Studies on the Physiology of Flowering of Timothy (Phleutm pratense L.): I. Influence of Daylength and Temperature on Initiation and Differentiation of the Inflorescence1

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Abstract
Spikelet initiation is advanced and the proportion of plants which attain the reproductive condition is increased in S. 48 timothy by lengthening the photo-period from 14½ to 24 hours. In shorter periods of light, reproduction is almost completely inhibited, and in 8-hour short days plants remain vegetative even after 35 weeks. Spikelet initiation at the shoot apex occurs after exposure to 3–5 long days followed by short days. Initiation also occurs when extended daylength is replaced by ‘light-breaks’ during long nights, or when a single leaf is photo-induced while the remainder of the plant receives short days. High temperatures promote spikelet initiation in continuous light; in photoperiods nearer the threshold for flowering this response is reversed and a rise in temperature from 55° to 75° F. increasingly inhibits reproduction. Once initiation has occurred, spike differentiation is hastened by increases in temperature or photoperiod. Internode elongation begins at the time of spikelet initiation, and is promoted by temperature and photoperiod. Elongated vegetative shoots may be produced when spikelet initiation fails in threshold photoperiods or high temperatures.