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Responses of transpiration and photosynthesis to reversible changes in photosynthetic foliage area in western red cedar

Summary

Published in Tree Physiology 22: 363-371

Experiments were conducted on 1-year-old western red cedar (Thuja plicata Donn.) seedlings to determine the response of illuminated foliage to reversible changes in total photosynthetic foliage area (La). Reductions in La were brought about by either shading the lower foliage or by reducing the ambient CO2 concentration (ca) of the air surrounding the lower part of the seedling. ln the latter case, the vapor pressure was also changed so that transpiration rates (E) could be manipulated independently of photosynthetic rates (A). We hypothesized that following such treatments, short-term compensatory changes would occur in stomatal conductance (gs) and A of the remaining foliage. These changes would occur in response to hydraulic signals generated by changes in the water potential gradient rather than changes in the distribution of sources and sinks of carbon within the seedling. When a portion of the foliage was shaded, there was an immediate reduction in whole-seedling E and a concomitant increase in gs, A and E in the remaining illuminated foliage. However, the inter-cellular CO2 concentration did not change. These compensatory effects were fully reversed after the shade was removed. When the lower foliage A was reduced to < 0 mmol m-2 s-I, by shading or lowering ca, and E was either unchanged or increased (by adjusting the vapor pressure deficit), there was no significant increase in gs and A in the remaining foliage. We conclude that compensatory responses in illuminated foliage occur only when reductions in LA are accompanied by a reduction in whole-plant E. The relationship between the reduction in whole-seedling E and the increase in A is highly linear (r2 = 0.68) and confirms our hypothesis of the strong regulation of gs by hydraulic signals generated within the seedling. We suggest that the mechanism of the compensatory effects is a combination of both increased CO2 supply, resulting from increased gs, and a response of the rate of carboxylation, possibly related to the activity of Rubisco.

Sector(s): 

Forests

Categorie(s): 

Scientific Article

Theme(s): 

Forestry Research, Forests, Silviculture

Author(s)

PEPIN, Steeve, Nigel J. LIVINGSTON and David WHITEHEAD

Year of publication :

2002

How to get the publication :

Available at the direction de la recherche forestière

Keywords :

compensatory responses, photosynthetic rate, stomatal conductance, transpiration rate, Thuja plicata, Western red cedar, foliage, growth, seedling, intercellular CO2 concentration, silviculture and yield of natural forests - softwood stands, sylviculture et rendement des forêts naturelles - peuplements résineux, sylviculture des forêts naturelles résineuses

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