Towards a more accurate future for chlorophyll a and b determinations: the inaccuracies of Daniel Arnon's assay

A recent publication (Esteban in New Phytol 217:341-342, 2018) describes how the use and citation of the assay of chlorophylls a and b extracted in aqueous 80% acetone by Arnon (Plant Physiol 2:1-15, 1949) is increasing, even in journals with high impact factors. This is a very disconcerting situation: the assay is outdated because it relies on the seriously under-estimated extinction coefficients of Mackinney (J Biol Chem 140:315-322, 1941), and the assay of chlorophylls is one of the most important, and much reported, procedures in studies of photosynthesis and related plant biological fields. Using the assay has led to the accumulation of masses of inaccurate data and confusion during the resolution of some plant biological problems. A summary not only of an accurate assay of chlorophylls in aqueous 80% acetone but also of a long-known method to correct the data obtained by Arnon's procedure (cf. Porra et al. in Biochim Biophys Acta 975:384-394, 1989) is briefly described below together with references to reliable assays in this and other solvents by other authors.