The phototropic response of Triticum aestivum coleoptiles under conditions of low gravity

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Abstract
Wheat (Triticum aestivum L. cv Broom) coleoptiles were stimulated by unilateral blue light pulses, the duration of which varied between 3 s and 30 min, under microgravity conditions on the IML‐1 Spacelab mission and in ground controls. The stimuli covered first positive, indifferent and second positive response regions. When phototropic responses were observed under low‐gravity conditions, slightly fewer seedlings responded compared with 1 g conditions. The latent period was similar in flight and 1g treatments (10–20 min), except for the response to 300,μmol m−2 in the indifferent response region, where a positive response in flight plants followed a 130 min latent period, while no response was observed in 1 g plants. First positive responses at Og were slightly enhanced, both in magnitude and in duration, compared to the 1 g controls, but not to the extent predicted by clinostat studies. The response kinetics in the second positive region at 1g showed a plateau at 120 min, in contrast to the single maximum at Og. The fluence‐response relationship was similar in both flight and ground controls. Only the responses to 4 and 6μmolm‐2 showed significantly greater curvatures at 9g. This contrasts with previous clinostat studies, which reported substantial response enhancement at all fluence levels.