EFFECTS OF ENHANCED UV_B RADIATION ON WATER USE EFFICIENCY, STOMATAL CONDUCTANCE, LEAF NITROGEN CONTENT AND MORPHOLOGICAL CHARACTERISTICS OF SPIRAEA PUBESCENS IN A WARM_TEMPERATE DECIDUOUS BROAD_LEAF FOREST

Abstract

Spiraea pubescens, a common shrub in the warm-temperate deciduous forest zone, is distributed in the Dongling Mountain area of Beijing, was exposed to ambient and enhanced ultraviolet-B (UV-B, 280-320 nm) radiation by artificially supplying a daily dose of 9.4 kJ•m^(-2) for three growing seasons, a level that simulated a 17% depletion in stratospheric ozone. The objective of this study was to explore the effects of long-term UV-B enhancement on stomatal conductance, leaf tissue δ 13C, leaf water content, and leaf area. Particular attention was paid to the effects of UV-B radiation on water use efficiency (WUE) and leaf total nitrogen content. Enhanced UV-B radiation significantly reduced leaf area (50.1%) but increased leaf total nitrogen content (102%). These changes were associated with a decrease in stomatal conductance (16.1%) and intercellular CO2 concentration/air CO2 concentration (C(subscript i)/C(subscript a)) (4.0%), and an increase in leaf tissue δ 13C (20.5‰), leaf water content (3.1%), specific leaf weight (SLW) (5.2%), and WUE (4.1%). The effects of UV-B on the plant were greatly affected by the water content of the deep soil (30-40 cm). During the dry season, differences in the stomatal conductance, δ 13C, and WUE between the control and UV-B treated shrubs were very small, whereas differences became much greater when soil water stress disappeared. Furthermore, the effects of UV-B became much less significant as the treatment period progressed over the three growing seasons. Correlation analysis showed that enhanced UV-B radiation decreased the strength of the correlation between soil water content and leaf water content, δ 13C, C(subscript i)/C(subscript a), stomatal conductance, with the exception of WUE that had a significant correlation coefficient with soil water content. These results suggest that WUE would become more sensitive to soil water variation due to UV-B radiation. Based on this experiment, it was found that enhanced UV-B radiation had much more significant effects on morphological traits and growth of S. pubescens than hydro-physiological characteristics.