Photothermal Time for Flowering in Faba Bean (Vicia faba) and the Analysis of Potential Vernalization Responses

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Abstract
One cultivar and one land-race of faba bean were subjected to 18 potentially vernalizing pre-treatments (constant temperatures of 1, 5 or 9 °C factorially combined with photoperiods of 8 or 16 h d−1 for 10, 30 or 60 d), and then transferred into four different growing regimes (‘day’/‘night’ temperatures of 18/5 °C or 24/13 °C factorially combined with photoperiods of 11 or 16 h d−1). Control plants were grown entirely in the latter four regimes. The times from sowing to appearance of first open flowers were recorded for all plants. Control plants of the land-race Zeidab Local flowered sooner in long days and in the warmer regime. Pre-treatment reduced the subsequent time to flower in the four growing-on regimes but most of the variation in the total time to first flower for the pre-treated plants was accounted for by differences in the combined photothermal time accumulated in the two successive environments - which was predicted by a simple photothermal model. Thus, there was neither a specific low-temperature nor a short-day vernalization response in this accession. Similarly, no true low-temperature or short-day vernalization response was detected in the cv. Maris Bead. However, this UK cultivar flowered later than predicted in the 24/13 °C regime, indicating that the 24 °C ‘day’ temperature was supraoptimal. Delays to flowering at 24/13 °C were, however, less evident when plants were grown in long days or following prolonged (30–60 d) pre-treatments at cool temperatures.