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Source: Data provided by D. Beadle of Venice, Florida.

Source: Data provided by D. Beadle of Venice, Florida.

Cambridge Books Online © Cambridge University Press, 2009

photoperiodic owering in Billbergia nutans. Day lengths extended with arti cial light delayed the onset of owering from mid to late January to about 1 April in one run. Another group of plants covered with black cloth to effect short days owered four days before the fully exposed controls. Subjects maintained in a vegetative state with a simulated summer regimen, nevertheless, produced ramets, thus engaging in what is usually a post- owering event. Photoperiodism may be especially important for B. nutans because it ranges farther poleward in Brazil than the other members of its genus.

Downs (1974) investigated photoperiodism among a selection of Bromeliaceae with "not very conclusive results except for deciduous Pitcairnia heterophylla (Fig. 2.12A). After each treated shoot had generated about 16 of the unarmed green leaves under 12-h or shorter days, the scalelike, spiny, nongreen foliage and bulbous base that usually presage owering began to appear. Fourteen-hour days prevented this transition, but not the formation of additional green leaves. Shoots induced to swell under short days reverted to the production of green leaves as if some additional requirement for owering remained unsatis ed. Neither simulated drought followed by heavy irrigation nor defoliation induced in orescences to appear. However, after producing additional linear leaves, and without altering the photoperiod, plants bulbed and owered. Apparently, oral induction requires short days, and, judging by the failure of short-day subjects with less foliage to reproduce, also the presence of more than 16 green leaves.

Inadvertent smoke-induced owering by pineapple plants in Hawaii rst alerted investigators to a pervasive chemical sensitivity (Downs 1974). Subsequent inquiry demonstrated that a variety of synthetic auxins (e.g., 2,4-dichlorophenoxyacetic acid, 1-naphthalene acetic acid, indole acetic acid), ethylene and related compounds stimulate diverse Bromeliaceae including Ananas. Beta-hydroxyethylhydrazine (BOH), an ethylene-gener-ating compound, continues in widespread commercial use, but the underlying mechanisms remain poorly understood. Ethylene and acetylene promote auxin synthesis during several other growth responses (e.g., hypo-cotyl unbending), and probably act similarly in responsive bromeliads, or these agents somehow sensitize shoot meristems to endogenous auxins and possibly other native growth factors.

Hydroperiod, broadly de ned as changes in the height of the water table and the arrival of dry or wet weather, coordinates the activities of many tropical plants. Phenology related to the second stimulus remains undocumented in Bromeliaceae, but certain behaviors are suspicious, for example the weather-related altered owering schedules exhibited by some of those phytotelm species comprising the described Costa Rican guild of ornitho-phils (Stiles 1978). A closer look at owering, bud break and, for the deciduous types, leaf turnover relative to moisture supply rather than some accompanying seasonal cue like photoperiod could prove rewarding.

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