You are correct to assume that the nodes on a new spike have not differentiated. This seems to be a function of temperature. I will assume we are discussing Phalaenopsis. In the fall of the year with cool night time temps, the plant will "set" the spike. This spike at this time will generate flowers and usually not keiki. As spring begins and the temperatures warm and therefor the day/night temp differences are more narrow vegetative growth begins. It is at this time that dormant nodes on the spike will more likely respond to hormonal application and produce a keiki.
This is the abstract of a paper written by Matthew G. Blanchard* and Erik S. Runkle
Department of Horticulture, Michigan State University, East Lansing, MI 48824-1325, USA
Journal of Experimental Botany, Vol. 57, No. 15, pp. 4043–4049, 2006
doi:10.1093/jxb/erl176 Advance Access publication 30 October, 2006
Abstract
Phalaenopsis orchids are among the most valuable
potted flowering crops commercially produced throughout
the world because of their long flower life and ease
of crop scheduling to meet specific market dates.
During commercial production, Phalaenopsis are usually
grown at an air temperature >28 C to inhibit flower
initiation, and a cooler night than day temperature
regimen (e.g. 25/20 C day/night) is used to induce
flowering. However, the specific effect of day and night
temperature on flower initiation has not been well
described, and the reported requirement for a diurnal
temperature fluctuation to elicit flowering is unclear.
Two Phalaenopsis clones were grown in glass greenhouse
compartments with constant temperature set
points of 14, 17, 20, 23, 26, or 29 C and fluctuating
day/night (12 h/12 h) temperatures of 20/14, 23/17, 26/14,
26/20, 29/17, or 29/23 C. The photoperiod was 12 h, and
the maximum irradiance was controlled to <150 mmol
m2 s1. After 20 weeks, >80% of plants of both clones
had a visible inflorescence when grown at constant 14,
17, 20, or 23 C and at fluctuating day/night temperatures
of 20/14 C or 23/17 C. None of the plants were
reproductive within 20 weeks when grown at a constant
29 C or at 29/17 C or 29/23 C day/night temperature
regimens. The number of inflorescences per
plant and the number of flower buds on the first inflorescence
were greatest when the average daily temperature
was 14 C or 17 C. These results indicate that
a day/night fluctuation in temperature is not required
for inflorescence initiation in these two Phalaenopsis
clones. Furthermore, the inhibition of flowering when
the day temperature was 29 C and the night temperature
was 17 C or 23 C suggests that a warm day
temperature inhibits flower initiation in Phalaenopsis.
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