In two phytotron experiments with different potato cultivars (experiment 1: cv. Atzimba and Van Gogh; experiment 2: cv. Spunta and Desiree), a study was made of the effects of temperature (15-27 degrees C) and photoperiod (long day, LD, or short day, SD) on sympodial development and shoot, leaf and flower production. In experiment 1, data on the entire shoot were collected, whereas in experiment 2 only leaf and flower production of the main and secondary stems were measured. In experiment 1, increasing the temperature in both SD and LD treatments, and increasing the photoperiod at 15 degrees C increased the number of lateral shoots, and the numbers of inflorescences and leaves of the sympodium and of the entire shoot. In experiment 2, the number of flower primordia and survival of flower primordia of individual inflorescences increased with the photoperiod and with temperature up to 23 degrees. At 27 degrees, flower development was suppressed. Total leaf and flower production per plant were largely a function of lateral shoot production. Increasing temperature and photoperiod increased the number of leaves of individual shoots in most treatments. However, the effects on leaf and flower production of individual shoots were relatively small, except for the effect of a temperature increase from 23 to 27 degrees in experiment 2. The photoperiodic response of the time till flowering of individual shoots was facultative SD or daylength-neutral, depending on the cultivar and stem position.