The effect of low root-zone temperature on nodulation of Lupinus angustifolius [L.] cv. Yandee was studied using glasshouse experiments in which the effects of temperature on nodule initiation and subsequent nodule development could be assessed separately. Low temperature (7 and 12degreesC compared with 25degreesC) reduced the growth of both uninoculated plants supplied with adequate mineral N and inoculated plants reliant on fixation alone for their N. However, even at 25degreesC, growth of inoculated plants compared with plants supplied with mineral N was limited, and at lower temperatures nodulation was severely inhibited. The most sensitive stage to low root-zone temperature was nodule initiation and there appeared to be a critical temperature between 7 and 12degreesC at which initiation did not take place. Increasing the number of bacteria in inocula (from 5 x 10(3) to 5 x 10(7) viable cells/mL) did not overcome inhibition. A number of diverse cultivars of L. angustifolius showed the same response as cv. Yandee. Low temperature inhibition of nodule initiation could be overcome by addition of culture solution collected from around the roots of symbioses established at 25degreesC. The culture solutions were only effective if the roots at 25degreesC were inoculated or, if collected from around uninoculated roots of plants grown with mineral N, they were first exposed to a Bradyrhizobium suspension and then sterilised before addition to cultures at low temperature. The data indicate that both plant and bradyrhizobial factors are required for nodule initiation and that exudation of plant factors at low root-zone temperature is insufficient to stimulate production of the nodulation factors from Bradyrhizobium. At 25degreesC, the nodulation zone of lupin roots bore many fractures in the epidermis and showed a high frequency of free root cap border cells, as well as a distinct matrix of extracellular material. These features were significantly reduced at 12degreesC and essentially absent at 7degreesC, indicating that at low temperature bacterial entry may be restricted.