Irreversible changes in barley leaf chlorophyll fluorescence detected by the fluorescence temperature curve in a linear heating/cooling regime

The chlorophyll fluorescence ( F) temperature curves in a linear time-temperature heating/cooling regime were used to study heat-induced irreversible F changes in primary green leaves of spring barley ( Hordeum vulgare L. cv. Akcent). The leaf segments were heated in a stirred water bath at heating rates of 0.0083, 0.0166, 0.0333, and 0.0500 degrees C s(-1) from room temperature up to maximal temperature T-m and then linearly cooled to 35 degrees C at the same rate. The F intensity was measured by a pulse-modulated technique. The results support the existence of the two critical temperatures of irreversible F changes postulated earlier, at 45-48 and 53-55 degrees C. The critical temperatures are slightly dependent on the heating rate. Two types of parameters were used to characterize the irreversibility of the F changes: the coefficient of irreversibility mu defined as the ratio of F intensity at 35 degrees C at the starting/ ending parts of the cycle and the slopes of tangents of linear parts of the F temperature curve. The dependence of mu on T-m revealed a maximum, which moved from 54 to 61 degrees C with the increasing heating/cooling rate nu from 0.0083 to 0.0500 degrees C s(-1), showing two basic phases of the irreversible changes. The Arrhenius and Eyring approaches were applied to calculate the activation energies of the initial increase in mu. The values varied between 30 and 50 kJ mol(-1) and decreased slightly with the increasing heating rate.