Assessing leaf pigment content and activity with a reflectometer

TLDR

The study examined whether leaf reflectometer-derived indices can quantify pigment content across leaves differing in light history, developmental stage, and functional type (annual vs evergreen). The authors used a leaf reflectometer to measure three indices—modified NDVI for chlorophyll, red/green ratio for anthocyanin, and ΔPRI for xanthophyll cycle activity. Reflectance indices revealed that xanthophyll cycle pigments scale with light intensity, with sun leaves containing twice as much as shade leaves and more de‑epoxidized forms midday, while annual and evergreen species show distinct pigment development trajectories—sun leaves rapidly accumulate chlorophyll and xanthophylls, whereas evergreen leaves develop pigments gradually with an anthocyanin peak—demonstrating that reflectometry can differentiate pigment activity patterns across light history and functional type.

Abstract

This study explored reflectance indices sampled with a ‘leaf reflectometer’ as measures of pigment content for leaves of contrasting light history, developmental stage and functional type (herbaceous annual versus sclerophyllous evergreen). We employed three reflectance indices: a modified normalized difference vegetation index (NDVI), an index of chlorophyll content; the red/green reflectance ratio ( R RED ∶ R GREEN ), an index of anthocyanin content; and the change in photochemical reflectance index upon dark–light conversions (ΔPRI), an index of xanthophyll cycle pigment activity. In Helianthus annuus (sunflower), xanthophyll cycle pigment amounts were linearly related to growth light environment; leaves in full sun contained approximately twice the amount of xanthophyll cycle pigments as leaves in deep shade, and at midday a larger proportion of these pigments were in the photoprotective, de‐epoxidized forms relative to shade leaves. Reflectance indices also revealed contrasting patterns of pigment development in leaves of contrasting structural types (annual versus evergreen). In H. annuus sun leaves, there was a remarkably rapid increase in amounts of both chlorophyll and xanthophyll 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 pigment content in newly expanding leaves. These temporal patterns of pigment development 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 distinguishing divergent patterns of pigment activity in leaves of contrasting light history and functional type.