Sexual dimorphism in resource allocation is expected to change during the life cycle of dioecious plants because of temporal differences between the sexes in reproductive investment. Given the potential for sex-specific differences in reproductive costs, resource availability may contribute to variation in reproductive allocation in females and males. Here, we used Rumex hastatulus, a dioecious, wind-pollinated annual plant, to investigate whether sexual dimorphism varies with life-history stage and nutrient availability, and determine whether allocation patterns differ depending on reproductive commitment. To examine if the costs of reproduction varied between the sexes, reproduction was either allowed or prevented through bud removal, and biomass allocation was measured at maturity. In a second experiment to assess variation in sexual dimorphism across the life cycle, and whether this varied with resource availability, plants were grown in high and low nutrients and allocation to roots, aboveground vegetative growth and reproduction were measured at three developmental stages. Males prevented from reproducing compensated with increased above- and belowground allocation to a much larger degree than females, suggesting that male reproductive costs reduce vegetative growth. The proportional allocation to roots, reproductive structures and aboveground vegetative growth varied between the sexes and among life-cycle stages, but not with nutrient treatment. Females allocated proportionally more resources to roots than males at peak flowering, but this pattern was reversed at reproductive maturity under low-nutrient conditions. Our study illustrates the importance of temporal dynamics in sex-specific resource allocation and provides support for high male reproductive costs in wind-pollinated plants.

Campbell, L. G., Teitel, Z., Miriti, M. N. and Snow, A. A. 2014. Context-specific enhanced invasiveness of Raphanus crop–wild hybrids: A test for associations between greater fecundity and population growth. Can. J. Plant Sci. 94: 1315–1324. Evolution by crop–wild hybridization may create plant lineages with greater population growth rates, dispersal, and persistence than weedy progenitors, depending upon plant density. We asked: (1) how does hybridization affect demography and population growth rate (lambda, λ) and (2) how does density affect demography and λ? Over 1 yr, we followed wild radish, Raphanus raphanistrum and crop-wild hybrid, Raphanus sativus×R. raphanistrum, plants grown in 18 experimental sub-populations (originally derived from three wild and three hybrid populations) to assess contributions of germination, survival, and seed production to λ. We explored genotypic differences in demographic characteristics of low- and high-density populations (from populations grown at unmanipulated densities for three generations), using a LTRE. Hybrid populations had greater λs than weedy progenitors when grown in low (but not high) densities. Seed production was more influential on λ than germination or survival, and seed mortality was least influential. Our results indicate weedy Raphanus populations may be best controlled by limiting seed production, rather than killing plants outright or preventing seeds from germinating. Furthermore, relative invasiveness of genotypes is density-dependent, low-density conditions improving potential invasiveness of hybrid populations. We emphasize that assessing invasive potential of a plant will be more successful if one uses a comprehensive demographic approach accounting for a weed's entire life history.

Cultivated plants are known to readily hybridise with their wild relatives, sometimes forming populations with weedier life-history strategies than their progenitors. Due to altered precipitation patterns from human-induced global climate change, crop-wild hybrid populations may have new and unpredictable environmental tolerances relative to parental populations, which would further challenge farming and land-management weed control strategies. To recognise the role of seed dormancy variation in weed invasion, we compared seedbank dynamics of two cross-type populations (wild radish, Raphanus raphanistrum, and crop-wild hybrid radish, R. raphanistrum 9 R. sativus) across a soil moisture gradient. In a seed-burial experiment, we assessed relative rates of seed germination, dormancy and seed mortality over two years across cross types (crop-wild hybrid or wild) and watering treatments (where water was withheld, equal to annual rainfall, or double annual rainfall). Weekly population censuses in 2012 and 2013 assessed the frequency and timing of seedling emergence within a growing season. Generally, germination rates were two times higher and seed dormancy was 58% lower in hybrid versus wild populations. Surprisingly, experimental soil moisture conditions did not determine seedbank dynamics over time. Yet, seed bank dynamics changed between years, potentially related to different amounts of annual rainfall. Thus, variation in seedbank dynamics may be driven by crop-wild hybridisation rates and, potentially, annual variation in soil moisture conditions.