Abstract
Enhanced ultraviolet-B (UV-B) radiation and elevated tropospheric ozone alone may inhibit the growth of agricultural crops. However, research regarding their combined effects on growth and biochemical properties of roots is still scarce. Using open top chambers, we monitored the response of growth, secondary metabolites, endogenous hormones and enzyme activities of soybean roots to elevated O3 and enhanced UV-B individually and in combination at stages of branching, flowering and podding. Our results indicated that the root biomass decreased by 23.6, 25.2, and 27.7%, and root oxidative capacity declined by11.2, 39.9, and 55.7% exposed to elevated O3, enhanced UV-B, and O3 + UV-B, respectively, compared to the control treatment. Concentrations of quercetin and ABA were significantly increased, while concentrations of total polyphenol and P-coumaric acid responded insignificantly to elevated O3, enhanced UV-B, and O3 + UV-B during the whole period of soybean growth. Elevated O3, enhanced UV-B and O3 + UV-B showed significant negative effects on superoxide dismutase (EC 1.15.1.1) activity at flowering stage, on activities of peroxidase (EC 1.11.1.7) and catalase (EC 1.11.1.6) at podding stage, on ascorbate peroxidase activity during the whole period of soybean growth. Moreover, compared to hormones and enzyme activity, secondary metabolisms showed stronger correlation with root growth exposed to elevated O3 and enhanced UV-B individually and in combination. Our study concluded that combined effects of O3 and UV-B radiation significantly exacerbated the decline of soybean root growth, and for annual legumes, the inhibited root growth exposed to O3 and/or UV-B radiation was mostly associated with secondary metabolisms (especially flavonoids).
Highlights
Tropospheric O3 has increased worldwide as a result of industrialization and anthropogenic activities, and its concentration has already exceeded the threshold levels for the protection of vegetation (Vingarzan, 2004)
Biomass of root and nodule as well as root activity were significantly lower in the treatments of elevated O3, enhanced UV-B radiation and O3 + UV-B than CK treatments during the whole period of soybean growth (Figure 2)
There were no significant differences in root biomass between elevated O3 treatment and enhanced UV-B treatment at stages of flowering and podding (Figure 2A)
Summary
Tropospheric O3 has increased worldwide as a result of industrialization and anthropogenic activities, and its concentration has already exceeded the threshold levels for the protection of vegetation (Vingarzan, 2004). The increase of tropospheric O3 concentration can reduce plant growth and crop yields by its direct injury on leaves and alteration of photosynthetic systems, and indirectly through its impacts on resource allocation (Goumenaki et al, 2010). Numerous studies have shown that enhanced UV-B radiation can affect physiological and biochemical processes of many plant species (Jansen et al, 1998; Newsham and Robinson, 2009; Bussell et al, 2012; Li X. et al, 2012; Mazza et al, 2013). Most of our knowledge today depends on the individual effects of O3 and UV-B radiation on the productivity and quality of some important agricultural crops and plants, and studies regarding their combined effect on crop yields and plant growth are still scarce (Ambasht and Agrawal, 2003; Tripathi et al, 2011; Rinnan et al, 2013; Tripathi and Agrawal, 2013)
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