Flavonoids accumulate in leaves and glandular trichomes of Phillyrea latifolia exposed to excess solar radiation

Experiments were conducted on Phillyrea latifolia plants grown under a dens e overstorey of Pinus pinea (shade plants) or on seashore dunes (sun plants ) in a coastal area of Tuscany (42 degrees 46' N, 10 degrees 53' E). Total integrated photon flux densities averaged 1.67 and 61.4 m mol m(-2) d(-1) f or shade and sun sites, respectively. A leaf morphological-structural analy sis, a qualitative and quantitative analysis of phenylpropanoids of leaf ti ssue and leaf surface, and a histochemical localization of flavonoids were conducted. The area of sun leaves reached 57%, of that of shade leaves, whe reas leaf angle (beta), sclerophylly index (ratio of leaf d. wt:leaf area), and trichome frequency (trichome number mm(-2)) were markedly greater in l eaves exposed to full solar radiation than in leaves acclimated to shade. T he total thickness of sun leaves was 78%, higher than that of shade leaves, mostly owing to a greater development of both palisade parenchyma and spon gy mesophyll. The concentration, but not the composition, of leaf tissue ph enylpropanoids varied significantly between sun and shade leaves, with a ma rked increase in flavonoid glycosides in sun leaves. Flavonoids occurred al most exclusively in the upper epidermal cells of shade leaves. By contrast, flavonoids largely accumulated in the upper and lower epidermis, as well a s in the mesophyll tissue of leaves that were acclimated to full sunlight. Flavonoid glycosides were found exclusively in the secretory products of gl andular trichomes of P. latifolia leaves exposed to high levels of light; l uteolin 7-O-glucoside and quercetin 3-O-rutinoside were the major constitue nts. By contrast, verbascoside and an unidentified caffeic acid derivative constituted 72% of total phenylpropanoids secreted by glandular trichomes o f shade leaves, whereas they were not detected in glandular trichomes of su n leaves. These findings suggest that the light-induced synthesis of flavon oids in glandular trichomes of P. latifolia probably occurs in situ and con comitantly inactivates other branch pathways of the general phenylpropanoid metabolism. This is the first report of the key role of glandular trichome s and of flavonoid glycosides in the integrated mechanisms of acclimation o f P. latifolia to excess light.