Abstract
Modulated fluorometry (PAM) was applied for probing the photosynthesis in cells of C. reinhardtii during sulfur deprivation. A significant (up to a fourfold) increase in chlorophyll fluorescence yield (parameters F(o) and F(m)) normalized to chlorophyll concentration was shown for deprived cells. An analysis of nonphotochemical quenching of chlorophyll fluorescence indicated a considerable modification of the energy deactivation pathways in PS II of sulfur-deprived cells. Thus, starved cells exhibited a lower deltapH-dependent quenching of excited states and a higher thermal dissipation of excess light energy in reaction centers of PS II, as well as the transition of the photosynthetic apparatus primarily to state 2. However, these changes cannot cause the elevation of chlorophyll fluorescence in the cells under sulfur limitation. The phenomenon observed may be due to a partial dissociation of light-harvesting complexes from reaction centers of PS II and/or dysfunction of the dissipative cycle in PS II with cytochrome b559 as an intermediate.
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