Morphological Changes and Factors Influencing Shoot Apex Development of Lilium longiflorum Thunb. during Forcing1

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Abstract
Procedures are described for critical point drying the shoot apical meristems of Easter lilies for viewing in the scanning electron microscope. This preparatory technique was superior to either freeze-drying or the use of freshly isolated meristems. Photomicrographs of the morphological development of the entire shoot apex from the vegetative through several reproductive stages are presented. The morphology of a single flower is also illustrated. Meristem diameter measurements revealed seasonal and bulb source variations. Diameter increased with increasing bulb size. When compared to non-cooled bulbs, low temperature treatments reduced meristem diameter prior to flower initiation. The average date of flower initiation of controlled temperature forced ‘Ace’ lilies over a 5-year period was January 21; whereas precooled lilies initiated about 1 week earlier. Greenhouse temperatures of 17−25°C accelerated the date of flower initiation when compared to 13°. A simplified technique for measuring the meristem diameter and observing the stage of development was developed. The Easter lily forms 2 classes of primary flowers, initial and raised. They can also form secondary flowers. The number of initial primary flowers was correlated with meristem diameter. Larger bulb sizes resulted in a greater number of raised primary flowers. In general, only large bulbs formed secondary flowers. A greenhouse temperature of 13°C promoted the formation of raised primary flowers, while 21° promoted the formation of secondary flowers. A measurement of the sprouting index showed seasonal and bulb source differences.

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