Tip-down anthocyanin in developing leaves of <em>Ailanthus altissima</em>
Nicole M. Hughes Wake Forest University Biology Winston-Salem North Carolina USA
Ailanthus altissima, Nicole M. Hughes, Conant Awards
One of the most conspicuous developmental changes observed in juvenile leaves as they mature is color change, with young leaves on new growth tips of many species first appearing red, purple, pink, or less commonly blue or white, and becoming greener with leaf age. The red-to-blue coloration of young leaves is most commonly due to the pigment anthocyanin, appearing within vacuoles of epidermal and/or mesophyll cells within hours to days of seedling germination, and then decreasing concomitantly with leaf expansion and maturation. The functional significance of this pigment in juvenile leaves still remains largely unresolved, as mixed support has been shown for it acting as a camouflage against herbivory, a fungicide, a signal indicating the presence of unpalatable phenolics, and also an antioxidant. Perhaps the most compelling current explanation is that the pigment act as a light-attenuating molecule (the plant-equivalent of sunscreen), protecting underlying cells from high irradiance through absorption of high energy blue-green (and possibly UV) wavelengths of the solar spectrum. Because immature leaves tend to be especially vulnerable to high-light stress due to immature chloroplast structure, reduced capacity of photosynthetic enzymes, and limited stomatal and cellular conductance of CO2, young leaves growing under high irradiances tend to photosynthetically saturate, as well as photoinhibit, under substantially lower sunlight levels than mature leaves. It is therefore generally beneficial for light capture to be down-regulated early in leaf development, until light-processing and carbon-fixation processes have matured to adequately balance energy capture with utilization. Ailanthus altissima represents one of many early successional species that exhibit anthocyanin in developing leaves.