Daily Changes in the Competence for Photo- and Gravitropic Response by Potato Plantlets

Competence for phototropic (PT) and gravitropic (GT) bending by potato plantlets grown in vitro manifests regular daily changes indicating possible involvement of circadian regulation. Unilateral stimulation of plantlets with blue light at dawn resulted in moderate PT response regarding both attained curvature and long lag phase. The PT response was the strongest between 8:00 and 12:00 h. Throughout the afternoon and in the evening, bending rate and maximal PT curvature declined significantly until 23:00 h. The GT response was fastest and strongest for plantlets stimulated early in the morning and late in the evening. During the rest of the day, GT competence did not change much apart from a minimum at 15:00. In conditions comprising either prolonged day or prolonged night, plantlets appeared to maintain rhythmicity of competence for PT and GT at least in the short-term. Introduction of a dark period prior to the tropic stimulation at 11:00 h when both PT and GT responses were strong resulted in the opposite effect: PT was depressed, and GT was enhanced. There was a time threshold of 60 min for the duration of the dark period so the plants can sense interruption in the daylight. Levels of relative expression of a PHOT2 gene indicate rhythmic daily changes. The PHOT2 gene was present at high levels during morning hours and late in the evening. As the mid-day and the afternoon hours approached, PHOT2 expression decreased and reached a daily minimum at 17:00 h. We believe that our data offer strong support for the conclusion that there is an involvement of circadian rhythms in control of both PT and GT.