Vascular functioning and the development of flowers and fruit

Background/Question/Methods

Flowers and fruit have long been considered central to the evolutionary success of the angiosperms.  In contrast with the more continuous or rhythmic growth of other sinks, particular phases of development of flowers and fruit are often rapid and discontinuous, with abrupt changes in form, growth rate or type of development between phases.  Examples include the opening of the corolla, the production of nectar at anthesis, initial rapid growth in ovary fresh weight after fertilization, and changes in the form and rate of accumulation of sugars or other storage compounds during fruit ripening.  Accompanying these changes there may be dramatic shifts in the demand for carbon and water supplied by the vascular tissues, suggesting that vascular functioning must be highly coordinated with each stage of reproductive development.  Here we explore the role of phloem and xylem in these processes, against a backdrop of advancing knowledge of phloem functioning.  

Results/Conclusions

While studies of flower and fruit development have usually occurred in isolation from each other, for both stages of development a general view has arisen that the phloem is the sole contributor to supply of water and carbon.  In theory, xylem disconnection hydraulically isolates sensitive reproductive tissues from daily and seasonal fluctuations in plant water status. This view now appears overly simplistic, with evidence accumulating that both the phloem and xylem contribute to each stage of development, and are intricately coordinated with each other.   During fruit development gradual reductions in surface conductance and increases in xylem hydraulic resistance represent a coordinated mechanism that maintains water potential gradients favourable for inward phloem and xylem flow and growth. Inter-specific variation in the balance between phloem and xylem contributions, the types of sugars transported in the phloem, and transitions in phloem unloading during development can all have implications for plant reproductive ecology.