Phototropic responses of potato under conditions of continuous light and subsequent darkness

Abstract

The phototropic (PT) bending capacity of potato plantlets in vitro was studied under conditions of continuous light (CL) and subsequent darkness (CLD). In CL, diurnal changes characteristic for the 16/8 h LD photoperiod were absent and the response, although complex, was constant at any time of the day. Graphic representation of shoot bending indicated the presence of two separate, partly overlapping PT responses. The first was a fast, time-limited process (FPR) with a 20 min lag phase and a maximum fixed at 70 min, followed by a steady bending process (SPR) starting 50–90 min after the beginning of blue light (BL) stimulation. The FPR response rapidly deteriorated in darkness and after 60–120 min it was replaced with BL augmented circumnutations. SPR was a constant feature in darkness and while both FPR and circumnutations were absent when darkness exceeded 120 min, the SPR response remained present in darkness irrespective of its duration. The abrupt end of circumnutations marked the beginning of the synchronized SPR bending response. Darkness lasting 120 min was also a pivotal point after which the PT capacity started to recover. After 8 h of CLD, the PT capacity was fully restored. The FPR response reappeared at dawn when the light was turned on. Potato plantlets also manifested fast solar tracking movements, correcting shoot position in relation to the horizontal displacement of the BL source. Solar tracking movements contrary to circumnutations comprised shoot torsion twisting (twining). Our results demonstrate that, in potato shoots, multiple PT responses are present and operational simultaneously.