In the early 1700s, fumes from fires used to heat pineapple houses were observed to force the induction of flowering and this finding led to the commercial use of smoke for this purpose. The active ingredient in smoke was shown to be ethylene (Rodriquez, 1932) and later work (Kerns, 1936) showed that acetylene also forced flower induction. This finding prompted the use of carbide as a source of acetylene, a method still widely used on small farms. A pea-sized amount of calcium carbide is dropped into the centre of the rosette of leaves of a vegetative plant of sufficient size. The carbide reacts with water to release acetylene, which is taken up by the leaves. Acetylene (Aldrich and Nakasone, 1975) and ethylene, which much research shows is the most effective forcing agent (Bartholomew and Criley, 1983), are both more effective if applied at night when the stomata are open.
Synthetic auxins such as a-naphthalene acetic acid (NAA) initiate flowering of pineapples by promoting the synthesis of ethylene by the plant (Burg and Burg, 1966). These materials significantly reduced the cost of forcing, while maintaining a high degree of forcing efficiency. The most widely used synthetic auxin was the sodium salt of naphthalene acetic acid (SNA). As little as 50 g ha-1 in 2800 l of water (20 g acre-1 in 300 gallons of water) could initiate flowering. Usually, two applications, a week apart, were made. This ensured that any plants not initiated by the first application were induced by the second.
The newest material used as a forcing agent is ethephon (2-chloroethylphosphonic acid). Ethephon is typically applied in combination with urea and, in some areas, sodium borate or calcium or sodium carbonate (Dass et al., 1976; Balakrishnan et al., 1978). The latter materials raise the solution pH, which hastens the rate of breakdown of ethephon into ethylene. For more details, see Bartholomew et al. (Chapter 8, this volume).
A variety of combinations of the above materials have been evaluated for commercial forcing. Best results go beyond just the percentages of plants that are forced and should include the uniformity of the harvest peak and the fruit yield. In this regard, commercial forcing with ethylene has shown outstanding results. However, this practice requires that 3000-4000 g ha-1 of ethylene gas be adsorbed on to activated carbon or a fine clay, such as bentonite. Typically, the ethylene and 22 kg of adsorbent are applied in 4500-7000 l ha-1 of water at night when the stomata are open. The large quantities of water required and the need for specialized equipment have generally limited this practice to the largest growing operations, although smaller farms in Côte d'Ivoire also force with ethylene. More generally, ethep-hon is used for knapsack and small and large boom applications, and the quantities being used vary with area and season and are limited by label requirements.
To determine the efficacy of the forcing operation, representative plants may be cut longitudinally through the apex, to observe the development of the young inflorescence. This is done at 6-8 weeks after forcing and may be repeated at weekly intervals until 95% or more of the plants have differentiated. Areas that do not attain 95% can be reforced.
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