The chl a fluorescence intensity is remarkably insensitive to changes in the chlorophyll content of the leaf as long as the chl a/b ratio remains unaffected

The effects of changes in the chlorophyll (chl) content on the kinetics of the OJIP fluorescence transient were studied using two different approaches. An extensive chl loss (up to 5-fold decrease) occurs in leaves suffering from either an Mg2+ or SO42− deficiency. The effects of these treatments on the chl a/b ratio, which is related to antenna size, were very limited. This observation was confirmed by the identical light intensity dependencies of the K, J and I-steps of the fluorescence rise for three of the four treatments and by the absence of changes in the F685 nm/F695 nm-ratio of fluorescence emission spectra measured at 77 K. Under these conditions, the F0 and FM-values were essentially insensitive to the chl content. A second experimental approach consisted of the treatment of wheat leaves with specifically designed antisense oligodeoxynucleotides that interfered with the translation of mRNA of the genes coding for chl a/b binding proteins. This way, leaves with a wide range of chl a/b ratios were created. Under these conditions, an inverse proportional relationship between the FM values and the chl a/b ratio was observed. A strong effect of the chl a/b ratio on the fluorescence intensity was also observed for barley Chlorina f2 plants that lack chl b. The data suggest that the chl a/b ratio (antenna size) is a more important determinant of the maximum fluorescence intensity than the chl content of the leaf.

► Mg2+ and sulfur deficiencies: lower leaf chl content, no effect on antenna size. ► Changes in leaf chl content (up to 80%) hardly affect F0 and FM. ► Antisense ODNs targeted to cab-mRNA can slow antenna formation down. ► Differences in antenna size (chl a/b ratio) have an important effect on FM. ► The optical properties of the leaf have little effect on the fluorescence kinetics.