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Impact of elevated atmospheric [CO2] on plants and crops |
Cahiers Agricultures. Volume 9, Number 1, 11-21, Janvier - Février 2000, Synthèse
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 Résumé
 Summary
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Author(s) : Marianne Mousseau, Laboratoire d’écophysiologie végétale, Université Paris-Sud, 61, avenue des Gobelins, 75013 Paris.. |
Summary : Since the eighties, as atmospheric and other anthropogenic gases are increasing in the atmosphere, scientists grouped in IPCC are warning that “there is a balance of evidence which suggests a discernable human influence on global climate”. One of the most important change for plants and terrestrial ecosystems is the increased [CO2] in the atmosphere. This increase, measured precisely since 1958 by Keeling, is reflecting the sentivity of the world vegetation as it is oscillating with its annual input and output. As the oscillation amplitude is increasing, and as the annual ring width of trees is showing an increasing trend, it is believed that there is a Northern hemisphere sink for atmospheric carbon, characterised by forest regrowth.
This induced intensive research on the effects of elevated [CO2] on plant ecophysiology. New techniques have been developed to study plant characteristics when grown over their full lifespan in a [CO2] enriched atmosphere. First, controlled cabinets were used in greenhouses, with potted plants. Then, plants were studied in the fields with “open top” chambers directly placed over crops or ecosystems. Then, to avoid increased temperatures induced by the walls of such chambers, free air FACE systems were deviced and used essentially for agricultural ecosystems. The special case of adult trees, which their too large size prevent from being studied in enclosures, was studied with bags placed around big branches.
The major effect of increased [CO2] is an increased photosynthetic CO2 fixation and its direct consequence: an increase in dry matter production. The almost universal response of short term exposure to elevated [CO2] is a stimulation of photosynthesis, compared with photosynthesis in ambient [CO2]. This is valid essentially on C3 plants for which the stimulation is coming from an increased carboxylation rate and a decreased photorespiration. Wether plants are able to sustain these increased rates of carbon assimilation over long periods of time, or show an “acclimation” of their photosynthetic rates, is essentially dependent on their nitrogen supply and on large enough sinks for extra carbon to be stored. The consequences of this increase in carbon assimilation are, among others, a change in dry matter repartition very often beneficial to roots and, as nutrition cannot keep up with increased carbon assimilation, an increase C/N ratio. This change in chemical composition in plants may lead to major changes in herbivore behaviour and, as a consequence, in crops yields. Another consequence may be a change in natural soil fertility, where microbial activity will depend upon the exsudation of extracarbon of CO2 enriched plant roots.
Elevated [CO2] ameliorates the water status of plants. Stomats are sensitive to atmospheric [CO2], and respond with a partial closure which decreases stomatal conductance. Moreover, plants which have developed in increased [CO2] often show a decrease in stomatal numbers. This results in an increased water use efficiency, plants needing less water to make the same quantity of dry matter. The effects of this relative closure of stomatas may be counteracted by an increase in total leaf area, thus increasing transpiring surface. Wether these facts are inducing a better resistance to drought is depending on the species evolued.
All these studies, conducted on various crops or wild species, have led to an extreme diversity of responses. An intraspecific variability has been described for many species, which let think that selection could take place, giving space for the adaptation of new varieties, either by natural evolution or by man. In general, crop species respond more than wild one’s, nitrogen fixing plants being the most responsive. Within wild species, fast-growing plants seem to have larger responses than slow growing ones and within trees, deciduous are more responsive than evergreens.
This brief overview is restricted to plant physiological responses to elevated [CO2]. Compensatory or additive responses to other environmental changes may occur, which make predictions for future yield very uncertain. |
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