4 DISCUSSION
Intraspecific genetic variation can have important ecological consequences for interspecific interactions (Ehlers, David, Damgaard, & Lenormand, 2016; Fridley & Grime, 2010; Genung, Bailey, & Schweitzer, 2012). Kin recognition and allelopathic interference are important mediators of interspecific or intraspecific interactions among plants (Bilas, Bretman, & Bennett, 2020; Dudley, Murphy, & File, 2013; Kong et al., 2018; Wang, Kong, Wang, & Meiners, 2020). From a model system of allelopathic rice interference with paddy weeds, this study presents the first evidence that intraspecific kin recognition may have consequences for interspecific allelopathic interference. In particular, this study makes the case for a novel mechanism: that reductions in the costs associated with intraspecific interactions with kin allow increased interspecific allelopathic inhibition of co-occurring weeds. The direct evidence and mechanisms were that, when grown with closely related cultivars, there was (1) increased weed inhibition, maintaining rice grain biomass; (2) reduced production of rice allelochemicals, lowering defense cost; (3) altered root placement towards weeds, reconciling niche partitioning and competitive ability; (4) generated similar soil microbial community structure; and (5) altered carbon and nitrogen partitioning of weed species.
A plant may interfere with neighboring plant growth and establishment directly through competition, allelopathy, or both. In allelopathic rice against paddy weeds, the weed-suppressive effect is a combination of allelochemical action and interspecific competition (Kong, Hu, Wang, & Wu, 2008). Allelopathy and competition are well known to regulate inter- and intra-specific interactions and community assembly (Fernandez et al., 2016; Inderjit, Wardle, Karban, & Callaway, 2011; Xia, Kong, Chen, Wang, & Wang, 2016). An increasing number of studies have shown that kin recognition in plants can also alter the outcome of intraspecific and interspecific competition and determine species coexistence (Dudley, Murphy, & File, 2013; Ehlers, David, Damgaard, & Lenormand, 2016; Yamawo & Mukai, 2020). In some plant communities, there is a reciprocal relationship between competition and intraspecific trait variation (Bennett, Riibak, Tamme, Lewis, & Partel, 2016). In particular, intraspecific genetic relatedness may be more important than variation among species in determining species coexistence (Ehlers, David, Damgaard, & Lenormand, 2016; Fridley & Grime, 2010). These studies have revealed the linkage between kin recognition and competition in plant coexistence and community assembly. In the current study, we extend this observation to both competition and allelopathy. Intraspecific genetic relatedness in rice enhanced root intrusive behavior towards paddy weeds, affecting competition and allelopathy. Furthermore, kin recognition reduced the production of allelochemicals in rice cultivar mixtures with closely related cultivars. Root inhibition of paddy weeds mainly result from allelochemicals and intrusive roots of allelopathic rice cultivars generating avoidance patterns in weed roots (Yang & Kong, 2017). This study highlights that kin mixtures result in stronger root inhibition and avoidance growth of paddy weeds than non-kin cultivar mixtures, even with reduced production of allelochemicals. Therefore, weed inhibition appears to be generated by targeted kin recognition through altered root placement, reconciling niche partitioning and competitive ability, rather than an overall upregulation of allelochemicals.
Allelopathic rice cultivars produce and release allelochemicals against paddy weeds. However, the production of allelochemicals must also incur a defense cost, subsequently generating a trade-off between growth and defense (Meiners, Kong, Ladwig, Pisula, & Lang, 2012). A plant can change its defensive strategy based on the identity of its neighbors (Broz et al., 2010; Metlen, Aschehoug, & Callaway, 2009; Pierik, Mommer, & Voesenek, 2013). The production of allelochemicals also depends on co-occurring plant species (Kong et al., 2018; Ormeno, Fernandez, & Mevy, 2007; Xia, Kong, Chen, Wang, & Wang, 2016). The production of allelochemicals may be decreased in the presence of a “good” neighbor like the kin cultivars in this study, representing a benefit to the allelopathic plant while maintaining a reduced cost to the kin.
