Influence des contraintes environmentales combinées sur la capacité photochimique et les flux de CO2 dans une prairie tempérée

Increase in agricultural production to insure food security and energy demand by 2050 might result in higher greenhouse gas emissions (GHG) from the agricultural sector. Managed grasslands, however, offer the opportunity to offset some of the GHG emission through the storage of carbon in terrestrial systems by photosynthesis. Photosynthesis, however, is highly sensitive to environmental conditions. Especially, plant ability to harvest and use light energy for photochemistry can be impaired by abiotic stresses. While numerous studies have focused on the impact of environmental constraints on ecosystem carbon fluxes, the influence on ecosystem photochemical capacity is understudied. The main goals of this thesis was to evaluate how environmental constraints impacted the grassland photochemical capacity and how variations in processes involved in light reactions of photosynthesis influenced ecosystem carbon fluxes. Frequent chlorophyll fluorescence measurements were conducted over a two-year period, on three grassland species (Lolium perenne L., Taraxacum sp., and Trifolium repens L.). The ecosystem photochemical capacity was estimated from measurements performed on the three grassland species. In addition, monitoring CO2 fluxes was performed by eddy covariance. Our results showed that photochemical capacity of the primary grasslands species exhibited diurnal and seasonal variations. The monocot L. perenne and the dicots (Taraxacum and T. repens) exhibited different acclimation strategies. All species exhibited the onset of energy dissipation mechanisms within the photosystem II but expressed contrasted response in the photosystem I efficiency. As a result, the ecosystem also exhibited variations in its ability to harvest and use photon energy. The strongest declines in photochemical capacity were observed in summer when abiotic stresses such as high light and high air temperature were combined. However, decrease in photochemical capacity did not result in a decreased ability to fix carbon in the grassland. The maintenance of carbon assimilation despite the onset of energy dissipation mechanisms can be explained by the higher availability of light energy under these conditions. In the final section of this PhD thesis, we discuss how future experiments can improve our knowledge in plants functional ecology and in the relationship between the photochemical capacity and ecosystem carbon fluxes. We also discuss how these results can benefit GHG mitigation strategies and how plants influence GHG balance through other routes than photosynthesis.CROSTVO

Impact of abiotic stresses on volatile organic compound production of field crops and grasslands

Abiotic and biotic stresses are known to alter biogenic volatile organic compound (BVOC) emission from plants. With the climate and global change, BVOC emissions are likely to increase. This increase on BVOC emissions could be driven by many environmental parameters like temperature, ozone and light availability for photosynthesis although it is still difficult to predict the impact of some environmental parameters, environmental controls on BVOC emission being species and BVOC-dependent. These BVOC are involved in a wide range of interactions of plants with their environment and these interactions could be affected by the global change. Moreover, BVOC also play a key role in the atmospheric chemistry and may contribute to ozone formation and an increase in methane lifetime, strengthening the global change. Yet, due to technical limitation, there are few studies examining the impact of multiple co-occurring stresses on BVOC emission at the ecosystem level although stress combination is probably more ecologically realistic in field. In the CROSTVOC (for CROp STress VOC) project, the impact of abiotic stresses (e.g. heat, drought, ozone and grazing) on BVOC emission will be investigated for field crops (maize and wheat) and grassland both at the ecosystem and plant scale.CROSTVO

Biogenic volatile organic compound emissions from senescent maize leaves and a comparison with other leaf developmental stages

Plants are the major source of Biogenic Volatile Organic Compounds (BVOCs) which have a large influence on atmospheric chemistry and the climate system. Therefore, understanding of BVOC emissions from all abundant plant species at all developmental stages is very important. Nevertheless, investigations on BVOC emissions from even the most widespread agricultural crop species are rare and mainly confined to the healthy green leaves. Senescent leaves of grain crop species could be an important source of BVOCs as almost all the leaves senesce on the field before being harvested. For these reasons, BVOC emission measurements have been performed on maize (Zea mays L.), one of the most cultivated crop species in the world, at all the leaf developmental stages. The measurements were performed in controlled environmental conditions using dynamic enclosures and proton transfer reaction mass spectrometry (PTR-MS). The main compounds emitted by senescent maize leaves were methanol (31% of the total cumulative BVOC emission on a mass of compound basis) and acetic acid (30%), followed by acetaldehyde (11%), hexenals (9%) and m/z 59 compounds (acetone/propanal) (7%). Important differences were observed in the temporal emission profiles of the compounds, and both yellow leaves during chlorosis and dry brown leaves after chlorosis were identified as important senescence-related BVOC sources. Total cumulative BVOC emissions from senescent maize leaves were found to be among the highest for senescent Poaceae plant species. BVOC emission rates varied strongly among the different leaf developmental stages, and senescent leaves showed a larger diversity of emitted compounds than leaves at earlier stages. Methanol was the compound with the highest emissions for all the leaf developmental stages and the contribution from the younggrowing, mature, and senescent stages to the total methanol emission by a typical maize leaf was 61, 13, and 26%, respectively. This study shows that BVOC emissions from senescent maize leaves cannot be neglected and further investigations in field conditions are recommended to further constrain the BVOC emissions from this important C4 crop species.CROSTVOC research project (T.0086.13