Light promotes an increase of cytokinin oxidase/dehydrogenase activity during senescence of barley leaf segments

Following a study of the relationship between cytokinin oxidase/dehydrogenase (CKX) and senescence in darkened barley leaf segments, we have now investigated the influence of light on the in vitro activity of CKX. Seedlings of Hordeum vulgare L. were grown for 8 d under a light/dark regime of 18 h white light and 6 h darkness. Then apical parts of 7 cm length were cut from the first foliage leaves and their bases were placed in water. In segments kept in the dark, the CKX activity measured by cleavage of N₆-(Δ₂-isopentenyl)adenine rose from 0.1 pkat (g FW)⁻¹ to 0.8 pkat (g initial FW)⁻¹ within the first 4 d of incubation. In contrast, in segments kept under the light/dark regime it reached a value of 8.6 pkat (g initial FW)⁻¹ over the same time period. The chlorophyll a content declined slightly slower during light/dark cycling than in darkness. In contrast to segments and isolated laminae, corresponding attached laminae exhibited less CKX activity after 2 d under light/dark conditions than after 2 d in the dark. The activity in attached laminae of first foliage leaves of plants growing in light/dark cycling increased strongly only when the plants were older than 4 weeks. In line with this, the CKX activity in attached laminae of flag leaves of barley growing in fields increased in a late developmental state. The senescence of darkened isolated laminae of Zea mays L. and Phragmites australis (Cav.) Trin. ex Steudel was associated with an enhancement of CKX activity too. Because in most cases a positive correlation between CKX activity and senescence was found, it is likely that the enzyme promotes senescence by destroying cytokinins, which help to keep Poaceae leaves green. Light may promote not only cytokinin degradation but also the formation of bioactive cytokinins in leaf segments.