Plant neighbor recognition and allelochemical responses have been well documented between species (Kong et al., 2018;Lankau & Strauss, 2007; Pierik, Mommer, & Voesenek, 2013), but less is known about intraspecific kin and non-kin interactions. Several studies have shown that relatedness of neighboring plants mediates the expression of indirect defense traits (Karban, Shiojiri, Ishizaki, Wetzel, & Evans, 2013; Kalske, Shiojiri, Uesugi, Sakata, & Kessler, 2019; Yamawo, 2015). However, these studies have been primarily in the context of herbivore-induced defenses rather than direct plant-plant interactions. Data generated in this study revealed that kin recognition also affect chemical defenses in plant-plant interactions, reducing the production of allelochemicals in the presence of close relatives, shifting allocation from defense into growth.
Plants differentiate their neighbors based on genetic relatedness and alter functional traits in response to intraspecific competition (Donohue, 2003; Dudley, Murphy, & File, 2013). Functional trait responses can result in kin facilitation and cooperative behavior (Ehlers, David, Damgaard, & Lenormand, 2016). Functional changes in response to the presence of kin particularly alter patterns of root growth and distribution (Semchenko, John, & Hutchings, 2007; Semchenko, Saar, & Lepik, 2014), ameliorating the negative effects of root competition (File, Murphy, & Dudley, 2012). Roots from distantly related rice plants may grow towards each other while closely related roots exhibit avoidance, minimizing intraspecific competition (Yang, Li, Xu, & Kong, 2018). Reduced competition among kin in cultivar mixtures allowed individuals to better cope with interspecific competition.
Soil microorganisms may contribute to the coexistence of plant species (Miki, Ushio, Fukui, & Kondoh, 2010). The effects of kin on rice performance and interaction with weeds may have been partly driven by soil microbial composition. Soil microbial composition is closely associated with plant species and their root exudate metabolites (Hu et al., 2018; Schnitzer et al., 2011). In particular, root exudate metabolites drive plant-soil feedbacks by shaping the rhizosphere microbiota (Hu et al., 2018), impacting plant growth and chemical defense traits (Formenti et al., 2021). In the presence of competing barnyardgrass, allelopathic rice generated cultivar-specific soil microbial communities that can induce positive feedbacks on growth and reproduction (Sun, Wang, & Kong, 2014). The present data show that kin cultivar mixtures are capable of recruiting and assembling a similar soil microbial community in which they are able to thrive. Kin effects mainly occur in the preferred soil where competitor relatedness alters local soil conditions, causing indirect soil effects on plant coexistence (Ehlers, David, Damgaard, & Lenormand, 2016). Therefore, kin cultivar mixtures may create a better soil microbial community for rice growth.
An additional source of fitness variation in plants growing with relatives arises from the trade-off in the allocation of plant carbon and nitrogen (File, Murphy, & Dudley, 2012). Recent studies have indicated the importance of nitrogen partitioning and cycling in kin recognition (Semchenko, Saar, S., & Lepik, 2017; Zhang, Liu, Tian, Xu, & Ouyang, 2016). In this study, kin cultivar mixtures always had stronger inhibition on barnyardgrass than non-kin cultivar mixtures, regardless of available nitrogen. However, nitrogen application generated carbon and nitrogen allocation shifts of barnyardgrass from shoots into roots. These results imply that nitrogen acquisition and partitioning may function independently in intraspecific kin recognition and interspecific allelopathy.
Allelopathic rice cultivars differentially respond to kin and non-kin by altering the release of nitrogen-rich allantoin, releasing more allantoin in the presence of distantly related cultivars (Yang, Li, Xu, & Kong, 2018). In this study, allantoin application appeared to alleviate inhibitory differences between kin and non-kin cultivar mixtures and did not affect the carbon and nitrogen partitioning of barnyardgrass. Allantoin may act as a transportable nitrogen or signal to protect plants from stresses (Nourimand & Todd, 2016), activating the jasmonic acid signaling pathway (Takagi et al., 2016). Rice cultivars produce and release allantoin in response to species interactions (Wang, Kong, Hu, & Xu, 2007), inducing changes in microbial diversity and community composition in soil (Wang, Kong, Sun, & Xu, 2010). In this study, allantoin was not a transportable source of nitrogen, but may serve as signaling chemical in response to kin recognition. However, the role of allantoin as a signal of relatedness still needs to be verified in cultivar specific or upstream signals. Allantoin may not be the signal but rather the effect of an unidentified, underlying signal (Wang, Kong, Wang, & Meiners, 2020).