Initiation and development of western red cedar cones in response to gibberellin induction and under natural conditions

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Abstract
Both seed and pollen cones in Thuja plicata Donn. are found at the tips of small lateral branches and form as a result of the transition of previously vegetative apices. The vegetative apex shows a cytohistological zonation similar to that found in other Cupressaceae and vegetative buds are not enclosed in scales. The first sign of pollen cone initiation occurred 13 days after the initial GA3 treatment under long days. Transition of the apex was marked by a slight increase in rate of cell division in all zones resulting in the formation of a mantle of small uniform cells several cells thick covering the surface of the apex. The apex increased in height and the long internode between the last-formed pair of leaf primordia and first pair of microsporophylls formed a short stalk at the base of the cone. The transition of a vegetative apex to a seed cone apex occurred 27 days after initial GA3 treatment and was similar to that for pollen cones. Bract primordia are initiated closer to the summit of the apex and are larger in initial stages than microsporophyll primordia, and longer internodes remain between successive pairs of bracts. Ovule initiation occurs when bracts have just begun upward growth. All microsporophylls, bracts, and ovules are formed during the few weeks after cone initiation and before cones become dormant. No anatomical changes occurred in either cone type during subsequent short-day cold treatment. A return to long-day warm conditions promoted normal cone development. Under natural conditions in the Victoria area, pollen cones are initiated early in June under long days and increasing day length while seed cones are initiated early in July under similar long days but decreasing day length. Development of both pollen and seed cones occurs during long days and decreasing day length. The possible relationship between sexuality of cones, gibberellin, auxin, and day length is discussed.

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