Leaf shape linked to photosynthetic rates and temperature optima in South African <Emphasis Type="Italic">Pelargonium</Emphasis> species |
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Authors: | A B Nicotra M J Cosgrove A Cowling C D Schlichting C S Jones |
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Institution: | (1) School of Botany and Zoology, The Australian National University, Canberra, ACT 0200, Australia;(2) Statistical Consulting Unit, The Australian National University, Canberra, ACT 0200, Australia;(3) Department of Ecology and Evolutionary Biology, University of Connecticut, Storrs, CT 06269, USA |
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Abstract: | The thermal response of gas exchange varies among plant species and with growth conditions. Plants from hot dry climates generally
reach maximal photosynthetic rates at higher temperatures than species from temperate climates. Likewise, species in these
environments are predicted to have small leaves with more-dissected shapes. We compared eight species of Pelargonium (Geraniaceae) selected as phylogenetically independent contrasts on leaf shape to determine whether: (1) the species showed
plasticity in thermal response of gas exchange when grown under different water and temperature regimes, (2) there were differences
among more- and less-dissected leafed species in trait means or plasticity, and (3) whether climatic variables were correlated
with the responses. We found that a higher growth temperature led to higher optimal photosynthetic temperatures, at a cost
to photosynthetic capacity. Optimal temperatures for photosynthesis were greater than the highest growth temperature regime.
Stomatal conductance responded to growth water regime but not growth temperature, whereas transpiration increased and water
use efficiency (WUE) decreased at the higher growth temperature. Strikingly, species with more-dissected leaves had higher
rates of carbon gain and water loss for a given growth condition than those with less-dissected leaves. Species from lower
latitudes and lower rainfall tended to have higher photosynthetic maxima and conductance, but leaf dissection did not correlate
with climatic variables. Our results suggest that the combination of dissected leaves, higher photosynthetic rates, and relatively
low WUE may have evolved as a strategy to optimize water delivery and carbon gain during short-lived periods of high soil
moisture. Higher thermal optima, in conjunction with leaf dissection, may reflect selection pressure to protect photosynthetic
machinery against excessive leaf temperatures when stomata close in response to water stress. |
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Keywords: | Dissection index Evaporative cooling Temperature response Photosynthesis Stomatal conductance |
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