Reproductive allocation and resource compensation in male-sterile and hermaphroditic plants of Plantago lanceolata (Plantaginaceae)

Gynodioecy is a breeding system in which hermaphrodites coexist with male steriles. Theoretical models predict that without any compensation in female fitness male steriles will disappear from a population due to their reproductive disadvantage. In the present study I investigated whether male-sterile (MS), partially male-sterile (IN), and hermaphroditic (H) plants of Plantago lanceolata differed in reproductive growth and allocation. Offspring of three interpopulation crosses segregating all three sex morphs were grown under nitrogen-limited conditions in a growth chamber. Independent of the genetic background MS plants attained a higher vegetative and reproductive dry mass and a higher reproductive output than H plants, whereas IN plants had intermediate values. When corrected for the mass of the pollen, the dry mass differences between the sex morphs were much reduced but still present. However, when whole-plant allocation was expressed on the basis of nitrogen, the differences between the sex morphs disappeared. Thus the sex morphs took up similar amounts of nitrogen but distributed them differently. The MS and IN plants used the nitrogen saved by not producing pollen for additional vegetative as well as reproductive growth. The data presented in this study suggest that resource compensation is one of the main mechanisms responsible for the maintenance of MS and IN plants in gynodioecious P. lanceolata.