“Resource allocation and seed size selection in perennial plants under pollen limitation”

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Qiaoqiao Huang, Martin Burd, and Zhiwei Fan

Pollen limitation plays an important role in the ecology and evolution of seed size and adult survival in plants

At any level of ovule fertilization rate, the optimal seed size maximizes the ratio of juvenile survival rate to the resource investment needed to produce one seed (including both ovule production and seed provisioning)

Pollen limitation of plant reproduction is becoming increasingly common in nature because of factors such as human disturbance, habitat loss or fragmentation, and climate change, which may reduce plant and pollinator abundance. Plants may adopt a variety of ways to cope with pollen limitation. For example, plants may try to attract more pollinator visits by increasing resource allocation to pollinator attraction. Plants may also overproduce ovules to capture the reproductive opportunities afforded by the rare flowers that receive unusually abundant pollen. However, the possibility that pollen limitation may change seed size has not been explored.

Here, Huang, Burd, and Fan examine the effect of pollen limitation on resource allocation and the optimal seed size using an evolutionarily stable strategy resource allocation model in outcrossing perennial iteroparous plants. They find that under density-independent population growth, pollen limitation (i.e., a reduction in ovule fertilization rate) should increase the optimal seed size. At any level of pollen limitation (including none), the optimal seed size maximizes the ratio of juvenile survival rate to the resource investment needed to produce one seed (including both ovule production and seed provisioning), that is, the optimum maximizes the fitness effect per unit cost. Seed investment may affect allocation to post-breeding adult survival. In the model, pollen limitation increases individual seed size but decreases overall reproductive allocation, so that pollen limitation should also increase the optimal allocation to post-breeding adult survival. Under density-dependent population growth, the optimal seed size is inversely proportional to ovule fertilization rate. However, pollen limitation does not affect the optimal allocation to post-breeding adult survival and ovule production. The effect of pollen limitation on the optimal seed size also applies to semelparous plants.

The model indicates that in addition to changes in resource allocation to pollen attraction and ovule production, plants may also increase seed size to cope with pollen limitation. Thus, the model introduces a novel element for understanding the consequences of the widespread phenomenon of pollen limitation. Because most plants have an ovule fertilization rate less than unity, alleviating pollen limitation may often decrease seed size and increase seed number. Read the Article