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 (25/158C) and constant temperature (208C) 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 25/158C or 208C in the presence of GA 3 reduced the mean base water potential [c b (50)] of the population to a similar extent, compared to that observed with seeds incubated at 208C without GA 3 . Irradiation with red light also reduced c 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 25/158C. 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.

Field experiments were carried out at the Facultad de Agronomía, Universidad de Buenos Aires, Argentina (34°25′S, 58°25′W), to evaluate the possibility of reducing weed seedling emergence through the use of alfalfa cultivars with low levels of winter dormancy and by increasing plant density from 200 to 400 plants m−2. It was hypothesized that these treatments would alter the temperature regime and the red (R)–far-red (FR) ratio of radiation to which seeds were exposed. Responses to management treatments were recorded for bull thistle, cotton thistle, plumeless thistle, tall rocket, mustard, curly dock, and pigweed. During the alfalfa establishment year, pigweed and curly dock emergence was reduced by the nondormant cultivar established at high density. This reduction disappeared when soil beneath the canopy was fitted with heaters that mimicked bare-soil temperatures. Crop canopy presence during the establishment year was not effective in reducing mustard, cotton thistle, bull thistle, plumeless thistle, and tall rocket emergence. During the second and third years after crop establishment, the canopy of the nondormant alfalfa cultivar was effective in reducing germination of weed seeds placed on the soil surface during fall and winter. In contrast, the winter-dormant cultivar allowed the establishment of weeds during the winter period. These reductions in weed emergence were associated with a modification in the R–FR ratio perceived by the seeds located at the soil surface and could largely be removed by using FR filters to increase the R–FR ratio. These results suggest that the selection of a nondormant cultivar combined with an increase in plant density could effectively reduce weed populations in alfalfa.

Summary Cynara cardunculus is a troublesome weed in temperate grazing lands. Cynara cardunculus achenes are usually dormant at dispersal and require alternating temperatures to terminate dormancy and germinate. Laboratory and glasshouse experiments were conducted to determine (i) the treatments able to terminate dormancy and (ii) the effect of environmental factors and agronomic practices on germination and emergence of non‐dormant (dry after‐ripened) achenes. Scarification, hydrogen peroxide and sodium hypochlorite promoted germination of dormant achenes. Dry after‐ripening and cold stratification were tested in two different populations. Dormancy of both populations was released from dormancy by dry after‐ripening. In contrast, cold stratification allowed dormancy release in just one of the populations, while the other was induced into secondary dormancy. Germination of non‐dormant (dry after‐ripened) achenes was maximum in a range of temperatures from 10 to 20°C and was inhibited at higher temperatures. Reduction of osmotic potential below −0.6 MPa led to a decrease in final germination. These results explain synchronic emergence of C. cardunculus seedlings in autumn after dormancy release during summer. Maximum seedling emergence was close to 60% at soil depths of 1 cm and only decreased as depth increased over 6 cm. In contrast, seedling emergence was not reduced by the presence of cover residues, while a flooding duration of 21 days was required to suppress emergence significantly. These results suggest that the deep burial of achenes and agronomic practices that take advantage of synchronic emergence of achenes could be useful tools leading to better long‐term management of C. cardunculus.

Fluctuating temperatures terminate seed dormancy in many species, including Cynara cardunculus (L.) var. sylvestris. Termination of physiological dormancy requires low ratios of abscisic acid (ABA)/gibberellins (GA). In a previous paper we have shown that physiological responses to fluctuating temperatures comprise a reduction of abscisic acid (ABA) content and sensitivity. However, a possible stimulation of GA synthesis was also suggested as part of the mechanism. That possible stimulation, as well as the identification of potential regulatory sites for ABA and GA metabolism and signalling involved in the termination of dormancy by fluctuating temperatures, are yet to be determined. In this study, we measured GA content and sensitivity in seeds incubated under constant and fluctuating temperatures. We also assessed the expression of several genes involved in ABA and GA metabolism and signalling. Our results show that fluctuating temperatures reduce ABA/GA ratios through a reduction in ABA accumulation during incubation but without altering GA synthesis as compared to that observed under constant temperatures. On the other hand, fluctuating temperatures did not increase sensitivity to GA. Fluctuating temperatures reduced the expression of CycaNCED and CycaABI5 (ABA synthesis and signalling genes) with a temporal pattern that coincides with the interruption of ABA accumulation that precedes germination of seeds incubated under fluctuating temperatures.However, fluctuating temperatures did not modify the expression of CycaCYP707A2 (ABA inactivation) as compared to that observed under constant temperatures. Consistent with our determinations of GA content and sensitivity, fluctuating temperatures did not modify the expression of GA synthesis (CycaGA3ox) and signalling genes (CycaRGL2 and CycaGAI) in relation to that observed at constant temperatures. These results show that fluctuating temperatures terminate dormancy in Cynara cardunculus seeds through an interruption in ABA accumulation and a reduction in ABA signalling exerted at the level of CycaNCED and CycaABI5 expression.