Photosynthesis Research 46: 393-397, 1995. © 1995 KluwerAcademic Publishers. Printed in the Netherlands. Regular paper Down regulation of photosynthesis in Artabotrys hexapetatus by high light U. Dwivedi, M. Sharma & R. Bhardwaj* School of Biochemistry, D. A. University, Khandwa Road, lndore 452 001, India; *Authorfor correspondence Received 17 May 1995;accepted in revisedform4 September 1995 Key words: photosynthesis, down-regulation, variable fluorescence, dark recovery, Artabotrys hexapetatus Abstract Using variable to maximum fluorescence (Fv/Fm) as the criterion, the down regulation of photosynthesis by high light stress was characterized in the detached leaves of Artabotrys hexapetatus. The decrease in Fv/Fm was co- related with the decrease in oxygen evolution by thylakoids isolated from high light exposed leaves. The decrease in Fv/Fm was linear with increasing time of exposure to high light. A comparison of recovery measured as Fv/Fm, in low light versus dark, revealed that the recovery in darkness was as significant as in low light. Since the relaxation of fluorescence was a rapid response after exposure to high light and the fact that the recovery occurs in total darkness, it is concluded that photoinhibition and down regulation of photosynthesis by high light are independent events. Abbreviations: Fpl- initial plateau; Fm- maximum fluorescence; Fo - prompt fluorescence; Fv - variable fluores- cence; PFD-photon flux density; PSI (II)-Photosystem I (II) Introduction Inhibition of photosynthesis is a direct consequence when plants encounter high light than that required for growth. The term photoinhibition of photosynthesis has generally been used to describe the high light- induced decrease in the quantum yield of Photosystem II (PS II) with simultaneous turnover of PS II proteins and in particular the reaction centre D1 protein (Tyys- tjarvi et al. 1994; see Dwivedi and Bhardwaj 1995). The synthesis and degradation of D 1 protein essential- ly requires light (Mattoo et al. 1984; Ohad et al. 1984). Thus, recovery from photoinhibition requires insertion of a fresh copy D1 protein to reactivate the damaged PS II reaction centre. Depending on the degree of inhibition, the recov- ery from photoinhibition takes several hours. It has been shown that when plants are exposed to high light (1500 #E m -2 s -1) for 2 to 4 h, 4 to 20 h are required for recovery in fresh water phytoplankton (Belay 1981) while in Phaseolus vulgaris, about 4 to 8 h are required for recovery (Powles et al. 1983). Low light (20 #E m -2 s -l) has been shown to be essential for recovery (Ohad et al. 1984). The recovery of variable fluores- cence (Fv) to near pre-inhibition level in leaves of N. oleander essentially required light (Powles and Bjork- man 1982). Charurvedi et al. (1985) have reported some recovery in the dark while Greet et al. (1986) observed a slower rate of recovery in the dark. The mathematical modelling studies of Tyystj~rvi et al. (1994) have found an excellent fit with the experimental data and Fv/Fm has been shown to be an excellent monitor of 'down-regulation' of photosyn- thesis which relaxes without the turnover of D1 pro- tein (Tyystjarvi et al. 1995). Accordingly, using Fv/Fm as the monitor, 'down-regulation' of photosynthesis was characterized in the intact leaves of Artabotrys hexapetatus. Data suggests that the degradation-repair cycle of PS II and the 'down-regulation' of photosyn- thesis are independent events and Artabotrys hexapeta- tus provides a good example of down-regulation of photosynthesis during high light stress.