Response to the “Comments to water-splitting activity of Photosystem II by far-red light in green plants” by H.-W. Trissl

Abstract

Two results of our papers [1,2] should be distinguished, one being an observation that surprisingly long-wavelength quanta up to 780 nm support oxygen evolution from plant leaves, and the second being a peculiar spectral behavior of the quantum yield of the PSII electron transport, showing a local maximum at ∼745 nm. This is the quantum yield spectral dependence and its interpretation that has been questioned by H.-W. Trissl in his Comments. Several arguments were presented in favour of the absorption from thermally populated vibrational sublevels of PS II chlorophylls for explanation of the photosynthetic activity under extreme red light, without appellation to special far-red PSII chlorophylls, the presence of which was hypothesized in [1,2]. One of the arguments used was a good fit of our experimental oxygen evolution rates with the calculated PSII absorption spectrum shown in Fig. 1b of the Comments. This agreement is, unfortunately, deceptive, because only a limited number of experimental points and arbitrary normalization in a narrow spectral range was applied. When, however, the whole set of experimental points is presented, as shown in Fig. 1 of this letter, obvious discrepancies appear, clearly suggesting an extra absorbance with respect to PSII chlorophylls in the 720–760 nm region. Thus, while the proposed interpretation is certainly an option, which is also discussed in our works [1,2], the evidence given in the Comments is circumstantial, based upon an idealized computational model [3]. Amongst other things, this model, for example, uses the Kennard–Stepanov relation for the calculation of the fluorescence spectrum. Yet as noted in [4], the thermal equilibration assumed by Kennard–Stepanov (KS) is hardly ever seen in practice. The KS anomalies are