In the open-air exposure field experiment with European aspen and silver birch, only a 0.8–1 °C elevation in ambient temperature induced notable changes in leaf structure. The most prominent changes in leaf structure under slightly elevated temperature appeared as larger and thinner leaves with thinner epidermis, reduced non-glandular trichome (leaf hairs) density and increased size of plastoglobuli in one or both deciduous tree species. In general, the observed changes in leaf structure, especially those in leaf surface and inner tissue structure, supposedly indicated modification of the leaf towards more mesomorphic leaf structure under warmer environment with sufficient water availability.
Relatively few O3-induced changes in leaf structural characteristics were observed. In the field experiment, slight elevation in ambient O3 concentration affected leaf structure mainly during the second growing season by reducing leaf size and increasing the palisade layer thickness in silver birch leaves. Certain O3 -induced alterations were no longer observed when the warming treatment was added, therefore suggesting that rising temperature may compensate O3 impacts in leaf structure. In the chamber experiment with crops, higher O3 exposure (50–100 ppb) significantly reduced the amount of starch in the chloroplasts. Both higher and lower O3 exposure tended to increase the number of mitochondria in the leaf mesophyll cells supposedly as response to increased oxidative stress.
Most studies concerning temperature effects on plant-derived VOC emission have focused on impacts of relatively high temperatures. Our studies showed that even a 0.8–1 °C increase in ambient temperature during the growing season considerably enhanced emissions of mono-, homo- and sesquiterpenes as well as GLVs from European aspen and silver birch. Increased VOC emissions may be involved in plant acclimation to rising temperature as the birch and aspen saplings in our studies mainly benefitted from the temperature rise. Under O3 exposure, terpene emissions remained unaffected but both chronic and acute O3 treatment rather decreased GLV emission. In some plant species, VOCs are stored in glandular trichomes at the leaf surface, and also vacuole is a potential storage for VOCs within the leaf cells. We were not able to confirm the direct relation between VOC emission and leaf structure but our results suggest that leaf structure (e.g. thinner epidermis and larger leaf size) could enhance diffusion of VOCs. Overall, altered VOC emissions, i.e. increased emissions at increasing temperature and reduced emissions under O3 elevation, can have considerable implications for various ecological and atmospheric processes, particularly for interactions between plants and insects, and formation of O3 and secondary organic aerosols in the atmosphere.
Solely VOC emissions or leaf structural characteristics may not be used as sensitivity indicators of plant species or genotypes/cultivars. However, together with other parameters (e.g. photosynthetic efficiency, antioxidant capacity) they may help to understand the processes beneath the plant acclimation and protection.
In our studies, particularly one oat cultivar differed from others by showing low
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amount of visible leaf injuries under O3 exposure. This could relate to its low stomatal conductance and high monoterpene production and leaf thickness, potentially offering protection against O3 stress. Thus, the plant defence and acclimation to changing climate, including rising temperature and O3 levels, are presumably obtained by a combination of protective mechanisms, VOCs and leaf structural characteristics apparently being part of these.
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