The arrangement of chloroplasts in cells influences the reabsorption of chlorophyll fluorescence emission. The effect of desiccation on the chlorophyll fluorescence spectra of Rhizomnium punctatum leaves

Chlorophyll fluorescence spectra measured with leaves are distorted by the effect of fluorescence reabsorption. A heterogeneous theoretical model simu lating the effect of chloroplast arrangement in a cell on the distortion of chlorophyll fluorescence spectra due to reabsorption was formulated. Desic cation of leaves of the moss Rhizomnium punctatum was carried out as a simp le model experiment. The parameters entering the model (maximal number of c hloroplasts forming columns in a cell, chloroplast size and chlorophyll con centration in a chloroplast) were estimated by means of light microscopy an d spectrophotometry. During the desiccation, a grouping of chloroplasts was observed by light microscopy and the chlorophyll fluorescence emission and excitation spectra of the leaves were measured at room temperature and at 77 K. The leaves were infiltrated with DCMU. The ratio F685/F735 of the mai n emission bands decreased by about 50% at room temperature and by about 30 % at 77 K upon decreasing the leaf water content. No significant changes we re found in the ratio E475/E436 of the bands of the leaf fluorescence excit ation spectra at 77 K for both 685- and 735-nm emission wavelengths. The ex citation spectra and mechanical dilution experiments indicated that no func tional changes appeared upon desiccation at the level of energy transfer. T heoretical simulations were in a good agreement with the experimental depen dencies. We were able to conclude that the grouping of chloroplasts in cell s may enhance the effect of chlorophyll reabsorption and thereby cause a si gnificant decrease of the F685/F735 ratio in the chlorophyll fluorescence s pectrum.