Role of Gibberellins in the Environmental Control of Stem Growth in Thlaspi arvense L.

Field pennycress (Thlaspi arvense L.) is a winter annual that requires a cold treatment for the induction of stem elongation. An inbred line was selected in which no stem elongation was observed in plants grown for 6 months at 21 degrees C regardless of the prevailing photoperiod. Increased exposure time of plants grown initially at 21 degrees C to cold (2 degrees C) induced a greater rate of stem elongation when the plants were returned to 21 degrees C. Moreover, longer cold treatments resulted in a greater maximum stem height and reduced the lag period for the onset of measurable internode elongation. The optimal temperature range for thermoinduced stem growth was broad: rates of stem growth in plants maintained for 4 weeks at either 2 degrees or 10 degrees C were virtually identical. However, a 4-week thermoinductive treatment at 15 degrees C resulted in a greater lag period for the initiation of stem elongation and a decreased growth rate. The rate of cold-induced stem elongation was greater in plants subjected to long days than for plants subjected to short days following the cold treatment. Thus, photoperiod does not control the induction of stem elongation, but does regulate stem elongation in progress. Exogenous gibberellin A(3) (GA(3)) was able to substitute for the cold requirement, but elicited a greater response in plants maintained under long days than short days. This indicates that photoperiod influences the plant's sensitivity to GAs. The GA biosynthesis inhibitor, 2-chloroethyltrimethyl ammonium chloride, inhibited low temperature-induced stem elongation, and this inhibition was completely reversed by exogenous GA(3). These results suggest that cold-induced stem elongation in field pennycress is mediated by some change in the endogenous GA status.