The effect of warming and enhanced ultraviolet radiation on gender-specific emissions of volatile organic compounds from European aspen.

Different environmental stress factors often occur together but their combined effects on plant secondary metabolism are seldom considered. We studied the effect of enhanced ultraviolet (UV-B) (31% increase) radiation and temperature (ambient +2 °C) singly and in combination on gender-specific emissions of volatile organic compounds (VOCs) from 2-year-old clones of European aspen (Populus tremula L.). Plants grew in 36 experimental plots (6 replicates for Control, UV-A, UV-B, T, UV-A+T and UV-B+T treatments), in an experimental field. VOCs emitted from shoots were sampled from two (1 male and 1 female) randomly selected saplings (total of 72 saplings), per plot on two sampling occasions (June and July) in 2014. There was a significant UV-B×temperature interaction effect on emission rates of different VOCs. Isoprene emission rate was increased due to warming, but warming also modified VOC responses to both UV-A and UV-B radiation. Thus, UV-A increased isoprene emissions without warming, whereas UV-B increased emissions only in combination with warming. Warming-modified UV-A and UV-B responses were also seen in monoterpenes (MTs), sesquiterpenes (SQTs) and green leaf volatiles (GLVs). MTs showed also a UV × gender interaction effect as females had higher emission rates under UV-A and UV-B than males. UV × gender and T × gender interactions caused significant differences in VOC blend as there was more variation (more GLVs and trans-β-caryophyllene) in VOCs from female saplings compared to male saplings. VOCs from the rhizosphere were also collected from each plot in two exposure seasons, but no significant treatment effects were observed. Our results suggest that simultaneous warming and elevated-UV-radiation increase the emission of VOCs from aspen. Thus the contribution of combined environmental factors on VOC emissions may have a greater impact to the photochemical reactions in the atmosphere compared to the impact of individual factors acting alone.