Plant responses to short- and long-term exposures to high carbon dioxide levels in closed environments

Physics

Scientific paper

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Scientific paper

When higher plants are exposed to elevated levels of CO_2 for both short- and long-term periods photosynthetic C-gain and photoassimilate export from leaves are generally increased. Water use efficiency is increased on a leaf area basis. During long-term exposures, photosynthesis rates on leaf and whole plant bases are altered in a species specific manner. The most common pattern in C_3 plants is an enhanced rate of whole plant photosynthesis in a well irradiated canopy. Nevertheless, in some herbaceous species prolonged exposure to high CO_2 results in remobilization of nitrogenous reserves (i.e., leaf protein degradation) and reduced rates of mature leaf photosynthesis when assayed at ambient CO_2 and O_2 levels. Both short- and long-term exposures to those CO_2 levels (i.e., 100 to 2,000 mul.l^-1) which modify photosynthesis and export, also modify both endogenous ethylene gas (C_2H_4) release, and substrate, 1-aminocyclopropane-1-carboxylic acid (ACC), saturated C_2H_4 release rates from irradiated leaves. Photosynthetically active canopy leaves contribute most of the C_2H_4 released from the canopy. Prolonged growth at high CO_2 results in a persistent increase in the rate of endogenous C_2H_4 release from leaves which can, only in part, be attributed to the increase of the endogenous pools of C_2H_4 pathway intermediates (e.g., methionine, M-ACC, and ACC). The capacity for increasing the rate of C_2H_4 release in response to short-term exposures to varying CO_2 levels does not decline after prolonged growth at high CO_2. When leaves, whole plants, and model canopies of tomato plants are exposed to exogenous C_2H_4 a reduction in the rate of photosynthesis can, in each case, be attributed to the classical effects of C_2H_4 on plant development and morphology. The effect of C_2H_4 on CO_2 gas exchange of plant canopies is shown to be dependent on the canopy leaf area index.

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