REGULATION OF TUBER INDUCTION IN POTATO BY DAYLENGTH AND PHYTOCHROME

Many potato species require short days for the induction of tuberization and will not tuberize if maintained in photoperiods longer than their critical daylength, or if the dark period of a short day regime is interrupted by a light treatment (what is termed a night-break). Phytochrome is known to be involved in photoperiodic responses such as tuberization and flowering because the effects of a red light night-break can be reversed by a far-red light treatment given immediately afterwards-a well- documented characteristic of phytochrome responses. Which of the several phytochromes that are known to exist is/are responsible for the photoperiodic response, however, has not been shown. Transgenic short-day requiring Solanum tuberosum ssp. andigena plants have been produced which have much lower levels of phytochrome B as a result of antisense technology. These plants are now able to tuberize in long days as well as short days. The reduction in phytochrome B levels has thus led to the loss of the photoperiodic control of tuberization in these plants and implicates phytochrome B in this response. Preliminary grafting experiments have indicated that a graft-transmissible tuberinducing signal is produced in the transgenic plants in long days as well as short days, whereas in wild-type plants this signal is only produced in short days. The existence of a possible inhibitor of tuberisation is also discussed.