Assessing leaf pigment content and activity with a reflectometer

This study explored reflectance indices sampled with a 'leaf reflectometer' as measures of pigment content for leaves of contrasting light history, de velopmental stage and functional type (herbaceous annual versus sclerophyll ous evergreen). We employed three reflectance indices: a modified normalize d difference vegetation index (NDVI), an index of chlorophyll content; the red/green reflectance ratio (R-RED:R-GREEN), an index of anthocyanin conten t; and the change in photochemical reflectance index upon dark-light conver sions (Delta PRI), an index of xanthophyll cycle pigment activity. In Helia nthus annuus (sunflower), xanthophyll cycle pigment amounts were linearly r elated to growth light environment; leaves in full sun contained approximat ely twice the amount of xanthophyll cycle pigments as leaves in deep shade, and at midday a larger proportion of these pigments were in the photoprote ctive, de-epoxidized forms relative to shade leaves. Reflectance indices al so revealed contrasting patterns of pigment development in leaves of contra sting structural types (annual versus evergreen). In H. annuus sun leaves, there was a remarkably rapid increase in amounts of both chlorophyll and xa nthophyll cycle pigments along a leaf developmental sequence. This pattern contrasted with that of Quercus agrifolia (coast live oak, a sclerophyllous evergreen), which exhibited a gradual development of both chlorophyll and xanthophyll cycle pigments along with a pronounced peak of anthocyanin pigm ent content in newly expanding leaves. These temporal patterns of pigment d evelopment in Q. agrifolia leaves suggest that anthocyanins and xanthophyll cycle pigments serve complementary photoprotective roles during early leaf development. The results illustrate the use of reflectance indices for dis tinguishing divergent patterns of pigment activity in leaves of contrasting light history and functional type.