Differential response to copper stress in the reproductive resources and allocation of metallophyte Kummerowia stipulacea

Abundant seed production is a key life history trait for plant to maintain the stability of the whole population in adverse environments such as heavy metal contaminated mine area. In the current studies, we hypothesize that mine (metallicolous) populations of metallophytes have formed specialized reproductive strategies to adapt themselves to the heavy metal contaminated habitats, and differ from normal (non-metallicolous) populations in reproductive allocation. To test this hypothesis, the differences in reproductive resources and reproductive allocation between the copper mine and non-copper mine populations of pseudo-metallophyte Kummerowia stipulacea were comparatively examined under controlled Cu exposure experiments. Compared to non-copper mine population, copper mine population shows an increased seed output and larger reproductive effort under Cu stress. The increase of reproductive allocation in metallicolous population depends on not only seed size but also seed number per plant. The plants of metallicolous population increase allocation to the reproductive organs at the expense of a curtailment of allocation to vegetative traits, resulting in plants with shorter height and fewer branch numbers. There is little evidence displaying effect of root nodule on the reproductive resources and allocation. In addition, plants in metallicolous population reduce the transfer of Cu from roots to aboveground parts. These data suggest that plants of metallicolous population tend to invest more resources to reproductive output and increase their reproductive allocation in the adaptive evolution to Cu-enriched mine soils.

► Cu stress increasing reproductive resources of metallicolous population while decreasing those of non-metallicolous population.
► Cu stress increasing reproductive allocation of metallicolous population while decreasing that of non-metallicolous population.
► Root nodules showing little effect on reproductive resources and allocation.
► Reducing Cu transport and allocation to aboveground parts in metallicolous population.
► Metallicolous population increasing their reproductive allocation in adaptive evolution in Cu contaminated environments for increasing fitness.