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Substrate Water Potential Constraints on Germination of the Strangler Fig Ficus aurea (Moraceae)

Eric N. Swagel, A. Van H. Bernhard and George S. Ellmore
American Journal of Botany
Vol. 84, No. 5 (May, 1997), pp. 716-722
Stable URL: http://www.jstor.org/stable/2445908
Page Count: 7
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Substrate Water Potential Constraints on Germination of the Strangler Fig Ficus aurea (Moraceae)
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Abstract

Palms are the most common support hosts for strangler fig (Ficus aurea) in the tropical dry forest. At Humming bird Cay Tropical Field Station in the Bahamas, viable F. aurea seeds scattered on open soil germinate within 6 d in the laboratory at 27⚬C in light or darkness. To test the hypothesis that water relations between seed and substrate can restrict the site of F. aurea establishment, fig seeds were imbibed in mannitol solutions with water potential (Ψ) from 0 MPa to -2.0 MPa. At Ψ > -1.0 MPa, germination ranges from 70 to 90%. Below -1.0 MPa, germination drops under 4%. Seedling growth rate slows linearly with decreasing Ψ, due to reduced cell enlargement in the radicle. Sensitivity of F. aurea seeds to Ψ stress is similar to that of mesophytic crops with "threshold" Ψ of -1.2 to -1.5 MPa, below which germination and/or seedling growth stops. F. aurea shows no evidence of xerophytic adaptation in germination physiology. In the field, Ψ of humus in palm leaf bases does not drop below -1.0 to -1.1 MPa, while humus in terrestrial sites reaches Ψ < -2.0 MPa. Seedling establishment of strangler fig in the dry forest depends on sustained moisture (Ψ \geq -1.2 MPa). In a climate marked by 4-7 mo dry seasons, the moisture requirement for F. aurea germination and seedling growth is met by humus in palm leaf bases, not by terrestrial sites. In dry forests, this restricts F. aurea establishment to the crown of palm trees.

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