Plant Science 153 (2000) 145–154 Antioxidant responses of cucumber (Cucumis satius ) to photoinhibition and oxidative stress induced by norflurazon under high and low PPFDs Sunyo Jung a, *, Jin Seog Kim a , Kwang Yun Cho a , Gun Sik Tae b , Bin G. Kang c a Screening Research Diision, Korea Research Institute of Chemical Technology, P.O. Box 107, Yusung, Taejon 305 -600, South Korea b Department of Biology, Dankook Uniersity, Chonan 330 -714, South Korea c Department of Biology, Yonsei Uniersity, Seoul 120 -749, South Korea Received 3 May 1999; received in revised form 8 November 1999; accepted 3 December 1999 Abstract Photooxidative damage is exacerbated by norflurazon (NF), which blocks carotenoid biosynthesis. This study examined the influence of photosynthetic photon flux density (PPFD) on the overall responses of both non-enzymatic and enzymatic antioxidants to NF-caused oxidative damage in leaves of cucumber (Cucumis satius ). Seven-day-old cucumber plants were exposed to NF under either low PPFD (30 mol m -2 s -1 ) or high PPFD (300 mol m -2 s -1 ) for 3 days. The NF plants exposed at high PPFD had lower levels of F v /F m ratio, quantum yield of electron transport, and 33-kDa protein of photosystem II as compared with the NF plants at low PPFD. In the NF plants, there was a reduction in total chlorophylls and carotenoids except newly formed zeaxanthin in either PPFD. The NF plants at high PPFD resulted in less level of photochemical quenching, q P , and Stern – Volmer quenching, NPQ, than those of the plants at low PPFD, whereas both plants had similar level of non-photochem- ical quenching coefficient, q N . However, the level of PPFD did not significantly affect the NF-caused induction of antioxidant enzymes including peroxidase, superoxide dismutase, glutathione reductase, and ascorbate peroxidase. © 2000 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Antioxidant enzymes; Cucumber (Cucumis satius ); Non-photochemical quenching; Norflurazon; Oxidative stress; Xanthophylls www.elsevier.com/locate/plantsci 1. Introduction Photoinhibition and photooxidation can occur when plants are exposed to stress. High light in synergy with other stress factors such as chilling, drought, or low carbon dioxide supply reduces the capacity of photosynthetic systems to utilize inci- dent radiation, leading to a photoinhibition pro- cess [1,2]. The photosynthetic electron transport system is the major source of active oxygen species (AOS) in plant tissues [3], having the potential to generate singlet oxygen ( 1 O 2 ) and superoxide (O 2 - ), which is favored under downregulation of metabolic pathways. Photosystem (PS) II has long been considered the primary target for photoinhi- bition [1,2] because PSI is more stable than PSII during strong light treatments [4]. Light-induced inactivation of PSI is suggested to be caused by AOS [5,6]. AOS are eliminated efficiently by an integrated system of non-enzymatic and enzymatic antioxidants that are concentrated in the chloro- plast [3]. The capacity of the antioxidative defense system is increased under adverse stress conditions but the imbalance between AOS production and antioxidant defenses ultimately leads to oxidative damage. The non-enzymatic reductants consist of ascor- bate, glutathione, -tocopherol, caroteniods, and phenolic compounds [7,8]. Among those, xantho- phyll cycle-dependent energy dissipation in the * Corresponding author. Present address: Department of Genetics, North Carolina State University, Raleigh, NC 27695-7614, USA. Tel.: +1-919-5155819; fax: +1-919-5153355. E-mail address: sjung2@unity.ncsu.edu (S. Jung) 0168-9452/00/$ - see front matter © 2000 Elsevier Science Ireland Ltd. All rights reserved. PII:S0168-9452(99)00259-9