Temperature dependence of the gibberellin response in lettuce hypocotyls.

The relationship between temperature and sensitivity to gibberellin A3 (GA3) was studied in lettuce seedlings (Lactuca sativa L. cv. Arctic). Dose/response curves for hypocotyl elongation (10(-4) mol l(-1) to 10(-8) mol l(-1)) were constructed for a range of temperatures and the slope of the linear portion of the plots used as an indication of the sensitivity to GA3. Hypocotyls were unresponsive to GA3 below 13°C but above this temperature sensitivity increased linearly. Plots of growth rate against temperature had inflexions between 12°C and 13°C, with slopes above this point which increased with increasing GA3 concentration. The Q10 value for response increased in a similar manner. Reaction rates of NAD-dependent malate dehydrogenase and peroxidase extracted from hypocotyls varied linearly with temperature whilst nonspecific tetrazolium reduction, a membrane based activity, showed an abrupt rate change above 14°C. Pre-exposure to GA3 had no effect on the temperature responses of soluble or particulate enzymes.