Hormonal nature of seed responses to fluctuating temperatures in Cynara cardunculus (L.)

Cynara cardunculus (L.) seeds require incubation at fluctuating temperatures to terminate dormancy. In this study, we analysed the physiological mechanisms underlying such a requirement, focusing on the role of abscisic acid (ABA) and gibberellin (GA). As a conceptual framework, we considered the possibility that fluctuating temperatures and light trigger a similar set of hormonal processes after stimulus perception. To test this possibility, we (1) carried out hydrotime analysis of germination in seeds exposed to fluctuating temperatures (25/15 degrees C) and constant temperature (20 degrees C) with or without gibberellin (GA(3)) or red light; (2) determined the responses of seeds incubated at fluctuating or constant temperature to ABA, GA(3), fluridone, an inhibitor of ABA biosynthesis, and paclobutrazol, an inhibitor of GA biosynthesis; and (3) determined the ABA content of seeds incubated at fluctuating or constant temperature. Incubation at 25/15 degrees C or 20 degrees C in the presence of GA3 reduced the mean base water potential [psi(b)(50)] of the population to a similar extent, compared to that observed with seeds incubated at 20 degrees C without GA(3). Irradiation with red light also reduced psi(b)(50) to a lesser extent than incubation in the presence of GA(3). At all concentrations tested, exogenously applied GA(3) did not promote germination of seeds incubated at 25/15 degrees C. However, paclobutrazol inhibited germination, suggesting that fluctuating temperatures terminate dormancy through de novo GA biosynthesis. Fluctuating temperatures enhanced seed germination in the presence of ABA, but ABA content did not differ between seeds incubated at fluctuating and constant temperatures. This study provides clear evidence for the involvement of hormonal regulation in dormancy termination by fluctuating temperatures.