Acclimation of barley to changes in light intensity: chlorophyll organization.

Barley seedlings (Hordeum vulgare L. cv. Boone) were grown at 20°C with a 16h/8h light/dark cycle of either high (H) intensity (550 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) or 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 and a 3 cm apical segment removed for analysis. H control plants had greater chlorophyll (Chl) per leaf area and higher Chl a/b ratios than L controls. Analysis of Chl-protein complexes revealed that H and L plants had the same percentage of total Chl (62-65%) associated with Photosystem II (PS II), but that the organization of Chl within PS II was different. H plants contained lower levels of light-harvesting complex (LHC-II) and higher levels of the PS II complex CPa compared with L plants. Leaf Chl content and Chl organization within PS II were sensitive to changes in light intensity. In H → L plants, leaf Chl content decreased, Chl a/b ratio decreased, and a redistribution of Chl from CPa to LHC-II occurred during acclimation to low light. Acclimation of L → H plants to high light involved an increase in leaf Chl content, an increase in Chl a/b ratio, and a decrease in LHC-II. In contrast, the level of photosystem I related Chl-protein complexes (CP1 + CP1a) was similar in all light treatments. The light acclimation process occurred slowly over a period of 6 to 8 d in H → L and L → H plants.