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Circadian regulation of sunflower heliotropism, floral orientation, and pollinator visits

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Science  05 Aug 2016:
Vol. 353, Issue 6299, pp. 587-590
DOI: 10.1126/science.aaf9793
  1. Fig. 1 The circadian clock regulates solar tracking.

    (A) Nighttime reorientation of stem and shoot apex. (B) Disruption of solar tracking by daily evening 180° rotation of experimental plants results in a 7.5% reduction in biomass (left) and an 11% reduction in leaf area (right) compared with 360°-rotated control plants [mean ± SEM (error bars), n = 80 plants, P = 0.01 (biomass) and 1 × 10–6 (leaf area), mixed effect linear regression models]. Numbers refer to leaf pairs. C, control; R, rotated. (C) Changes in orientation anticipate dawn and dusk transitions in both fall (left y axis) and summer (right y axis) [mean ± SEM, n = 10 plants]. (D) Persistence of rhythmic movements after transfer from field to continuous light and temperature conditions. In (C) and (D), white areas denote daytime; dark and light gray areas represent night and subjective night, respectively [mean ± SEM, n = 3 plants]. (E) The onset of “eastward” movement in a growth chamber equipped with four directional lights is consistently phased with lights being turned off in 24-hour T-cycles (left and right) but is erratic in 30-hour T-cycles (center). Time zero indicates dawn [(C) and (D)] or the beginning of the first T-cycle (E). Angles <90° and >90° represent inclination toward east and west, respectively. White areas, day; gray, night.

  2. Fig. 2 Solar tracking is driven by opposing growth rhythms on the east and west sides of stems.

    (A) Changes in stem elongation and (B) the angle of curvature of the shoot apex relative to the horizon in control (green) and gibberellin-deficient dw2 plants (purple). dw2 mutants were treated twice with 2 μM of the gibberellin GA3 (gibberellic acid), with the last treatment on day 0. Data in (A) were fitted to centered second-order polynomial equations to aid visualization. (C) Timing of elongation for east and west sides of stems of solar tracking field-grown plants [mean ± SEM, n = 42 plants]. (D) Timing of stem elongation of plants growing vertically in a top-lit environmental control chamber [mean ± SEM, n = 9 plants]. Asterisks indicate that the east and west sides of the stem (C) or the daytime and nighttime means (D) significantly differ (*P < 0.05, Student’s t test). (E to H) Differential gene expression on the east and west sides of solar tracking stems assessed by quantitative reverse transcription polymerase chain reaction (*P < 0.05, **P < 0.01, ***P < 0.001; orientation by time-point effect, single linear mixed model). Time zero indicates dawn [(C) and (E) to (H)] or when lights were turned on (D). White areas, day; gray, night.

  3. Fig. 3 Eastward orientation of sunflower heads after anthesis is due to gating of light responses by the circadian clock and enhances pollinator visits.

    (A) Amplitude of solar tracking and changes in stem growth of mature plants nearing floral anthesis. Petals were first observed during day 5 (mean ± SEM, n = 2 plants). (B) Stem curvature of juvenile plants entrained in 16L:8D cycles and then exposed to unidirectional blue light for 4 hours at the indicated times (mean ± SEM, n = 11 plants). Different letters indicate significantly different curvature values in pairwise comparisons (P < 0.05, Student’s t test). (C) FLIR images of east-facing (E) and west-facing (W) floral disks at hourly intervals. (D) Pollinator visits to east- and west-facing plants (mean ± SEM, n = 4 days, eight plants per treatment) during 45-min intervals at three times of day. (E) Temperature (mean ± SEM, n = 6 plants per condition) of sunflower disks with east or west (with or without supplemental heat) orientations. (F) Pollinator visits in the morning to the inflorescences with temperatures reported in (E). In (D) to (F), *P < 0.05, t test with unequal variances; comparisons are for: (D) E versus W, (E) W versus W + heat, and (F) all three values. Time zero indicates dawn [(A) and (C) to (E)] or lights on (B).

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