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JXB Advance Access published online on November 29, 2004
Journal of Experimental Botany, doi:10.1093/jxb/eri037
© 2004 by Oxford University Press
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Received May 10, 2004
Accepted September 22, 2004

Light Stress in Plants: Mechanisms and Interactions Special Issue Article

Photoinhibition and drought in Mediterranean woody saplings: scaling effects and interactions in sun and shade phenotypes

Fernando Valladares 1*, Iker Dobarro 1, David Sánchez-Gómez 1, and Robert W. Pearcy 2

1 Centro de Ciencias Medioambientales, CSIC, Serrano 115 dpdo, E-28006 Madrid, Spain
2 Section of Evolution and Ecology, Division of Biological Sciences, University of California Davis, CA 95616, USA

* To whom correspondence should be addressed.
Fernando Valladares, E-mail: valladares{at}ccma.csic.es


  Abstract

Interacting effects of high light and drought on the performance of sun and shade phenotypes were experimentally undertaken following survival, chlorophyll fluorescence and gas exchange in 2-year-old saplings of four Mediterranean trees (Quercus ilex and Q. coccifera as water-saving species, and Pistacia lentiscus and P. terebinthus as water-spending species). Half of the saplings were grown in full sunlight and the other half in the shade (6% sunlight). Half of each combination of species-phenotype was exposed to high light during a simulated late-summer drought. Light absorptance and gas exchange were scaled up to the whole plant with the 3-D geometrical model, Y-Plant. Quercus species were more plastic and tolerated high light and water stress better than Pistacia species, surviving longer and in drier soils, and exhibiting a less pronounced photoinhibition. There was no evidence of disadvantage for shade phenotypes under high light with increasing drought. By contrast, shade phenotypes survived longer despite larger initial decreases in photochemical efficiency and higher sensitivity to drought than sun phenotypes. The enhanced control of transpiration during drought in water-saving versus water-spending species (and also in shade versus sun phenotypes in three out of the four species) allowed extended survival. Photoinhibition reduced whole crown carbon gain in high light by c. 3% and affected significantly more the shaded leaves of a given plant (reducing their carbon gain by up to 7%) than those exposed to direct sunlight. Despite this apparently minor impact, whole plant carbon gain reduction by photoinhibition negatively correlated with survival and drought tolerance. The implications for succession and forest regeneration in arid environments, particularly under a global change scenario, are discussed.

Keywords: Carbon gain; crown architecture; facilitation; forest regeneration; global change; interactions between stresses; phenotypic plasticity; photoinhibition; succession; water-use efficiency.
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