We studied the emission of a-pinene from Quercus ilex leaves. Only the abaxial side of the hypostomatous Q. ilex leaf emits a-pinene. Light induced photosynthesis and a-pinene emission. However, the response of photosynthesis to dark-to-light transitions was faster than that of a-pinene, suggesting that ATP controls the emission. The emission was higher at 30 than at 20"C, whereas photosynthesis did not change. Therefore, the relationship between photosynthesis and a-pinene emission does not always hold. When CO, was removed from the air, transpiration was stimulated but photosynthesis and a-pinene emission were inhibited. a-Pinene inhibition was more rapid under low O,. When CO, in the air was increased, photosynthesis was stimulated and transpiration was reduced, but a-pinene emission was unaffected. Therefore, the emission depends on the availability of photosynthetic carbon, is not saturated at ambient CO,, and is not dependent on stomatal opening. The pattern of a-pinene emission from Q. ilex is different from that of plants having specialized structures for storage and emission of terpenes. We suggest that a-pinene emitted by Q. ilex leaves is synthesized in the chloroplasts and shares the same biochemical pathway with isoprene emitted by isoprene-emitting oak species.
l h e carbon of the four main monoterpenes emitted by Quercus ilex L. leaves was completely labeled with 13C after a 20-min feeding with 99% "CO,. This labeling time course is comparable with the labeling time course of isoprene, the terpenoid emitted by other Quercus species and synthesized in leaf chloroplasts. It is also comparable with that of phosphoglyceric acid. Our experiment therefore provides evidence that monoterpenes emitted by Q. ilex are formed from photosynthesis intermediates and may share the same synthetic pathway with isoprene. By analyzing the rate and the distribution of labeling in the different fragments, we looked for evidence of differential carbon labeling in the a-pinene emitted. However, the labeling pattern was quite uniform in the different fragments, suggesting that the carbon skeleton of the emitted monoterpenes comes from a unique carbon source.
Photosynthetic electron transport drives the carbon reduction cycle, the carbon oxidation cycle, and any alternative electron sinks such as nitrogen reduction. A chlorophyll fluorescence- based method allows estimation of the total electron transport rate while a gas-exchange-based method can provide estimates of the electron transport needed for the carbon reduction cycle and, if the CO2 partial pressure inside the chloroplast is accurately known, for the carbon oxidation cycle. The gas-exchange method cannot provide estimates of alternative electron sinks. Photosynthetic electron transport in flag leaves of wheat was estimated by the fluorescence method and gasexchange method to determine the possible magnitude of alternative electron sinks. Under non-photorespiratory conditions the two measures of electron transport were the same, ruling out substantial alternative electron sinks. Under photorespiratory conditions the fluorescence-based electron transport rate could be accounted for by the carbon reduction and carbon oxidation cycle only if we assumed the CO2 partial pressure inside the chloroplasts to be lower than that in the intercellular spaces of the leaves. To further test for the presence of alternative electron sinks, carbon metabolism was inhibited by feeding glyceraldehyde. As carbon metabolism was inhibited, the electron transport was inhibited to the same degree. A small residual rate of electron transport was measured when carbon metabolism was completely inhibited which we take to be the maximum capacity of alternative electron sinks. Since the alternative sinks were small enough to ignore, the comparison of fluorescence and gas-exchange based methods for measuring the rate of electron transport could be used to estimate the mesophyll conductance to CO2 diffusion. The mesophyll conductance estimated this way fell as wheat flag leaves senesced. The age-related decline in photosynthesis may be attributed in part to the reduction of mesophyll conductance to CO2 diffusion and in part to the estimated decline of ribulose 1,5-bisphosphate carboxylase amount.
The direct and indirect impact of ozone on Populus alba was studied by exposing leaves enclosed in specially designed cuvettes for 30 days to high ozone (150 ppb, 11 h per day), while leaves developing above the cuvettes were exposed to ambient ozone. Gas exchanges and histo‐anatomical parameters were measured to specifically understand whether ozone indirectly affects the anatomy and physiology of leaves. Three leaf classes were investigated: (1) those expanding above the cuvettes (A leaves); (2) those already developed inside the cuvettes (B leaves) and (3) those developing inside the cuvettes, since the beginning of the ozone treatment (C leaves). The anatomy and morphology of the first leaf developing outside the cuvette (A1) were strongly affected by ozone, whereas photosynthesis was not perturbed. However, in leaves of ozone‐treated plants developing after A1, a large reduction of starch accumulation was observed, which suggests a delayed biosynthesis, or a very rapid export of starch toward other sinks. Isoprene emission was higher and isoprene synthase messenger RNA was more expressed in ozone‐treated A1 leaves than in control leaves with similar ontogeny. This indicates that isoprene synthesis is stimulated by ozone, and reveals that isoprene emission is controlled at a transcriptional level. Leaves already developed inside the cuvette (B leaves) rapidly sensed ozone stress, which inhibited photosynthesis, stomatal conductance and isoprene emission. The observation that new leaves were developing inside the cuvettes during the treatment (C leaves) suggests that resistance to ozone may be acquired by plants. Leaves C showed a more packed and thinner mesophyll than controls of similar development, which may help reduce ozone penetration inside cells. They also showed a lower photosynthesis in comparison to controls and to other leaf classes, probably because of ribulose 1,5‐bisphosphate carboxylase/oxygenase activity limitation, as inferred from photosynthesis response to intercellular CO2. However, isoprene emission was slightly stimulated also in C leaves, confirming that a large fraction of carbon is invested into isoprene formation under ozone stress.
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