Optical detection of pigment contents in leaves: intercomparison of several handheld devices

Handheld devices have become popular to determine epidermal contents of UV-A absorbing compounds in intact leaves. For the comparability of the results between different labs, it is important that all instruments show the same or at least similar results. Therefore, the signals from seven different Dualex Scientific optical leaf clips from five laboratories were compared to each other, alongside a prototype of an LSA-2050 leaf state analyzer and a UV-A-PAM fluorometer. The values of the Flav index, which is used to quantify epidermal UV-A absorbing compounds such as flavonoids, corresponded well with each other (r² > 0.989), except for one instrument. A higher variability among instruments was found for the chlorophyll (Chl) index, again with one instrument showing a significant offset. Although one Dualex instrument showed an excellent linear calibration for the anthocyanin (Anth) index relative to extract concentrations when red hazelnut leaves were investigated (r² = 0.969), the Dualex instruments had all an offset against each other and at Chl index values below 30, erroneous Anth index values were observed. While Flav index results appeared quite reliable, it is recommended to calibrate a Dualex instrument against leaf extracts for exact determination of chlorophyll and anthocyanin contents. Simple tests for the functionality of a Dualex instrument are suggested.