Abstract
Understanding urban tree responses to drought, salt stress, and co-occurring stresses, as well as the capability to recover afterward, is important to prevent the cited stresses’ negative effects on tree performance and ecological functionality. We investigated the impact of drought and salinity, alone and in combination, on leaf water potential, gas exchange, chlorophyll a fluorescence, xanthophyll cycle pigments, and isoprene emission of the urban tree species Liquidambar styraciflua L. Generally, drought had a rapid negative impact, while the effect of salt stress was more long lasting. Both stressors significantly decreased photosynthesis, transpiration, and stomatal conductance, as well as the maximum quantum efficiency of photosystem II (Fv/Fm) and the photochemical efficiency of PSII (ΦPSII), but increased nonphotochemical quenching (NPQ). Under stress conditions, a strong negative correlation between the PSII efficiency and the xanthophyll cycle pigment composition indicated a nocturnal retention of zeaxanthin and antheraxanthin in a state primed for energy dissipation. Drought and salt stress inhibited isoprene emission from leaves, although its emission was less responsive to stresses than stomatal conductance and photosynthesis. Full recovery of photosynthetic parameters took place after rewatering and washing off of excess salt, indicating that no permanent damage occurred, and suggesting downregulation rather than permanent impairment of the photosynthetic apparatus. Sweetgum trees were capable of withstanding and surviving moderate drought and salt events by activating defense mechanisms conferring tolerance to environmental stresses, without increasing the emission in the atmosphere of the highly reactive isoprene.
Highlights
The wide range of ecosystem services provided by urban trees and their benefits to the environment, climate, and human population are well recognized [1]
We addressed some of these knowledge gaps, investigating how the effects of moderate levels of drought, salt stress, and their combination affect leaf water potential, the efficiency of photosynthetic apparatus, and isoprene emission in Liquidambar styraciflua L
This study highlights the importance of the physiological responses of L. styraciflua to cope with water deficit and salt stress occurring in cities at different intensities throughout the growing season
Summary
The wide range of ecosystem services provided by urban trees and their benefits to the environment, climate, and human population are well recognized [1]. Trees can ameliorate the urban environment by reducing the heat island effect, sequestering CO2 , and lowering atmospheric pollutants [2,3]. Urban tree performance and functionality are decreased by human activities, limited sunlight, environmental pollutants, restricted soil volume and water availability, extreme temperatures, and soil compaction, some of which can be exacerbated by climate change [4]. These stressors are becoming more widespread in many regions and their severity is predicted to increase [7]. Drought stress is induced by both enhanced evapotranspiration and restricted soil volume, which prevent a sufficient amount of water from being retained in soils [8]
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