P. tuberosus, a native species of the Amazon region, was cultivated under different thermal regimes and photoperiods in an attempt to relate these stimuli to flower initiation and tuberous root formation. It is shown that P. tuberosus is an intermediate-day plant (flowers only under photoperiods above 9 h and below 16 h). With regard to tuberization P. tuberosus may be considered a short-day plant: tuberization occurs only in photoperiods below 16 h. High thermal regimes 30/25 °C (13 h day/11 h night) delay and reduce flowering and completely inhibit tuberous root formation. Thermal regimes of 25/20 °C and 20/15 °C (13 h day/11 h night) were the most suitable for flowering and tuberization. © 1989 Annals of Botany Company.644411414