Effects of Photoperiod and Temperature on Growth and Flowering in Norwegian and British Timothy Cultivars(Phleum PratenseL.)

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Abstract
Four Norwegian and 2 British timothy cultivars were grown in a controlled environment to study their flowering and growth response to photoperiod, light flux and temperature. All cultivars responded as obligatory long day plants with no cold requirement before photoperiodic induction. The critical photoperiod for flower initiation varied from 16 h in the N. [northern] Norwegian cultivars Engmo and Bodin to 14 h in ''Grindstad'' of S. [southern] Norwegian origin. The British cultivars S.48 and S.51 had no shorter critical photoperiods than those of S. Norwegian origin. High temperature and reduced light flux interacted to inhibit flower initiation and heading in long photoperiods. The response was closely related to the latitude of origin of the cultivars, Engmo and Bodin being very sensitive and the British cultivars rather tolerant in this respect. Indeterminate or barren determinate shoots were formed in long days under conditions of high temperature and reduced light flux. Associated with flower initiation was a specific photoperiodic effect on dry matter production. At the end of a 5-wk experimental period the average dry weight for all cultivars and temperatures was 46% higher in 24-h than in 10-h photoperiods under conditions of nearly identical daily light energy. This photoperiodic effect could compensate for > 3.degree. C difference in temperature. Apparently, creation of greater sink capacity is the primary effect of long days triggered by a sudden surge in stem and leaf growth which are then in turn augmented by increased source capacity resulting from increased leaf area. Dry matter content, leaf growth, tillering and haplocorm size were all greatly affected by photoperiod and temperature, usually with significant interaction of these factors.