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Diurnal Temperature Cycling Influences Flowering and Node Numbers of Broccoli


Broccoli plants (Brassica oleracea L. var. italica) were subjected to two diurnal chilling temperature cycles that varied between 2° and 14°C and from 14 to 52 days. Cycles differed in the length of time in transition between the minimum and maximum temperatures. In two experiments, plants were grown in an air-conditioned greenhouse phytotron at 22°/18° (day/night temperature) for 7 to 21 days before chilling. Plants (and germinating seeds) placed in the chilling environment for 28 to 49 days from 0 to 21 days from seeding, flowered as much as 13 days earlier than plants not chilled. Further, plants 14 days from seeding and chilled for an additional 28 days flowered 16 days earlier than controls. Similarly, chilled plants had fewer nodes at flowering attributable to the temperature cycling regimes, except in a third experiment when seeds in pots were placed immediately into the chilling cycles for 31 to 52 days. ‘Futura’ flowered earlier than ‘Early One’ but neither was different from ‘Bravo’ or ‘Apollo’ in this third experiment.