THE PROTECTIVE EFFECT OF HEAT-SHOCK PROTEINS AGAINST PHOTOINHIBITION UNDER HEAT-SHOCK IN BARLEY (HORDEUM-VULGARE)

Barley leaves, when analyzed by variable fluorescence measurements of PS II activity, have been found to possess a considerably higher resistance against photoinhibition under heat stress conditions after a pretreatment by heat shock. This preconditioning, protecting 2 h heat shock treatment could be applied either 24 h or immediately prior to the combination of heat- and photoinhibitory light stress at a light intensity of 1000 W m-2. Surprisingly, resistance against light stress under heat shock of heat-pretreated plants was even somewhat higher than in unshocked plants which were photoinhibited under normal growth temperature. The preformed, accumulated heat shock proteins (HSP) did not protect the leaves against photoinhibition under normal growth temperatures. Heat shock treatments under light intensities used for normal growth did not impair activity of PS II. Inhibition of protein synthesis by cycloheximide during the first heat shock abolished almost completely the protective effect of the conditioning heat shock. Plants pretreated with heat in the presence of chloramphenicol were as resistant against light stress during the first hour as were the controls which were heat treated in the absence of the drug; however, under prolonged conditions of light stress the chloramphenicol treated plantlets showed a rapid decline in PS II activity.