Photosynthesis Research 61: 65–75, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands. 65 Regular paper Effects of growth-light quantity, growth-light quality and CO 2 concentration on Rubisco deactivation during low PFD or darkness Jerriann Ernstsen 1 , Ian E. Woodrow 2 & Keith A. Mott 1,∗ 1 Department of Biology, Utah State University, Logan, UT 84322-5305, USA; 2 School of Botany, The University of Melbourne, Parkville, VIC 3052, Australia; ∗ Author for correspondence Received 9 March 1999; accepted in revised form 25 May 1999 Key words: photosynthesis, photosynthetic induction, sunflecks Abstract The effects of CO 2 concentration and the effects of growth-light conditions on Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) deactivation were examined for Spinacea oleracea (spinach). Rubisco deac- tivation kinetics and the degree that Rubisco activation limited the rise in photosynthesis following an increase in photon flux density (PFD) were determined from gas-exchange time courses. There were no significant differences in the apparent relaxation time for Rubisco deactivation among leaves exposed to high or low CO 2 (50 or 1000 µmol mol −1 ) and low PFD (170 µmol m −2 s −1 ) or darkness. However, when PFD was increased to 1700 µmol m −2 s −1 following a period of low PFD or darkness, leaves exposed to low CO 2 × low PFD showed a lower contribution to the photosynthetic induction process by the activation of Rubisco than leaves exposed to the other treatments. For the growth-light experiments, spinach was grown under high PFD × high red:far-red ratio (R:FR), low PFD × high R:FR, or low PFD × low R:FR light environments. Leaves that matured under the low PFD × low R:FR treatment showed a lower percent change in photosynthesis due to Rubisco activation than leaves exposed to the other growth-light treatments. However, there were no significant differences among the growth- light treatments in the maximum contribution of Rubisco activation to the induction response or in the apparent relaxation time for Rubisco deactivation during shade events. Abbreviations: A – CO 2 assimilation rate; c i – intercellular CO 2 concentration; c a – ambient CO 2 concentra- tion; PFD – photon flux density; Rubisco – ribulose 1,5-bisphosphate carboxylase/ oxygenase; RuBP – ribulose 1,5-bisphosphate Introduction In most environments, plants experience fluctuating light caused by cloud movement and by overstory gap formation and closure (Knapp and Smith 1990). Cloud cover, for example, can reduce light quantity by 90% on fully overcast days, yet does not signific- antly modify light quality (Smith 1982). Overstories, in comparison, not only decrease light quantity, but also substantially change light quality (Turnbull and Yates 1993). Because direct and filtered light is con- tinuously changing in quantity and quality, the success of a plant may be related to its ability to quickly acclimate to these changes by optimizing light-use efficiency (Walters and Horton 1995). Following an increase in photon flux density (PFD), photosynthetic induction is characterized by an initial rapid (<1 min) increase in photosynthesis followed by a slower increase until steady state is attained many minutes later. This physiological phe- nomenon consists of three phases (Pearcy 1990). The first phase (RuBP phase) responds rapidly to fluc- tuations in PFD and primarily reflects activation of stromal fructose 1,6-bisphosphatase and sedoheptu- lose 1,7-bisphosphatase (Sassenrath-Cole and Pearcy 1994). The second phase (Rubisco phase) responds