Rooting with neighbours: Detection affects growth and reproductive strategies of plants

Abstract

Evolutionary game-theoretical studies have indicated that plant populations with maximum seed production per unit area can be invaded by a mutant or intruder that grows more leaves, is taller or produces more roots, and that an evolutionarily stable vegetation is therefore less than maximally productive. Meanwhile, a growing body of research discovers that plant roots are able to detect the presence, even recognize the relatedness of their belowground neighbours. Root morphological traits and distribution patterns can be modified by neighbour presence independently of nutrient status. Such responses often lead to an overproliferation in roots at the expense of investments in reproduction. In line with evolutionary game theory, it is argued that neighbour detection enables plants to engage in resource competition with non-self-roots for a greater capture of shared resources, but at the same time to avoid non-profitable competition with self-roots. However, the mechanisms underlying such detection are still unclear. More importantly, the evidence of neighbour detection and its consequent root overproliferation scenarios in some studies have been subjected to debate, mainly arising from concerns about the methods employed in those studies. Therefore, the central topic addressed in this thesis is to determine the extent to which there is evidence of belowground neighbour detection with improved methodology and examine the possible occurrence of the consequent game theoretical scenarios. After a general introduction in Chapter 1, Chapter 2 provides an extensive review that summarizes the results of numerous ecological studies dealing with self/non-self and kin/non-kin root interactions, and proposes putative mechanisms for both types of recognitions. In Chapter 3, I show that corrections for rooting volume and plant size reveal Pisum sativum plants allocated less mass to roots in the presence of a neighbour. A major part of neighbour presence and rooting volume effects on plant growth was indirectly mediated through the influence on plant size. In Chapter 4, I demonstrate that growth differences between severed ramet pairs and intact ramet pairs in Potentilla reptans were likely to be caused by the disruption of source-sink relationship. With a novel design that avoids severing connections, I find that ramets grown with genetically identical disconnected neighbours produced similar amounts of root mass but less stolon mass than those grown with connected neighbours, irrespective of the distance along the clone. In Chapter 5, I show that Phaseolus vulgaris plants produced proportionally more large seeds when exposed to the roots of a neighbour, independently of the status of resource availability in seed production. Finally, in Chapter 6, I integrate the results of previous chapters, and conclude that (1) container-based experiments must carefully control plant rooting volume; (2) research on plant responses to environmental factors need to take plant size into account; (3) it is time to explore the underlying mechanisms of the belowground neighbour detection; (4) the responses of root growth to the presence of belowground neighbours seem to be environmentally dependent; (5) neighbour presence affects both vegetative growth and reproductive strategies of plants.