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

Wheat (Triticum aestivum L, cv Broom) coleoptiles were stimulated by u nilateral blue light pulses, the duration of which varied between 3 s and 30 min, under microgravity conditions on the IML-1 Spacelab missio n and in ground controls, The stimuli covered first positive, indiffer ent and second positive response regions, When phototropic responses w ere observed under low-gravity conditions, slightly fewer seedlings re sponded 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 res ponse in flight plants followed a 130 min latent period, while no resp onse was observed in 1 g plants, First positive responses at Og were s lightly 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 plate au at 120 min, in contrast to the single maximum at 9 g. The fluence-r esponse relationship was similar in both flight and ground controls, O nly the responses to 4 and 6 mu mol m(-2) showed significantly greater curvatures at 9 g, This contrasts with previous clinostat studies, wh ich reported substantial response enhancement at all fluence levels.