Thermodormancy in Celery Seeds and its Removal by Cytokinins and Gibberellins

Imbibition of celery (Apium graveolens L.) seeds at 32°C for up to 96 h lowered the upper temperature limit for germination. If this high temperature treatment was given in the light, these seeds germinated slightly earlier than those treated in the dark although the final percentage germination was similar for both treatments. The inhibitory effect of the high temperature treatment was completely removed by allowing the seeds to imbibe in a mixture of the gibberellins A₄ and A₇ (GA_4/7) and partially removed by the cytokinin N⁶-benzylaminopurine (BA). GA_4/7 was less effective when added before rather than after the high temperature treatment, whereas the opposite was true of BA. At constant temperatures more GA_4/7 was required to promote germination as the temperature was raised but addition of BA reduced the concentration of GA_4/7 required. A model is proposed for the control of celery seed germination by light and temperature through the action of endogenous cytokinins and gibberellins.