Photo- and Gravitropic Bending of Potato Plantlets Obtained In Vitro from Single-Node Explants

Etiolated and light-grown plantlets obtained from potato shoot cultures were shown to perform vigorous tropic movements. Unilateral blue irradiation actively induced phototropic curvature of the shoots toward the light source, although etiolated plantlets required ten times longer stimulation than the light-grown plantlets to achieve a 90° angle. The fluence requirements for induction of second positive phototropism (PT) of light-grown plantlets spanned almost five orders of magnitude (~30–1.7?×?10⁵ μmol/m²). Upon responding to unilateral blue light by curving, plantlets entered the process of straightening after irradiation ended. Straightening also occurred in plantlets placed on a clinostat but it was of lower magnitude. Compared to early-morning and day-time hours, plantlets exhibited a significantly lower PT response in the late afternoon (5 p.m.) and gravitropic (GT) response at the end of the day (11 p.m.), suggesting that these responses may be under the control of circadian rhythms. GT was also recorded for both light-grown and etiolated plantlets. Ninety-degree stimulation, used to induce GT in etiolated plantlets, needed to be 50?% longer than stimulation used for light-grown plantlets to induce a similar response. Straightening was also recorded for the shoots that exhibited GT but was smaller when plantlets were placed on a clinostat compared to straightening exhibited by those plantlets left standing in an upright position for 2?h.