ACCLIMATION OF BARLEY TO CHANGES IN LIGHT-INTENSITY - PHOTOSYNTHETIC ELECTRON-TRANSPORT ACTIVITY AND COMPONENTS

Barley seedlings (Hordeum vulgare L. Boone) were grown at 20°C with 16 h/8 h light/dark cycle of either high (H) intensity (500 μmole m-2 s-1) or low (L) intensity (55 μmole m-2 s-1) white light. Plants were transferred from high to low (H → L) and low to high (L → H) light intensity at various times from 4 to 8 d after leaf emergence from the soil. Primary leaves were harvested at the beginning of the photoperiod. Thylakoid membranes were isolated from 3 cm apical segments and assayed for photosynthetic electron transport, Photosystem II (PS II) atrazine-binding sites (QB), cytochrome f(Cyt f) and the P-700 reaction center of Photosystem I (PS I). Whole chain, PS I and PS II electron transport activities were higher in H than in L controls. QB and Cyt f were elevated in H plants compared with L plants. The acclimation of H → L plants to low light occurred slowly over a period of 7 days and resulted in decreased whole chain and PS II electron transport with variable effects on PS I activity. The decrease in electron transport of H → L plants was associated with a decrease in both QB and Cyt f. In L → H plants, acclimation to high light occurred slowly over a period of 7 days with increased whole chain, PS I and PS II activities. The increase in L → H electron transport was associated with increased levels of QB and Cyt f. In contrast to the light intensity effects on QB levels, the P-700 content was similar in both control and transferred plants. Therefore, PS II/PS I ratios were dependent on light environment. © 1990 Kluwer Academic Publishers.