Effects Of Enhanced UV-B Radiation On Hydro-physiology, Nitrogen Use And Morphological Characteristics Of Spiraea Pubescens In Warm-temperate Deciduous Broad-leaved Forest

Water is one of the main factors limiting plants' growth and distribution in most areas of North China. What's more, dramatic increase in solar ultraviolet-B radiation (UV-B, 280-320 nm) reaching the Earth's surface will occur due to the ozone depletion. Under the interactive effects of the two environmental factors, plants will develop the corresponding strategy to deal with the stress. Spiraea pubescens, one of the popular shrubs in warm-temperate deciduous forest zone, distributing in Dongling Mountain area of Beijing, was exposed to ambient and enhanced ultraviolet-B (UV-B, 280-320nm) radiation by artificially supplying daily dose of 9.4kJ/m2 for three growth seasons in simulating 17% depletion in stratospheric ozone on the experimental site. The objective of this study was to explore the effects of long-term UV-B enhancement on hydro-physiology, nitrogen use and morphological characteristics of Spiraea pubescens. The investigated parameters include stomatal conductance, leaf tissue δ¹³ C, leaf water content and leaf area. Particular attention was paid to the effects of the UV-B radiation on the contribution of water use efficiency (WUE) and leaf total nitrogen content, leaf nitrogen resorption. Enhanced UV-B radiation significantly reduced leaf area (50.1%) but increased leaf total nitrogen content (102%), nitrogen resorption (50.9%). These changes were associated with the decrease in stomatal conductance (16.1%) and intercellular CO2 concentration/ambient CO₂ concentration (Ci/Ca) (4.0%), and the increase in leaf tissue δ¹³ C (20.5‰), leaf water content (3.1%), specific leaf weight (SLW) (5.2%), and water use efficiency (4.1%). The effects of the UV-B were greatly affected by the water content in deep soil (30-40cm). During the water deficient season, the differences in the stomatal conductance, δ13 C, WUE between the control and UV-B treated shrubs were very small, whereas the differences became much more obvious when soil water stress disappeared. Furthermore, the effects of UV-B became much less significant as the treatment period lasted longer. Correlation analysis was shown that enhanced UV-B radiation decreased the correlation coefficients between the soil water content and leaf water content, δ13C, Ci/Ca, stomatal conductance, with the exception of WUE, its correlation coefficient with soil water content was significant. It seemed that WUE would become more sensitive to soil water variation due to the enhanced UV-B radiation. Based on this experiment, it was found that enhanced UV-B radiation had much more significant effects on its morphological traits and growth of Spiraea pubescens than the hydro-physiological characteristics.