Differential reward in male versus female pollen of functionally dioecious Solanum

Premise Five to six percent of angiosperm species exhibit a dioecious sexual system, with unisexual male or female flowers borne on separate plants. The consequent need for inter-individual pollen exchange is a special challenge for taxa where pollen is the sole pollinator reward. Dioecious Australian Solanum assure visits from pollen-foraging bees via production of inaperturate pollen in functionally female (morphologically bisexual) flowers. Biochemical composition of pollen from Australian Solanum has not been assessed nor compared to porate pollen from staminate flowers to reveal whether these flowers differ in their pollinator reward potential. Methods Porate pollen from male flowers and inaperturate pollen from functionally female flowers of two functionally dioecious Australian species were compared for protein and amino acid content. We also assessed pollen from bisexual and staminate flowers of a closely related andromonoecious species, in which all pollen is porate, as a comparison across co-occurring sexual systems. Results In both functionally dioecious species, porate pollen grains from staminate flowers had significantly higher levels of proteins and amino acids than inaperturate pollen grains from functionally female flowers. Levels of proteins and amino acids were highest in bisexual and staminate flowers of the andromonoecious species. Conclusions Higher levels of proteins and amino acids in porate pollen of male flowers in our functionally dioecious Solanum species suggests a greater nutritive reward for bees foraging on male plants than for those foraging on functionally female plants. Greater reward in porate pollen (including andromonoecious species) may be connected to the potential to generate a pollen tube.

Automated summary

The study compared protein and amino acid content of porate pollen from male flowers and inaperturate pollen from functionally female flowers of two functionally dioecious Australian species. It found that porate pollen grains from staminate flowers had higher levels of proteins and amino acids compared to inaperturate pollen grains from functionally female flowers. Additionally, the andromonoecious species showed the highest levels of proteins and amino acids in pollen from bisexual and staminate flowers.