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Reproductive success and sexual allocation in Stylidium armeria
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posted on 08.02.2017by Brookes, Rowan
Reproductive success and the patterns of sexual allocation in flowering plants are affected by the intrinsic and extrinsic influences that vary within and between plants. I used the sequentially flowering, perennial herb, Stylidium armeria to explore reproductive success and sexual allocation over two flowering seasons at Lake Mountain, Victoria, Australia.
The Haig and Westoby model predicts that plants should be simultaneously limited by both pollen and resources. I tested this model using a fully factorial experiment that manipulated both pollen and resources. Seed number increased at the whole-plant level when pollen and resources were added simultaneously, but not when added separately, a result in accord with the predictions of the Haig and Westoby model.
Internal effects can also influence reproductive success: earlier flowers can pre-empt resources that could be allocated to later flowers, or architectural constraints could occur when shoot ontogeny creates constraints. Distal flowers and fruits within inflorescences tended to have lower biomass. Racemes with a large initial investment in basal flowers allocated less towards ovules and seeds in distal flowers compared to racemes with a smaller initial investment, suggesting a trade off between initial and subsequent allocation, consistent with the resource pre-emption hypothesis. The magnitude of the trade-off was altered by resource manipulations, but the trade-off was never removed. Together with the decline in floral and fruit biomass, this decline implies that architectural effects occur. This pattern suggests that investment in S. armeria is jointly constrained by both resources and architectural effects.
The mating environment hypothesis predicts that the sex ratio of a population will greatly influence reproductive success in dichogamous plants. To test this, I observed the phenology of S. armeria and asked whether changes in the pollen/ovule ratio throughout the flowering season influenced female fitness. Protandry resulted in the floral sex ratio becoming more female as the flowering season progressed. In addition, flowers had relatively greater male allocation towards the end of the season, and those plants with a late first flowering date were more male, in agreement with theoretical predictions for protandrous plants. Consistent with the mating environment hypothesis, I found that when there was lower competition for male flowers in the population, the probability of fruit set increased.
Phenological patterns can affect sex allocation, as plants with different first flowering date (FFD) or flowering duration can be exposed to different selective pressures. I found that plants with an earlier FFD had a greater calyx mass and seed number and invested more in seed and fruit mass. These results suggest that it might be adaptive for maternal fitness to flower earlier in the season.
Female fitness in S. armeria is the result of intrinsic resource and architectural constraints and extrinsic competition in the mating environment. The study has contributed to our knowledge on the interaction of pollen and resource on plant reproduction and furthered our understanding of the contribution of mating environment in determining female fitness.