Effect of root contact on pollen competitive ability in a hermaphroditic winter-annual herb

Presence of a root neighbor can induce changes in root allocation and pollen traits, but only a limited number of studies have investigated such effects on pollen. To learn more about effects of root contact on pollen competitive ability, we studied plants of the hermaphroditic winter-annual Collinsia heterophylla, native to California. We cultivated plants in two-pot treatments with roots kept either separate or intermingled with the same amount of resources. Pollen-tube growth rate, as an indication of pollen competitive ability, was affected by root treatment but the response varied among competing plant families. The response to root-treatment was not an effect of differential resource uptake of the two competitors. Root biomass was significantly higher when roots were intermingled compared to separate. This finding adds to the number of species with a strategic root response in the presence of competitors, but could also be a consequence of a larger rooting volume. Allocation to pollen performance versus roots in the presence of a competitor was lower in small plants and higher in large plants, potentially implying high costs of producing competitive pollen. We conclude that our study demonstrated that pollen tube growth rate is highly sensitive to the root environment in C. heterophylla.