Interaction with gravitropism, reversibility and lateral movements of phototropically stimulated potato shoots

Abstract

Phototropic (PT) and gravitropic (GT) bending are the two major tropic movements that determine the spatial position of potato shoots. We studied PT bending of potato plantlets grown under long-day photoperiods in several prearranged position setups providing different interactions with the GT response. Starting with the standard PT stimulation setup composed of unilateral irradiation of vertically positioned shoots, experiments were also done in antagonistic and synergistic setups and in treatments with horizontal displacement of the light source. In the standard setup, PT bending suppressed the GT bending, which could occur only if the PT stimulation was cancelled. The antagonistic position, with phototropism and gravitropism attempting to bend shoots in opposite directions, showed phototropism and gravitropism as independent bending events with the outcome varying throughout the day reflecting diurnal changes in the competence of individual tropic components. Whilst gravitropism was constant, phototropism had a marked daily fluctuation of its magnitude with a prominent morning maximum starting an hour after the dawn in the growth room and lasting for the next 6 h. When phototropism and gravitropism were aligned in a synergistic position, stimulating shoot bending in the same direction, there was little quantitative addition of their individual effects. The long period of morning PT bending maximum enabled multiple PT bending events to be conducted in succession, each one preceded by a separate lag phase. Studies of secondary PT events showed that potato plantlets can follow and adjust their shoot position in response to both vertical and horizontal movements of a light source. PT bending was reversible, since the 180° horizontal change of a blue light (BL) source position resulted in reversal of bending direction after a 20-min-long lag phase.

Keywords: Gravitropism; Phototropism; Reversibility; Solanum tuberosum L.; Tropic interactions.

Fig. 1

Positional setup arrangements used in the study. The standard position setup (STA) is the standard position of the phototropic (PT) stimulation used in most tropic studies. Culture flasks and plantlets within were in the vertical position, and the unilateral blue light (BL) of ~470 nm was coming at 24 μM m⁻² s⁻¹. Prior to PT stimulation, cultures were grown in a long-day growth room with fluorescent lamps positioned above the cultures, providing 70 μM m⁻² s⁻¹, 4500° K. Standard gravitropism was obtained by placing culture flasks with plantlets in a horizontal position. The antagonist position setup was obtained by placing the culture flasks with plantlets horizontally, and a BL source was positioned beneath the culture flasks illuminating cultures at 24 μM m⁻² s⁻¹. The synergistic setup position was also obtained by placing culture flasks with plantlets horizontally; only the BL source illuminates cultures from above. The lateral displacement setup (LAT) is a modification of the STA position in which some 50–60 min after the beginning of BL stimulation, the BL source was laterally displaced to some desired angle, such as 10°, 15°, 30°, 45°, 60° or 90° in relation to the original BL source position. The reversion position setup (REV) is a special case of LAT where the lateral displacement angle is 180°. Arrows indicate direction of tropic forces; shoots bend in the opposite direction. Lateral displacement was done by rotating the culture flasks in front of the BL source on a simplified turntable. REV and LAT positions have two distinct lag phases; the second one starts after the lateral light displacement

Fig. 2

a In the synergistic setup position (SYN), phototropism and gravitropism join forces to bend the shoot upward. b In the antagonist setup position (ANT), the two tropic forces work against each other, and the outcome is variable depending mostly on the time of day. SYN and ANT graphs present mean values for shoot tip curvatures ± standard error, n ≥ 30 shoots per treatment

Fig. 3

Formation of S-shaped shoots in a culture vessel with plantlets mounted in ANT setup position started 4 h after the dawn. Culture vessel diameter 62 mm

Fig. 4

Shematic presentation of shoot tropic bending in the ANT position mounted at three different times of day (dawn, dawn +4 h and dawn +12 h). Shoots are initially in horizontal position. Downward bending denotes phototropic (PT) and upward bending gravitropic (GT) response of shoot tips and middle segments

Fig. 5

a Phototropic (PT) bending in the standard setup position subdues and prevents manifestation of the gravitropic bending. At the end of 120 min PT bending, plantlets can maintain the obtained horizontal shoot position for hours. However, if the blue light (BL) is turned off, then PT bending decays as shoots enter straightening. Turning the BL on after 25 min of darkness restarts PT bending but only after a new lag phase. Graphs present mean values ± stardard errors, n ≥ 30 shoots per treatment, b Phototropic (PT) bending in the lateral displacement setup induces a secondary lag phase after lateral displacement of the light source. The figure presents percentages of shoots ending secondary lag phase and entering lateral PT bending after 90° light displacement at 5 min intervals, n = 63 plantlets

Fig. 6

Lateral displacement setup position bending of a group of plantlets in their culture vessel toward blue light (BL) coming from the left side (arrows in a, d and g). Following the initial lag phase, shoots bend to the left (b, c), but after 60 min, the turntable with the culture flasks is turned 90° clockwise (d), starting the secondary lag phase. For the first 15 min after lateral light reallocation, (e) shoots bend by inertia in the wrong direction, but 5 min later (f) they start to correct their position until they again get aligned with the new BL source position (g, h). Culture vessel diameter 62 mm

Fig. 7

a Following lateral displacement of the light source, plantlets adjust the spatial position of their shoots by phototropic (PT) bending, both in the vertical and horizontal planes. The graph presents rates of horizontal position corrections after 10°, 15°, 30°, 45°, 60° and 90° angle displacements of the BL source following a 60-min long standard position setup PT bending pre-treatment. Zero at the X axis is time of light source lateral displacement. Points in the graphs are mean values of shoot tip lateral angle correction ± stadard error, n ≥ 30 shoots per treatment, b In the reversion setup position, phototropic (PT) bending was first done for 50 min toward the blue light (BL) source on the left, which was then displaced horizontally to the opposite right side (180°). A secondary lag phase appeared lasting until the shoots started to revert their bending direction. Bending to the second light source continues in the same vertical plane but in the opposite direction. To differentiate the two PT bending stages presented in the graph, REV bending was shown with negative (–) values for bending to the first (left-positioned) light source and with positive (+) values for bending to the second (right-positioned) BL source. Graph points are mean values for shoot tip curvatures ± standard error, n ≥ 30 shoots per treatment

Fig. 8

Phototropic bending (PT) in a culture vessel with plantlets mounted in the reversion setup position. Plantlets were bending first to the left side (a, b), and then 50 min later the blue light source was moved to the opposite (right) side. For the next 20 min, corresponding to the secondary lag phase, c shoots continue to bend to the left by inertia, and then they revert their bending direction (d–f), entering a long stretch of linear bending. Culture vessel diameter 62 mm