Flower Petal Cell Walls: Changes Associated with Flower Opening and Senescence*

Flowers are prized as objects of great beauty and diversity, and are commercially valuable (~US$4.5 billion in international trade yearly) and highly perishable. Biologically, flower petals have an important role in the lifecycle of plants, as they protect immature reproductive structures, then provide the attraction and accessibility needed for pollination to occur. Flower petal cell wall construction, maintenance, and breakdown are important factors in the life of flowering plants as well as being related to the visual quality of commercial cut flowers. The petal is generally a thin structure, with a mesophyll-type cell layer between an ordered upper and lower epidermis. Petal cell walls are constructed in such a way as to be able to accommodate large and quite fast increases in cellular turgor during petal expansion and flower opening. There is some variety in the way the flower lifecycle may end once pollination has occurred, with petal wilting, shattering, abscission, almost complete autolysis of petal tissue, and the development of a papery shell all occurring in different species. The senescence of flower petals can be accompanied by increased activity of cell wall hydrolases, similar to that found during fruit ripening, and degradation of pectic and hemicellulosic polysaccharides. For some flowers, however, wall dissolution is restricted to depolymerisation of hemicelluloses and loss of neutral sugars, particularly galactose and arabinose. The few species in which the metabolism of cell wall polysaccharides in flower petals has been studied in depth include carnation, sandersonia, and daylily.