Interaction of [(3)H]gibberellin A 1 with a sub-cellular fraction from lettuce (Lactuca sativa L.) hypocotyls : I. Kinetics of labelling.

The distribution of [(3)H]gibberellin A1 ([(3)H]GA1) amongst sub-cellular fractions from lettuce (Lactuca sativa L.) hypocotyl sections was examined by differential centrifugation. Material pelleting at 2000 g (2KP) accounted for 2-5% of the total tissue radioactivity and no other significant interactions were detected. After rigorous washing procedures, the 2KP fraction was found to consist mainly of cell wall material and the radioactivity remained associated during sucrose density-gradient centrifugation. 2KP-labelling increased linearly with time and was only partially removed by chasing for up to 40 h in the presence of unlabelled GA1. A linear plot was obtained for the relationship between external concentration and the log of 2KP GA1 content with a maximum percentage incorporation at 10(-5) mol 1(-1). Incorporation of radioactivity was reduced by chasing with unlabelled GA1 or GA9, but not by GA8. Labelling of the 2KP fraction increased uniformly with temperature between 15°C and 30°C but showed little change below 15°C. Growth rate and 2KP labelling were highly correlated (r=0.989). The possible significance of these findings is discussed.