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Initiation of Inflorescences in Cool-season Perennial Grasses¹

The objectives of this paper are to discuss pertinent literature and to present some recent research concerning induction of the vegetative primordia and initiation of floral development in perennial cool-season grasses. The amount of exposure to short photoperiod and cool temperatures under field conditions necessary for induction of three forage grasses and four cultivars of Kentucky bluegrass (Poa pratensis L.) was studied. Plants were moved from the field at 15-day intervals from September 15 to March 15 and grown under long photoperiod and warm temperatures for the production of inflorescences.

The number of inflorescences produced indicated that a high level of induction for smooth bromegrass [Bromus inermis (Leyss.)], crested wheatgrass [Agropyro desertorum (Fisch. ex. Link) Schult.], and orchardgrass (Dactylis glomerata L.) had been reached by October 15, November 1, and November 15, respectively. The four Kentucky bluegrass cultivars showed a significant linear response to exposure. Inflorescence production increased as the exposure time increased. Seed production for Kentucky bluegrass cultivars followed essentially the same pattern as inflorescence production in response to exposure. The time required for inflorescence emergence for all cultivars and species had a negative association with the length of exposure to inductive conditions. If the rate of inflorescence emergence was used as the index, induction intensity was increasing up to March 15.

Observations of dissected tillers at weekly intervals indicated the initiation of floral development for smooth bromegrass by February 12, and for orchardgrass and crested wheatgrass by February 19. The apices of the Kentucky bluegrass cultivars show a transition to floral primordia by January 22 for Delta and by February 5, 12, and 26 for Newport, Cougar, and Merion, respectively. Transition of vegetative apices to floral development occurred under conditions of cold temperature and short photoperiod for all species and cultivars.

Key Words: Induction • Photoperiod • Temperature

² Research Agronomist, Plant Science Research Division, Agricultural Research Service, US Department of Agriculture, Pullman, Wash.

³ Post-doctoral Fellow, Department of Botany, University of Western Ontario, London, Ontario, Canada (formerly Graduate Student, Washington State University, Pullman, Wash.

⁴ Associate Professor of Agronomy, Kansas State University (formerly Assistant Professor of Agronomy, Washington State University, Pullman, Wash.).

Received for publication April 15, 1971.

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