Investigating UV screening in leaves by two different types of portable UV fluorimeters reveals in vivo screening by anthocyanins and carotenoids

Two portable instruments, designed to evaluate epidermal UV screening in leaves, were compared: the Dualex and the UV-A-PAM fluorimeter. Both instruments excite chlorophyll fluorescence at the same UV wavelengths but reference excitation is in the red and the blue spectral range in the former and the latter fluorimeter, respectively. When analyzing green leaves, general agreement of the data is obtained with the two instruments. In the presence of anthocyanins, the UV-A-PAM fluorimeter provided higher estimates for epidermal UV transmittance than the Dualex fluorimeter, which was attributed to absorption of blue excitation light by anthocyanins. By comparing data from the instruments, anthocyanin-dependent transmittance of 50% was determined in abaxial sides of some autumn leaves, and also in abaxial sides of tropical shade plants. Further, with leaves of chlorophyll b-less mutants of H. vulgare, unusually high epidermal UV transmittance was detected but this was attributed to the lack of chlorophyll b absorption and, in addition, to absorption of blue radiation by xanthophylls which are not functionally connected to photosystems.