Effects of epidermal cell shape and pigmentation on optical properties of Antirrhinum petals at visible and ultraviolet wavelengths

We used the Mixta(+) and mixta(-) lines of Antirrhinum majus as a model system to investigate the effects of epidermal cell shape and pigmentation on tissue optical properties in the visible and ultraviolet (UV) spectral regions. Adaxial epidermal cells of Mixta(+) flowers have a conical-papillate shape; in the mixta(-) line the cells are slightly domed. Mixta(+) cells contained significantly more anthocyanin and other flavonoids than mixta(-) cells when plants were grown under either high- or low-UV conditions. Mixta(+) cells focused light (3.5-4.7 times incident) within their pigmented interiors, whereas mixta(-) cells focused light (2.1-2.7 times incident) in the unpigmented mesophyll. UV light penetrated the epidermis (commonly 20-50% transmittance at 312 nm) mainly through the unpigmented peripheral regions of the cells that were similar for the two lines, so that overall penetration through Mixta(+) and mixta(-) epidermises was equal. However, maximum UV absorption in the central region of epidermal cells was slightly greater in Mixta(+) than mixta(-), and intact Mixta(+) flowers reflected less light in the spectral regions with intermediate flavonoid absorbance. In both cases, about 50 to 75% of the difference could be attributed to cell shape and resulting changes in the optical pathlength or focusing.