THE PHOTOTROPIC RESPONSE OF TRITICUM-AESTIVUM COLEOPTILES UNDER CONDITIONS OF LOW GRAVITY

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 1 g treatments (10-20 min), except for the response to 300 mu mol 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 1 g showed a plateau at 120 min, in contrast to the single maximum at 9 g. The fluence-response relationship was similar in both flight and ground controls, Only the responses to 4 and 6 mu mol m(-2) showed significantly greater curvatures at 9 g, This contrasts with previous clinostat studies, which reported substantial response enhancement at all fluence levels.