Chlorophyll fluorescence temperature curve on Klebsormidium flaccidum cultivated at different temperature regimes

Abstract

Summary The alga Klebsormidium flaccidum from the Moroccan desert was studied using the measurement of the Fluorescence Temperature Curve - FTC (a dependence of chlorophyll fluorescence intensity on linearly increasing temperature) during the cell cycle. The green alga was cultivated at an alternating temperature regime (35 °C - light/5 °C - dark) simulating natural desert conditions, or at a constant temperature (20 °C). Pronounced reversible changes in FTC were detected during the diurnal cycle with alga cultivated at alternating temperatures. The critical temperature (TC) of the steep fluorescence rise up to the FTC maximum (the M1 maximum) and the temperature of the M1 FTC maximum (TP), which are usually used as the indicators of photosystem II (PSII) thermostability, gradually increased by about 2.5 °C during the light phase. The ratio of the height of M1 FTC maximum to the fluorescence intensity at 30 °C (M1/F(T30) ratio), usually reflecting PSII photochemistry, also significantly increased during the light phase of the diurnal cycle. However, the changes in the parameters corresponded with changes in the slope of the fluorescence rise to the M1 FTC maximum, and with changing temperature dependence of the maximal fluorescence - FM. Analysis of the data led to the conclusion that the change in the TC parameter and the M1/F(T30) ratio need not merely reflect the changes in the PSII thermostability and PSII photochemistry, respectively. The causes of these discrepancies are discussed.