Effects of abscisic Acid treatment on the thermostability of the photosynthetic apparatus in barley chloroplasts.

Thermostability of the photosynthetic apparatus of abscisic acid (ABA)-treated seedlings of barley (Hordeum vulgare) was studied by light-scattering and by fluorescence measurements of isolated chloroplasts. ABA treatment markedly decreased heat damage of the chloroplast ultrastructure; an exogenous ABA concentration of 10(-5) molar was most effective. Heat-induced increase of the 77 kilodalton fluorescence ratio F(740)/F(685) was also smaller at this ABA concentration. The heat-induced increase of the initial chlorophyll fluorescence level (F(o)) was virtually eliminated in ABA-treated (10(-5) molar) chloroplasts up to 45 degrees C and slightly increased at 50 degrees C, relative to control chloroplasts where F(o) increased even at 35 degrees C and reached its maximal value at 45 degrees C. In control chloroplasts, F(o) increased with a 5-minute pretreatment temperature, an effect observed as low as 35 degrees C. F(o) was maximal at 45 degrees C. In contrast, chloroplasts treated with 10(-5) molar ABA did not exhibit a heat-induced increase in F(o) until 50 degrees C.