Pigment distribution, light reflection and cell structure in petals

Abstract

Petal structure and the distribution of pigments in petals were studied in relation to the functional anatomy of petals and the ways in which petals absorb and reflect light. We examined 201 species from 60 angiosperm families. Anthocyanins, betalains and ultraviolet-absorbing flavonoids were normally confined to the epidermal cells, occurring in solution in the vacuole; carotenoids were found in the epidermis and in smaller quantities in the mesophyll, normally in chromoplasts. In a few species, mainly blue-flowered members of the Boraginaceae and Liliaceae-Scilleae, anthocyanins were confined to the mesophyll. Six basic kinds of petal epidermis anatomy were found, sometimes in combination; papillate (112 species) and multiple-papillate (13 species), in which the conical-papillate form of the cells traps incident light and scatters emergent light, with surface striations aiding these functions in many cases; reversed-papillate (4 species), multiple reversed-papillate (29 species), lenticular (32 species) and flat (11 species), all with surface striations in some cases. Light is usually reflected from petals mainly by an aerenchymatous unpigmented reflective mesophyll; in certain species this is replaced by a reflective layer of starch grains in the upper mesophyll.