within the shoot under calcareous conditions is a major factor contributing to limeresistance in heathers.

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