Elevated atmospheric CO2 suppresses silicon accumulation and exacerbates endophyte reductions in plant phosphoru

Many temperate grasses are both hyper-accumulators of silicon (Si) and hosts of Epichloë fungal endophytes; functional traits which may alleviate environmental stresses such as herbivore attack. Si accumulation and endophyte infection may operate synergistically, but this has not been tested in a field setting, nor in the context of changing environmental conditions. Predicted increases in atmospheric CO2 concentrations can affect both Si accumulation and endophyte function, but these have not been studied in combination.

We investigated how elevated atmospheric CO2 (eCO2), Si supplementation, endophyte-presence and insect herbivory impacted plant growth, stoichiometry (C, N, P and Si), leaf gas exchange (rates of photosynthesis, stomatal conductance, transpiration rates) and endophyte production of anti-herbivore defences (alkaloids) of an important pasture grass (tall fescue; Lolium arundinaceum) in the field.

eCO2 and Si supplementation increased shoot biomass (+52% and +31%, respectively), whereas herbivory reduced shoot biomass by at least 35% and induced Si accumulation by 24%. Shoot Si concentrations, in contrast, decreased by 17–21% under eCO2. Si supplementation and herbivory reduced shoot C concentrations. eCO2 reduced shoot N concentrations which led to increased shoot C:N ratios. Overall, shoot P concentrations were 26% lower in endophytic plants compared to non-endophytic plants, potentially due to decreased mass flow (i.e. observed reductions in stomatal conductance and transpiration). Alkaloid production was not discernibly affected by any experimental treatment. The negative impacts of endophytes on P uptake were particularly strong under eCO2.

We show that eCO2 and insect herbivory reduce and promote Si accumulation, respectively, incorporating some field conditions for the first time. This indicates that these drivers operate in a more realistic ecological context than previously demonstrated. Reduced uptake of P in endophytic plants may adversely affect plant productivity in the future, particularly if increased demand for P due to improved plant growth under eCO2 cannot be met.

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多数温带草本植物既是硅（silicon, Si）超富集植物，同时也是香柱菌（Epichloë）真菌内生菌的宿主——这两类功能性状可缓解植食性动物取食等环境胁迫。硅富集与内生菌侵染可能存在协同效应，但目前尚未在田间环境或变化的环境条件下得到验证。据预测升高的大气CO₂浓度会同时影响硅富集与内生菌功能，但二者的联合效应尚未得到研究。

本研究以重要牧草物种高羊茅（Lolium arundinaceum）为研究对象，在田间条件下探究了大气CO₂浓度升高（eCO2）、硅添加、内生菌定植与昆虫植食作用对植物生长、化学计量特征（碳C、氮N、磷P及硅Si）、叶片气体交换参数（光合速率、气孔导度、蒸腾速率）以及内生菌合成的抗植食防御物质（生物碱）的影响。

大气CO₂浓度升高与硅添加分别使地上生物量提升了52%与31%，而植食作用则使地上生物量至少降低35%，同时诱导硅富集量提升24%。与之相反，在大气CO₂浓度升高条件下，植株地上部硅浓度降低了17%~21%。硅添加与植食作用均降低了地上部碳浓度。大气CO₂浓度升高会降低地上部氮浓度，进而导致地上部碳氮比升高。总体而言，内生菌定植植株的地上部磷浓度较未定植植株低26%，这可能与物质运输速率下降有关（即观测到的气孔导度与蒸腾速率降低）。所有实验处理均未对生物碱合成产生显著可观测的影响。内生菌对磷吸收的负面效应在大气CO₂浓度升高条件下尤为显著。

本研究首次结合部分田间条件，证实大气CO₂浓度升高与昆虫植食作用分别会降低与促进硅富集。这表明这些调控因子在比此前研究更贴近真实的生态场景中发挥作用。内生菌定植植株的磷吸收能力下降，可能会在未来对植物生产力产生不利影响，尤其是当大气CO₂浓度升高下植物生长提升所带来的磷需求增加无法得到满足时。

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