Physiologia Plantarum 134: 609–623. 2008 Copyright ª Physiologia Plantarum 2008, ISSN 0031-9317 Senescence progression in a single darkened cotyledon depends on the light status of the other cotyledon in Cucurbita pepo (zucchini) seedlings: potential involvement of cytokinins and cytokinin oxidase/dehydrogenase activity Kalina Ananieva a , Evgue ´ ni D. Ananiev b, *, Snejana Doncheva a , Katya Georgieva a , Nikolina Tzvetkova c , MiroslavKamı´nek d , Va ´ clav Motyka d , Petre Dobrev d , Silvia Gajdos ˇova ´ d,e and Jiri Malbeck d a Acad. M. Popov Institute of Plant Physiology, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Bl. 21, Sofia 1113, Bulgaria b Department of Plant Physiology, Faculty of Biology, St. Kl. Ohridski University of Sofia, 8 Dragan Tsankov bld., 1164 Sofia, Bulgaria c University of Forestry, 10 Kliment Ohridski av., Sofia 1756, Bulgaria d Institute of Experimental Botany, Academy of Sciences of the Czech Republic, Rozvojova ´ 135, CZ-16502 Prague 6, Czech Republic e Department of Plant Physiology, Faculty of Science, Charles University, Vinic ˇna ´ 2, CZ-12844 Prague 2, Czech Republic Correspondence *Corresponding author, e-mail: ananiev@bio21.bas.bg Received 20 June 2008; revised 18 July 2008 doi: 10.1111/j.1399-3054.2008.01161.x Darkness mediates different senescence-related responses depending on the targeting of dark treatment (whole plants or individual leaves) and on the organs that perceive the signal (leaves or cotyledons). As no data are available on the potential role of darkness to promote senescence when applied to individual cotyledons, we have investigated how darkness affects the progression of sene- scence in either a single or both individually darkened cotyledons of young 10-day-old Cucurbita pepo (zucchini) seedlings. Strong acceleration of senes- cence was observed when both cotyledons were darkened as judged by the damage in their anatomical structure, deterioration of chloroplast ultrastructure in parallel with decreased photosynthetic rate and photochemical quantum efficiency of PSII. In addition, the endogenous levels of cytokinins (CKs) and IAA were strongly reduced. In a single individually darkened cotyledon, the structure and function of the photosynthetic apparatus as well as the contents of endogenous CKs and IAA were much less affected by darkness, thus suggesting inhibitory effect of the illuminated cotyledon on the senescence of the darkened one. Apparently, the effect of darkness to accelerate/delay senescence in a single darkened cotyledon depends on the light status of the other cotyledon from the pair. The close positive correlation between CK content and the activity of CK oxidase/ dehydrogenase (CKX; EC 1.4.3.18/1.5.99.12) suggested that CKX was essentially involved in the mechanisms of downregulation of endogenous CK levels. Our results indicated that CKX-regulated CK signaling could be a possible regulatory mechanism controlling senescence in individually darkened cotyledons. Introduction Leaf senescence is a programmed and highly regulated developmental process comprising the final stage of plant/organ development leading to death and abscission (Noode ´n 1988a, Smart 1994). The onset of leaf senescence represents an age-dependent genetically controlled Abbreviations – CK, cytokinin; cis-Z, cis-zeatin; cis-ZR, cis-zeatin riboside; CKX, cytokinin oxidase/dehydrogenase; DHZ, dihydrozeatin; FW, fresh weight; iP, N 6 -(2-isopentenyl)adenine; iPR, N 6 -(2-isopentenyl)adenine 9-riboside; Z, trans-zeatin; ZR, trans-zeatin 9-riboside; ZOG, trans-zeatin O-glucoside; ZROG, trans-zeatin 9-riboside O-glucoside; ZRMP, trans-zeatin 9-riboside- 5#-monophosphate; (abbreviations for cytokinins according to Kamı´nek et al. 2000). Physiol. Plant. 134, 2008 609