The functional ecology of shoot architecture in sun and shade plants of Heteromeles arbutifolia M. Roem., a Californian chaparral shrub |
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Authors: | Fernando Valladares Robert W Pearcy |
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Institution: | (1) Centro de Ciencias Medioambientales, CSIC, Serrano 115 dpdo. 28006 Madrid, Spain Fax: 34 1 5640800; email: valladares@fresno.csic.es, ES;(2) Division of Biological Sciences, Section of Ecology and Evolution, University of California at Davis, CA 95616, USA, US |
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Abstract: | The functional roles of the contrasting morphologies of sun and shade shoots of the evergreen shrub Heteromeles arbutifolia were investigated in chaparral and understory habitats by applying a three-dimensional plant architecture simulation model,
YPLANT. The simulations were shown to accurately predict the measured frequency distribution of photosynthetic photon flux
density (PFD) on both the leaves and a horizontal surface in the open, and gave reasonably good agreement for the more complex
light environment in the shade. The sun shoot architecture was orthotropic and characterized by steeply inclined (mean = 71o) leaves in a spiral phyllotaxy with short internodes. This architecture resulted in relatively low light absorption efficiencies
(E
A) for both diffuse and direct PFD, especially during the summer when solar elevation angles were high. Shade shoots were more
plagiotropic with longer internodes and a pseudo-distichous phyllotaxis caused by bending of the petioles that positioned
the leaves in a nearly horizontal plane (mean = 5o). This shade-shoot architecture resulted in higher E
A values for both direct and diffuse PFD as compared to those of the sun shoots. Differences in E
A between sun and shade shoots and between summer and winter were related to differences in projection efficiencies as determined
by leaf and solar angles, and by differences in self shading resulting from leaf overlap. The leaves exhibited photosynthetic
acclimation to the sun and the shade, with the sun leaves having higher photosynthetic capacities per unit area, higher leaf
mass per unit area and lower respiration rates per unit area than shade leaves. Despite having 7 times greater available PFD,
sun shoots absorbed only 3 times more and had daily carbon gains only double of those of shade shoots. Simulations showed
that sun and shade plants performed similarly in the open light environment, but that shade shoots substantially outperformed
sun shoots in the shade light environment. The shoot architecture observed in sun plants appears to achieve an efficient compromise
between maximizing carbon gain while minimizing the time that the leaf surfaces are exposed to PFDs in excess of those required
for light saturation of photosynthesis and therefore potentially photoinhibitory.
Received: 8 June 1997 / Accepted: 2 November 1997 |
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Keywords: | Functional plant architecture Heteromeles arbutifolia Light harvesting Phenotypic plasticity Photosynthesis |
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