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.