Bouchra El Omari

Climate induced upslope migration of alpine plants is accompanied by a decline in atmospheric pressure, raising the question of whether plant performance at higher elevations is constrained by carbon availability and/or by water-related stress. Decreasing air pressure alters gas diffusivity, evaporative demand, and the partial pressure of CO₂, potentially shifting limitation from carbon acquisition towards plant water relations. However, in the field the impact of reduced air pressure is hardly to detect because it covaries with temperature, humidity, and radiation along natural elevational gradients.We thus addressed this question using ecotron experiments that isolate air-pressure effects from co-varying climatic factors. Mountain grassland species and model plants with contrasting functional strategies (Arabidopsis thaliana, Trifolium pratense, Hieracium pilosella, Brachypodium rupestre) were grown under atmospheric pressures corresponding to ~1,500–4,000 m a.s.l. (85–62 kPa), while temperature, radiation and air humidity were kept equal among the treatments. We quantified traits related to gas exchange (stomatal conductance, carbon isotope discrimination), carbon acquisition and allocation (photosynthetic efficiency, carbohydrate storage, biomass production), and nutrient status (leaf nitrogen and chlorophyll).Across species, reduced air pressure consistently lead towards higher photosynthetic energy-use efficiency and increased leaf carbohydrate pools (up to +40%), while aboveground biomass decreased. Gas-exchange revealed species-specific strategies: stomatal conductance increased or remained stable under low pressure in forb species, whereas grass responses depended on interactions with water availability. After 4 weeks results indicated a decreased carbon assimilation efficiency under low air pressure and a higher vulnerability to drought because of the higher Vapour-pressure deficit (VPD)These short- term results suggest that reduced air pressure is a relevant parameter for upwards-migrating mountain plants and may play an underestimated role in shaping composition and performance of alpine ecosystems.

Climate change is a major factor shaping the distribution of plant species. A well-documented response consequence is the upward shift of plant species to higher elevations as they track their thermal niches. However, plants migrating upward face complex environmental changes shaped by multiple interacting factors. Among these, reduced air pressure remains relatively understudied, its effects are often confounded with other covarying parameters. This study investigated the direct impact of reduced air pressure on the eco-physiological responses of two plant species (Hieracium pilosella L. and Trifolium pratensis L.). The plants were grown for 4 weeks in controlled climatic chambers under different air pressures (85, 75, and 62 kPa), while all other environmental parameters were kept constant. At the end of the experiment, photosynthesis, chlorophyll fluorescence, growth, carbohydrate content, carbon stable isotopes, and plant nitrogen concentrations were determined. Reduced air pressure decreased growth, carbon isotopic discrimination and chlorophyll content, but increased CO2 fixation efficiency and carbohydrate accumulation in the leaves. These results suggest that reduced air pressure impacts plant performance during upslope migration and may, in turn, contribute to shaping future distribution patterns in alpine ecosystems.

This paper examines the dependence of whole leaf hydraulic conductance to liquid water (K_L) on irradiance when measured with a high pressure flowmeter (HPFM). During HPFM measurements, water is perfused into leaves faster than it evaporates hence water infiltrates leaf air spaces and must pass through stomates in the liquid state. Since stomates open and close under high versus low irradiance, respectively, the possibility exists that K_L might change with irradiance if stomates close tightly enough to restrict water movement. However, the dependence of K_L on irradiance could be due to a direct effect of irradiance on the hydraulic properties of other tissues in the leaf. In the present study, K_L increased with irradiance for 6 of the 11 species tested. Whole leaf conductance to water vapour, g_L, was used as a proxy for stomatal aperture and the time-course of changes in K_L and g_L was studied during the transition from low to high irradiance and from high to low irradiance. Experiments showed that in some species K_L changes were not paralleled by g_L changes. Measurements were also done after perfusion of leaves with ABA which inhibited the g_L response to irradiance. These leaves showed the same K_L response to irradiance as control leaves. These experimental results and theoretical calculations suggest that the irradiance dependence of K_L is more consistent with an effect on extravascular (and/or vascular) tissues rather than stomatal aperture. Irradiance-mediated stimulation of aquaporins or hydrogel effects in leaf tracheids may be involved.

The total antioxidant activity (TAA) and the accumulation of small heat shock proteins (sHsps) were analysed under field conditions in Quercus ilex with regard to organ ontogeny and the physiological state of the plant. The results point toward the participation of sHsps and an increase of TAA in the general adaptation syndrome (GAS) of woody Mediterranean evergreens. In leaves and stems, there was a definite TAA seasonal pattern but no effect from diurnal variations or from the current stage of organ ontogeny. TAA was about twice as large in summer as in spring and winter and the hydrophilic antioxidant content was about 16 times greater than that of lipophilic antioxidants. The accumulation of sHsps in leaves also showed a seasonal pattern, but no effect from diurnal variations or from leaf ontogeny. In summer days, the sHsps content remained invariable even during the daylight hours in which the leaves were physiologically recovered. However, the accumulation of sHsps in stems did vary in relation to organ ontogeny. Old stems had a high accumulation of sHsps throughout the year, whereas in young stems, accumulation of sHsps was detected only in summer. This is probably due to a higher quantity of lignified and suberized tissues in the older stems. Using two‐dimensional immunodetection for leaves and stems, two sets of Hs protein species (17 and 10 kDa regions) were observed. In stems, there was an increase in 10 kDa proteins from winter to summer. These results are discussed and the possible role of the two types of polypeptides in the face of environmental and endogenous oxidative stress are considered.

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