An integrated overview of physiological and biochemical responses of Celtis australis to drought stress

Trees in Mediterranean areas frequently face severe drought stress events, due to sudden decreases in soil water availability associated to intense heat waves. The knowledge of strategies adopted by plants to cope with the environmental pressures associated to Mediterranean climate is crucial for reforestation strategies and planning future urban greening. Here we investigated the physiological and biochemical adjustments activated by Celtis australis in response to drought stress during summer. Despite widely used for reforestation in Southern Mediterranean, how C. australis responds to the severe challenges imposed by Mediterranean climate has not investigated yet. In our study, we performed analyses of water relations, gas exchange and PSII performance, the concentration of photosynthetic pigments, the activity and the concentration of primary antioxidants in plants exposed to drought stress of increasing severity. Data of our study reveal that C. australis displays both conservative water use and isohydric behavior in response to drought, and diffusive resistance mostly limits photosynthesis even at severe drought. Our study also reveals an effective down-regulation rather than permanent impairment of PSII photochemistry in response to drought stress of increasing severity, since excess electron transport due to declines in photosynthesis (-61% at severe stress, compared to control) was matched by an increase in nonphotochemical quenching (+71% at severe stress, compared to control). However, our study highlights that under severe drought, zeaxanthin (and neoxanthin) increased by 75% (and 25%), likely served an important function as chloroplast antioxidant, other than sustaining nonphotochemical quenching. Antioxidant enzymes and ascorbate also increased (+132% on average for superoxide dismutase, ascorbate peroxidase, and catalase) and contributed in countering oxidative stress in severely droughted plants. Large adjustments in the suite of physiological and biochemical traits may effectively enable C. australis to gain carbon at appreciable rates while avoiding irreversible damage to the photosynthetic apparatus even when challenged by severe drought stress, thereby making this species an excellent candidate for forest and urban plantings in sites experiencing extended periods of drought stress

Plants' responses to novel environmental pressures

Plants have been exposed to multiple environmental stressors on long-term (seasonal) and short-term (daily) basis since their appearance on land. During the last decades, however, plants have been frequently exposed to sudden changes in their environment (imposed by global change) which indeed involves the acclimation/adaptation syndrome of living organisms. The frequency of these unpredictable \u2018stress\u2019 events is expected to increase further in the near future. Such severe, even transient alterations in environmental stimuli (variables) represent new challenges to plants, which do not possess the \u2018flight\u2019 strategy usually displayed by other organisms. Plants have developed, however, a multiplicity of highly integrated adjustments, involving morpho-anatomical, physiological and biochemical traits, to cope with challenges imposed by novel, harsher environments: these constitute the \u2018flight strategy of sessile organisms\u2019. Interestingly, several habitats threatened by the novel stresses are biodiversity hotspots. For example, Mediterranean basin, in which high light growing plants face heat waves coupled with the scarcity of rainfall of increasing frequency and severity, represents just 2% of the earth\u2019s land area, but account for 16% of the world\u2019s plant species. This implies that plants have been and are capable to display a wide range of acclimation/adaptation strategies to cope with most unfavorable environments. Nonetheless, the unpreceded rate at which climate changes may exceed the capacity of plants to acclimate and adapt successfully to the novel environmental pressures, further exacerbated by an increase in anthropogenic pressure. Understanding the mechanisms through which plants respond to new challenges posed by the concurrent effect of different stress agents is crucial, as obvious, to develop strategies of biodiversity conservation and ecosystem functionality. This is exactly the focus of this Research Topic. Review, Opinion as well as Original Research articles are welcome covering basic and applied research on plant functioning under adverse environmental conditions. The frequency of extreme stress events, mostly due to the concurrent effects of different stressors, is increasing particularly in the arid and semi-arid regions, which represent indeed among the most fragile ecosystems worldwide. Papers dealing with the effects of multiple stress agents on plant functioning are, therefore, particularly welcome. We are, however, also interested to receive contributions dissecting response mechanisms (from molecular to organism and whole-plant levels) of plants to a wide range of individual stressors, with a view to a rapidly changing climate, covering plant responses from other regions of the world. These include, but are not limited to drought and heat stress, excess light stress (including UV radiation), cold, ozone and rising CO2 concentration, and their combinations. Theories that predict the plant behavior, acclimation and plant plasticity are also inside the scope of this topi

Functional and structural leaf plasticity determine photosynthetic performances during drought stress and recovery in two platanus orientalis populations from contrasting habitats.

In the context of climatic change, more severe and long-lasting droughts will modify the fitness of plants, with potentially worse consequences on the relict trees. We have investigated the leaf phenotypic (anatomical, physiological and biochemical) plasticity in well-watered, drought- stressed and re-watered plants of two populations of Platanus orientalis, an endangered species in the west of the Mediterranean area. The two populations originated in contrasting climate (drier and warmer, Italy (IT) population; more humid and colder, Bulgaria (BG) population). The IT control plants had thicker leaves, enabling them to maintain higher leaf water content in the dry environment, and more spongy parenchyma, which could improve water conductivity of these plants and may result in easier CO2 diffusion than in BG plants. Control BG plants were also characterized by higher photorespiration and leaf antioxidants compared to IT plants. BG plants responded to drought with greater leaf thickness shrinkage. Drought also caused substantial reduction in photosynthetic parameters of both IT and BG plants. After re-watering, photosynthesis did not fully recover in either of the two populations. However, IT leaves became thicker, while photorespiration in BG plants further increased, perhaps indicating sustained activation of defensive mechanisms. Overall, our hypothesis, that plants with a fragmented habitat (i.e., the IT population) lose phenotypic plasticity but acquire traits allowing better resistance to the climate where they became adapted, remains confirmed