It could be established, that biennial caraway really require some cold effect for generative differentiation. The results show, that the constant 22/15°C could not induce any flower development within the studied period. Lower temperature, as constant 15/10°C had a moderate induction effect, it stimulated flowering in nearly 20% of the individuals. The vernalization at 8/5°C day/night temperatures resulted in the highest proportions of flowering individuals, the longer this period, the more flowering plants (Figure 2). After the 7 week induction period the whole population developed stems. Thus, it seems, that by this latter treatment the optimal vernalization circumstances could be approached. According to the data, caraway optimal induction regime might lay between 5°C and 8°C, which is effective when lasting more than 2 weeks. Both a shorter period as well as higher temperatures up to 15°C result in partial flowering. However, lower effective temperatures can not be excluded on the basis of the existing results.
Beside temperature, illumination length may also play a basic role in flower initiation (Runger 1977). In case of several species those inductive factors stay in tight correlation with each other (Booij and Meurs 1994). The most frequent reaction types are: short day treatment, short day treatment before or during the cold period, long day treatment after a cold period, etc.
Illumination may act on flowering through its length during the day (number of light and dark hours), its length during the plant life (number of cycles of photoperiodic induction) and sometimes its intensity.
In case of caraway scientific data on photoperiodic reaction are very few. Putievsky (1983) examined the effect of daylength and temperatures on the flowering of three Umbelliferous species: caraway, dill and coriander. The three spices exhibited different reactions to the treatments. Caraway developed flowers under all the experimental circumstances (18/12°C or 24/12°C day and night temperatures, with 10h or 16h photoperiods). However, except plant height, the yield components (number of branches, seed weight, etc.) were all reduced by the long day treatment. Although flowering occurred about 3 weeks earlier on long day and higher temperatures, seed yield decreased by more than 30%. The experiment indicated, that caraway does not need any special photoperiod for flowering, however a longer vegetative growth at lower temperatures and short day may enhance generative mass.
As for the related Umbelliferous species, a reduction of time needed for flowering and an advantageous effect of long days on dry weight of the plant was established for the related dill (Halva et al. 1993). In Apium graveolens L. var. rapaceum short day treatment did not prevented flowering but did so for stem elongation (Booij and Meurs 1995). In case of Foeniculum vulgare, it was proved, that the critical photoperiod for umbel initiation is longer, than 13 hours. For 100% initiation at least 25 photoperiodic cycles are required (Peterson et al. 1993).
In our investigations biennial caraway individuals of different age were exhibited to a short day treatment. 10 hours dark periods and 14 hours light periods alternated during 28 days (Nemeth et al. 1997). The temperature during the whole treatment lie at 22/15°C (day/ night). Four age groups were studied, which could be characterised by different sizes of the individuals. The smallest ones were 3 months old with 5.4mm rootstock diameter, the others 4, 5, 6 months old with 7.0, 8.4 and 9.6mm diameters respectively.
However, in neither plant group not any flowering stems appeared during the five months observation period. It might mean, either, that caraway does not need any short day induction for flower initiation at all, or any photoperiodic response is effective only with interaction of low temperatures. Besides, the success of a longer application of the mentioned treatment is not excluded.
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