In situ measurement of leaf chlorophyll concentration: analysis of the optical/absolute relationship

Abstract

In situ optical meters are widely used to estimate leaf chlorophyll concentration, but non‐uniform chlorophyll distribution causes optical measurements to vary widely among species for the same chlorophyll concentration. Over 30 studies have sought to quantify the in situ/in vitro (optical/absolute) relationship, but neither chlorophyll extraction nor measurement techniques for in vitro analysis have been consistent among studies. Here we: (1) review standard procedures for measurement of chlorophyll; (2) estimate the error associated with non‐standard procedures; and (3) implement the most accurate methods to provide equations for conversion of optical to absolute chlorophyll for 22 species grown in multiple environments. Tests of five M inolta (model SPAD‐502) and 25 O pti‐S ciences (model CCM‐200) meters, manufactured from 1992 to 2013, indicate that differences among replicate models are less than 5%. We thus developed equations for converting between units from these meter types. There was no significant effect of environment on the optical/absolute chlorophyll relationship. We derive the theoretical relationship between optical transmission ratios and absolute chlorophyll concentration and show how non‐uniform distribution among species causes a variable, non‐linear response. These results link in situ optical measurements with in vitro chlorophyll concentration and provide insight to strategies for radiation capture among diverse species.