Root competition influences pollen competitive ability in Viola tricolor: effects of presence of a competitor beyond resource availability?
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1. It has recently been suggested that not only nutrients but also presence of a competitor can influence competitive ability in plants, for example, by strategic changes in allocation to roots (a 'tragedy of the commons' effect). Such strategic changes might occur also in pollen, because this has potential to increase siring success during pollen competition in the pistil. 2. I tested the new hypothesis that pollen competitive ability - a trait often strongly affected by resources of the pollen parent - was influenced by presence of a root competitor using glasshouse-grown Viola tricolor in 2 years. Plants of two maternal families were combined in pairs with their roots either separated or intermingled in the same amount of resources. 3. Maternal families varied in response to root competition, with pollen performance increasing in some families, decreasing in others and unchanged in others. An increase did not mirror a decrease in the competing family, suggesting an explanation beyond differential ability to gain resources. The responses to competitive environment were often consistent across three independent competitors. 4. There was a positive correlation between family responses in pollen performance and family responses in plant size to root competitors. Larger plants did not produce better pollen per se, indicating that the change in pollen performance was not a pure side effect of altered plant size. There was no support for a 'tragedy of the commons' effect on root production. With the experimental design used, an effect of rooting volume could not be completely ruled out. 5. The currently unknown mechanism of the below-ground interactions between plants of V. tricolor was strong enough to change relative pollen competitive ability. These interactions might thus promote variation in the outcome of pollen competition. 6. Synthesis. The results of this study indicate the presence of a competitor on pollen competitive ability beyond the effect of nutrients. Even though the underlying mechanism needs to be explored further, the detected link between pollen competition and soil competition suggests a phenomenon well worth investigating. Further studies may lead to increased understanding of the evolutionary consequences of selection operating in response to interactions with neighbours.