1 INTRODUCTION

Silicon (Si), taken up from soil as monosilicic acid and deposited in plant tissues as silica (SiO₂·nH₂O), increases plant resistance to a wide range of biotic and abiotic stresses (e.g. water stress, metal toxicity, pathogens and herbivory) (Cooke & Leishman, 2016; Debona et al., 2017; Hartley & DeGabriel, 2016; Massey & Hartley, 2006) and confers mechanical strength to plants (Epstein, 1994; Raven, 1983). The essentiality of Si for plants remains challenging to assess (Coskun et al., 2019; Epstein, 1994) but increased resistance to herbivores and stress alleviation following Si fertilisation can lead to increased plant primary productivity and crop yields (Liang et al., 2015; Savant et al., 1999; Tubana et al., 2016; Xu et al., 2020). Because graminoid crop species can exhibit very high Si concentrations ([Si]) (e.g. up to 20% of SiO₂ in rice; Klotzbücher et al., 2018), the beneficial role of Si in agriculture is well recognised, and Si is routinely applied to croplands in many countries (e.g. China, Japan, USA, Brazil) (Datnoff et al., 2001; Yan et al., 2018). Thus, it is important to understand the factors affecting plant Si nutrition but, to date, we still have limited knowledge of how soil nutrient availability and interactions between plants affect Si concentration.

Increasing evidence suggests that plant Si concentration depends on soil nutrient status (de Tombeur, Laliberté, et al., 2021; Johnson et al., 2021; Minden et al., 2021; Quigley et al., 2020). In particular, decreases in Si concentration and resulting Si-based defences following nitrogen (N) fertilisation have recently been reported for different grassland/pasture species (Johnson et al., 2021; Minden et al., 2021; Quigley et al., 2020) (but see Moise et al., 2019). This has been attributed to the investment in ‘cheap’ Si versus relatively ‘more expensive’ carbon (C) (Raven, 1983) during N stress (Johnson et al., 2021; Minden et al., 2021) and reflects trade-offs between plant growth rate and carbon- or Si-based defences within Poaceae family (Massey et al., 2007). However, past studies have generally focused only on a single, non-cultivated genotype (Johnson et al., 2021; Minden et al., 2021). Significant genotypic variation in Si concentration has been reported in rice and wheat (Ma et al., 2007; Merah et al., 1999; Talukdar et al., 2019), so the plasticity (i.e. production of multiple phenotypes from a single genotype depending on environmental conditions; Miner et al., 2005) of leaf [Si] in response to N fertilisation might differ among genotypes, but this has not yet been tested.

So far, the influence of plant–plant interactions on plant Si nutrition has received surprisingly little attention in the literature (but see Garbuzov et al., 2011; Ning et al., 2017, 2021), especially compared with other nutrients (Li et al., 2014). At the interspecific level, Ning et al. (2021) showed that rice accumulates significantly more Si when grown with water spinach (Ipomoea aquatica Forsk)—a low Si-accumulating species—compared with a rice monoculture, possibly through the effect of root exudates on soil Si mobilisation (de Tombeur, Cornelis, et al., 2021; Ning et al., 2021). However, when two grasses with high Si-concentration (Poa annua and Lolium perenne) were investigated, such interspecific facilitation on Si concentration was not observed (Garbuzov et al., 2011). The influence of plant–plant interactions on Si concentration at the intraspecific level, to our knowledge, has received no attention, either in intra-genotypic cultures or intergenotypic mixtures. It is important to consider both intra- and intergenotypic cultures because facilitation for Si uptake in the rhizosphere might prevail over competition when genotypes are functionally different (e.g. they contrast in nutrient-acquisition strategies and/or Si demand). Furthermore, both types of genotypic cultures should be considered because intragenotypic stands are typical of modern agriculture, but there is increasing interest in the role of genetic diversity in increasing the sustainability of agriculture as greater intraspecific diversity may increase productivity and resistance to pests and pathogens (Barot et al., 2017; Hajjar et al., 2008; Litrico & Violle, 2015; Montazeaud et al., 2022).

Finally, leaf Si has been linked to different plant architecture traits that could in turn influence competition for light capture, including decreasing leaf insertion angle and leaf arc/straightness (Ando et al., 2002; de Tombeur, Cooke, et al., 2021; Yamamoto et al., 2012; Zanão Júnior et al., 2010), and increasing plant height (Gong et al., 2003; Ma et al., 1989; Zanão Júnior et al., 2010). As such, we might expect some relationships between the Si concentration of a genotype and the outcomes of plant–plant interactions (i.e. in this case, biomass loss or gain when mixed with a neighbour). It remains challenging to predict potential links between Si and competition outcomes, since greater plant height might increase competition intensity (Falster & Westoby, 2003; Violle et al., 2009), but decreasing leaf insertion angle and arc reduces the light extinction coefficient inside the canopy and may thus decrease competition intensity (Ando et al., 2002). Nevertheless, studies on Si benefits against biotic and abiotic stresses have greatly expanded during the last 10 years (Coskun et al., 2019), and investigating previously overlooked functions of silicification, such as its influence on plant architecture and potential impact on plant–plant interactions, is thus needed.

Here, we studied 19 genotypes of durum wheat (Triticum turgidum ssp. durum), a major staple crop, which we grew in pots, either alone, in intragenotypic culture or in intergenotypic culture, at two levels of N availability. We quantified plant above-ground biomass, plant height and leaf [Si] to (a) evaluate intraspecific variation in leaf [Si] among the 19 genotypes, (b) estimate plasticity of leaf [Si] in response to N fertilisation and plant–plant interactions and (c) explore potential relations between leaf [Si] and competition outcomes. The variation of leaf [Si] among genotypes, as well as plasticity in leaf [Si] in response to N fertilisation, was tested on genotypes grown alone to avoid a neighbour effect. How plant–plant interactions affect leaf [Si], either in intra- or intergenotypic cultures and with or without N addition, was tested by comparing the leaf [Si] of plants alone with that of plants in interaction. Finally, we tested correlations between genotype leaf [Si] and their response to competition in terms of biomass/height losses/gains to explore potential links between [Si] and competition outcomes. We hypothesised a decrease in leaf [Si] following N fertilisation. We further hypothesised that wheat genotypes would vary in both their Si concentrations, and in their response to N fertilisation and plant–plant interactions.