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

Spikelet initiation is advanced and the proportion of plantswhich attain the reproductive condition is increased in S. 48timothy by lengthening the photo-period from 14 to 24 hours.In shorter periods of light, reproduction is almost completelyinhibited, and in 8-hour short days plants remain vegetativeeven after 35 weeks. Spikelet initiation at the shoot apex occursafter 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 singleleaf is photo-induced while the remainder of the plant receivesshort days. High temperatures promote spikelet initiation incontinuous light; in photoperiods nearer the threshold for floweringthis response is reversed and a rise in temperature from 55°to 75° F. increasingly inhibits reproduction. Once initiationhas occurred, spike differentiation is hastened by increasesin temperature or photoperiod. Internode elongation begins atthe time of spikelet initiation, and is promoted by temperatureand photoperiod. Elongated vegetative shoots may be producedwhen spikelet initiation fails in threshold photoperiods orhigh temperatures.