1 INTRODUCTION
Understanding the mechanisms of co-occurring species interactions in plant coexistence and community assembly has been of great interest to ecologists for decades (Maron, Hahn, Hajek, & Pearson, 2020; Tilman & Pacala, 1993). When one or more species occur together and interact, plants may detect and recognize their neighbors, potentially differentiating interspecific individuals from conspecific individuals (Gruntman & Novoplansky, 2004), and even intraspecifically between kin and non-kin individuals (Dudley & File, 2007). Following neighbor detection and recognition, plants may adjust their growth and defense strategies, altering plant coexistence and community assembly (Wang, Kong, Wang, & Meiners, 2020).
The role of plant-plant interactions in regulating coexistence has been a fundamental issue within ecology and evolution, focusing predominately on interspecific and intraspecific competition (Bennett, Riibak, Tamme, Lewis, & Partel, 2016; Genung, Bailey, & Schweitzer, 2012; Yamawo & Mukai, 2020). In recent decades, this issue research has expanded to include both kin recognition and allelopathy (Bilas, Bretman, & Bennett, 2020; Kong et al., 2018). Kin recognition may allow plants to optimize competitive strategies, resulting in less intraspecific competition and more cooperation among plants, maximizing stand performance (Bilas, Bretman, & Bennett, 2020; Dudley, Murphy, & File, 2013). Kin recognition in crop cultivars may also be exploited to improve crop yields (Anten, & Chen, 2021; Murphy, Swanton, van Acker, & Dudley, 2017). In contrast to the altruism of intraspecific kin recognition, allelopathy is an interference mechanism in which plants produce and release allelochemicals exerting a mostly negative effect on interspecific or intraspecific neighbors. Such allelopathic interferences can have profound effects on the performance of neighboring plants and alter local plant coexistence (Meiners, Kong, Ladwig, Pisula, & Lang, 2012; Zhang, Liu, Yuan, Weber, & van Kleunen, 2020). Noteworthily, different from competition for taking up shared resources, allelopathy is a chemical strategy in the defense of plant against competing neighbors, which results from the allelochemicals produced and released from plants themselves (Macías, Mejías, & Molinillo, 2019).
Both kin recognition and allelopathy have been investigated extensively in natural and managed ecosystems (Anten, & Chen, 2021; Inderjit, Wardle, Karban, & Callaway, 2011; Macías, Mejías, & Molinillo, 2019; Serra, Shanmuganathan, & Becker, 2021). However, kin recognition and allelopathic interference are usually studied separately, with no attention to their potential linkage. Plants often grow in mixtures of kin, non-kin conspecifics, and individuals from different species, and thus kin recognition and interspecific allelopathic interference may often occur simultaneously. Accordingly, kin recognition may interact with allelopathic interference within a heterogenous community context. Ultimately, understanding the interactions between kin recognition and allelopathic interference in diverse natural systems will depend on initial efforts from simplified agroecosystems.
Crop plants have coexisted with weeds in agroecosystems for thousands of years and can act as models for addressing eco-evolutionary dynamics in plant coexistence (Baucom, & Holt, 2009). Weeds pose an important biological constraint to crop productivity while only a few crop cultivars are very competitive against weeds. However, the weed-suppressive effects have not been fully explained by considering just the physical characteristics of being strong in competition for resources (Bastiaans, Kropff, Kempuchetty, Rajan, & Migo, 1997; Olofsdotter, Navarez, Rebulanan, & Streibig, 1999). Subsequent studies have shown the participation of allelopathy. In particular, allelopathic crop cultivars can detect the presence of weeds and respond by increasing allelochemicals to inhibit weeds, maximizing their own growth (Kato-Noguchi, 2011; Kong et al., 2018; Kong, Li, Hu, Xu, & Wang, 2006). Therefore, crop-weed interactions are not only of fundamental ecological interest, but also exceedingly important from an applied perspective in sustainable agriculture.
Rice (O ryza sativa ) is one of the principal food crops in the world, containing both allelopathic and non-allelopathic cultivars. Allelopathic rice cultivars can produce and release allelochemicals to inhibit the growth and establishment of paddy weeds, acting as an efficient component of integrated weed management (Kong, Hu, Wang, & Wu, 2008; Patni et al., 2018; Serra, Shanmuganathan, & Becker, 2021). Recent studies have shown kin recognition in rice cultivars (Fang et al., 2013) and allelopathic rice cultivars with the ability for kin recognition can alter root behavior and biomass allocation, increasing grain yields (Yang, Li, Xu, & Kong, 2018). Therefore, allelopathic rice interference with paddy weeds represents a well-characterized model system to understand the combined roles of kin recognition and interspecific allelopathy in plant-plant interactions.
There is a wealth of information on allelopathic rice interference with paddy weeds under field situations and controlled conditions (Gealy, Rohlla, & Boykin, 2019; Kato-Noguchi, 2011; Kong, Hu, Wang, & Wu, 2008; Li, Zhao, & Kong, 2020). However, when it comes to the role of kin recognition in allelopathic rice interference with paddy weeds, there is a lack of data. Such information is critical for understanding the consequences and mechanisms of plant coexistence in the given ecosystem. In this study, we identify the potential linkage between kin recognition and allelopathic interference in a rice paddy model system, to test two specific hypotheses (1) whether kin recognition can alter the consequences of interspecific allelopathic interference, and (2) whether relatedness allows allelopathic plants to discriminate between neighboring collaborators (kin) and competitors by altering biochemical plasticity and then adjust their growth, competitiveness, and chemical defense. To do this, we examined the effects of allelopathic rice cultivars with the ability for kin recognition from two sets of indica-inbred and indica-hybrid rice lines against paddy weeds in field and controlled conditions. Furthermore, we addressed potential mechanisms through documenting changes in root behavior, allelochemical production, associated soil microbial communities, and the carbon and nitrogen partitioning of weeds. Together, these efforts provide a thorough assessment of the contribution of kin recognition to allelopathic interference, and its implications for plant coexistence and community assembly.