Like those of many horticultural crop species, the growth and leaf gas exchange responses of carrot (Daucus carota L.) to salinity are poorly understood. In this study ion accumulation in root tissues (periderm, xylem and phloem tissues) and in leaves of different ages was assessed for carrot plants grown in the field with a low level of salinity (5.8 mM Na+ and 7.5 mM Cl-) and in a glasshouse with salinity ranging from 1-80 mM. At low levels of salinity (1-7.5 mM), in both the field and glasshouse, carrot leaves accumulated high concentrations of Cl- (140-200 mM); these appear to be the result of a high affinity for Cl- uptake and a low retention of Cl- in the root system. However, Cl- uptake is under tight control, with an 80-fold increase in external salinity resulting in only a 1.5-fold change in the Cl- concentration of the shoot and no increase in the Cl- concentration of the root xylem tissue. In contrast to Cl-, shoot Na+ concentrations were comparatively low (30-40 mM) but increased by seven-fold when salinity was increased by 80-fold. Growth over the 56-d treatment period in the glasshouse was insensitive to salinity less than 20 mM, but at higher concentrations the yield of carrot tap roots declined by 7 % for each 10 mM increase in salinity. At low levels of salinity the accumulation of high concentrations of Cl- (150 mM) in carrot laminae did not appear to limit leaf gas exchange. However, photosynthesis and stomatal conductance were reduced by 38 and 53 %, respectively, for plants grown at a salinity of 80 mM compared with those grown at 1 mM. Salinity-induced reductions in both p(i) and carbon isotope discrimination (Delta) were small (2.5 Pa and 1.4 parts per thousand, respectively, at 80 mM) indicating that the reduction in photosynthesis was only marginally influenced by CO2 supply. At a salinity of 80 mM the photosynthetic capacity was reduced, with a 30 % reduction in the CO2-saturated rate of photosynthesis (A(max)) and a 40 % reduction in both the apparent rate of RuBP-carboxylase-limited CO2 fixation (V-cmax) and the electron transport rate limiting RuBP regeneration (J(max)). This study has shown that carrot growth and leaf gas exchange are insensitive to the high leaf Cl- concentrations that occur at low levels (1-7 mM) of salinity. However, growth is limited at salinity levels above 20 mM and leaf gas exchange is limited at salinity levels above 8 mM. (C) 2002 Annals of Botany Company.