Abstract
Ground-level ozone (O3) is a widespread air pollutant causing extensive injuries in plants. However, its effects on perennial energy crops remain poorly understood due to technical difficulties in cultivating fast-growing shrubs for biomass production under O3 treatment on the field. Here we present the results of a two-year evaluation in the framework of which willow (Salix sachalinensis F. Schmid) shrubs were exposed to ambient (AOZ) or elevated (EOZ) O3 in two successive growing seasons (2014, 2015) and treated with 0 (EDU0) or 400 mg L−1 (EDU400) ethylenediurea spray in the second growing season. In 2014, EOZ altered the chemical composition of both top young and fallen leaves, and a novel mechanism of decreasing Mg in fallen leaves while highly enriching it in young top leaves was revealed in shrubs exposed to EOZ. In 2015, EDU400 alleviated EOZ-induced decreases in leaf fresh mass to dry mass ratio (FM/DM) and leaf mass per area (LMA). While EDU400 protected against EOZ-induced suppression of the maximum rate at which leaves can fix carbon (Amax) in O3-asymptomatic leaves, it did not alleviate EOZ-induced suppression of the maximum rates of carboxylation (VCmax) and electron transport (Jmax) and chlorophylls a, b, and a + b in the same type of leaves. In O3-symptomatic leaves, however, EDU400 alleviated EOZ-induced suppression of chlorophylls a and a + b, indicating different mode of action of EDU between O3-asymptomatic and O3-symptomatic leaves. Extensive herbivory occurred only in AOZ-exposed plants, leading to suppressed biomass production, while EOZ also led to a similar suppression of biomass production (EDU0 × EOZ vs. EDU400 × EOZ). In 2016, carry-over effects were also evaluated following cropping and transplantation into new ambient plots. Effects of EOZ in the preceding growing seasons extended to the third growing season in the form of suppressed ratoon biomass production, indicating carry-over effect of EOZ. Although EDU400 protected against EOZ-induced suppression of biomass production when applied in 2015, there was no carry-over effect of EDU in the absence of EDU treatment in 2016. The results of this study provide novel mechanistic understandings of O3 and EDU modes of action and can enlighten cultivation of willow as energy crop.
Highlights
Willows (Salicaceae) have been an integral part of the human societies since the ancient times due to their multiple uses, such as the use of their bark to alleviate pain (Vlachojannis et al 2009) and the utilization of their wood to create willow coracles, which dates back to Herodotus in the fifth century BC (Karp et al 2011)
Plants in VA-brown forest soil (BF) had 33.0% and 37.9% more Fe and Cr in fallen leaves relative to plants in BF. For both Fe and Cr, the only difference among means was between the means of BF and VA-BF in EOZ, i.e. plants grown in VA-BF had 78.5% and 65.7% more Fe and Cr in shed leaves, compared with plants grown in BF, in EOZ
Such toxicities were mainly observed in older leaves, which were exposed to EOZ for a longer time, and evaluation of young top leaves revealed no significant effect of EOZ on SPAD and leaf mass per area (LMA)
Summary
Willows (Salicaceae) have been an integral part of the human societies since the ancient times due to their multiple uses, such as the use of their bark to alleviate pain (Vlachojannis et al 2009) and the utilization of their wood to create willow coracles, which dates back to Herodotus in the fifth century BC (Karp et al 2011). Being affected by transboundary long-range transport (Gao et al 2020; Erickson et al 2020), which downplays national efforts to decrease O3 concentrations, O3 levels may remain elevated for several decades to come (Sicard et al 2017) Such elevated O 3 levels can lead to inhibited photosynthesis and suppressed woody biomass production and yields (Paoletti 2006; Karnosky et al 2007a; Koike et al 2013; Jolivet et al 2016; Li et al 2017; Cotrozzi 2021), suggesting increased risks to tree productivity (Sicard et al 2017; Feng et al 2019a; Proietti et al 2021; Sacchelli et al 2021). It remains unknown whether elevated O 3 suppresses the biomass production of shrub/tree energy crops cultivated on the field with no root limitation due to pot effect, yet such a root limitation would influence the response of plants to gaseous treatments (Oksanen 2003; Karnosky et al 2007b; Wang et al 2013)
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