Implications for ecological weed control in agricultural crops
Farmers generally aim to maximize performance (e.g. yields or resource-use efficiency) at the crop stand (hence, plant population) level. This may be in conflict with the fact that in dense vegetation natural selection favours the most competitive plants rather than those maximizing group performance (see Anten & Vermeulen 2016; Denison, 2011). As kin selection can lead to more cooperative traits in plants and greater group performance, its potential use in crop breeding could be considered (Anten & Chen, 2021; Murphy, Van Acker, Rajcan, & Swanton, 2017). The results of Xu et al. are particularly interesting in this regard. Among all biotic stress types (i.e. pathogens, pests and weeds), weeds have the largest potential for yield reduction (larger than the other two stress types combined) (Oerke, 2006). Chemical control of weeds is coming under increasing pressure due to concerns about their health- and environmental impacts and increasing herbicide resistance of weeds. This in turn, has led to an increased interest in ecological weed control, of which development of weed-suppressive crops can be an important part (Bastiaans, Paolini, & Baumann, 2008). Thus, if kin discrimination can reduce intraspecific competition among crop plants in favour of interspecific competition with weeds as shown by Xu et al., this could be part of a strategy for ecological weed control.
The question however is whether kin selection is the most effective means of obtaining cooperative crop types. Conceptually, kin discriminating plants can be considered as facultative co-operators, that is, they exhibit cooperative behaviours only in the presence of kin and not in the presence of non-kin. In a crop variety mixture such a facultative co-operator may do worse than an obligate co-operator, i.e., a crop type that expresses cooperative behaviours irrespective of whether it interacts with kin or non-kin (Anten & Chen, 2021). Thus, the question arises whether preferential root placement towards the roots of weeds rather than towards con-specific crop plants could also be selected as a constitutive trait.