Chlorophyll b reduction during senescence of barley seedlings

During senescence of flowering plants, only breakdown products derived from chlorophyll a were detected although b disappears, too (Matile et al., 1996, Plant Physiol 112: 1403-1409). We investigated the possibility of chlorophyll b reduction during dark-induced senescence of barley (Hordeum vulgare L.) leaves. Plastids isolated from senescing leaves were lysed and incubated with NADPH. We found 7(1)-hydroxy-chlorophyll a, 7(1)-hydroxy-chlorophyllide a, and, after incubation with Zn-pheophorbide b, also Zn-7(1)-hydroxy-pheophorbide a, indicating activity of chlorophyll(ide) b reductase. The highest activity was found at day 2 of senescence when chlorophyll breakdown reached its highest rate. Chlorophyllase reached its highest activity under the same conditions only at days 4-6 of senescence. Based on the chlorophyll b reductase activity of plastids at day 2.5 of senescence (=100%), the bulk of activity (83%) was found in the thylakoids and only traces (5%) in the envelope fraction. Chlorophyll b reduction is considered to be an early and obligatory step of chlorophyll b breakdown.