Photosynthesis and Light Activation of Ribulose 1,5-bisphosphate Carboxylase in the Presence of Starch

Abstract

Limitation of photosynthesis and light activation of ribulose, 1,5-6/.sphosphate carboxylase (RuBPCO) were examined in the 5th leaf of seedlings of red clover (Trifolium pratense L. cv. Renova) for 5 d following an increase in photosynthetic photon flux density (PPFD) from 200 to 550fttnol quanta m" 2 s" l . Net photosynthesis and its stimulation at 20 kPa O 2 , initial activity of rapidly extracted RuBPCO, standard activity of RuBPCO after incubation of the extracts in the presence of CO 2 , Mg 2 + , and inorganic phosphate and contents of soluble protein, starch, soluble sugars, and various photosynthetic metabolites were determined. Photosynthesis decreased and starch content increased. No decrease in photosynthesis was found if, when PPFD was increased, all leaves except the investigated 5th leaf were removed, suggesting that the decrease in photosynthesis was due to accumulated carbohydrates. The stimulation of photosynthesis at 2-0 kPa O 2 did not decrease and the ratio of the total foliar steady-state contents of triose phosphate to 3-phosphoglycerate increased suggesting that the decrease in photosynthesis was not due to limiting inorganic phosphate in chloroplasts. Intercellular CO 2 partial pressure and RuBP content were not decreased. Nevertheless, the ratio of photosynthesis to initial RuBPCO activity decreased, suggesting that the catalysis per active RuBPCO site was decreased. The increase in PPFD in the growth cabinet and the PPFD at which leaves were preconditioned for 1 h, affected not only initial activity but also the standard activity of RuBPCO. The results suggest that a varying proportion of RuBPCO was bound to membranes and was contained in the insoluble fraction of the extracts. A comparison of photosynthesis with extracted RuBPCO activity suggested that membrane bound RuBPCO did not contribute to photosynthetic CO 2 fixation and that the binding and release to and from membranes modulated actual RuBPCO activity in vivo.

An evaluation of direct and indirect mechanisms for the ?sink-regulation? of photosynthesis in spinach: Changes in gas exchange, carbohydrates, metabolites, enzyme activities and steady-state transcript levels after cold-girdling source leaves

“…However the mechanism is not understood, and there are discrepancies between the various investigations, e.g. whereas Rubisco activation increased in our studies and in red clover (Grub and Mfichler 1990), it declined in soybean but could be reactivated in vitro by incubation with Pi (Sawada et al 1990). Further studies are needed to resolve these discrepancies, and to relate them to known mechanisms for regulating Rubisco.…”

Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase Activase Deficiency Delays Senescence of Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase but Progressively Impairs Its Catalysis during Tobacco Leaf Development

“…They suggested that the activity of carbamylated Rubisco sites was being reduced by an unknown mechanism in response to carbohydrate accumulation induced by cold girdling. Grub and Machler (1990) also reported a reduction in the catalytic effectiveness of activated Rubisco while carbohydrates were accumulating following transfer of low-light-grown red clover to a higher light intensity. The similarity between our present observations and those of Krapp and Stitt (1995) and Grub and Machler (1990) encourages speculation that there might be a common underlying mechanism.…”

“…Grub and Machler (1990) also reported a reduction in the catalytic effectiveness of activated Rubisco while carbohydrates were accumulating following transfer of low-light-grown red clover to a higher light intensity. The similarity between our present observations and those of Krapp and Stitt (1995) and Grub and Machler (1990) encourages speculation that there might be a common underlying mechanism. Perhaps in some unknown way, carbohydrate accumulation induces by another means the same reduction in the ability of activase to stimulate catalysis by carbamylated Rubisco that we observe as a result of reducing the content of activase itself.…”

Decreased ribulose-1,5-bisphosphate carboxylase-oxygenase in transgenic tobacco transformed with ?antisense? rbcS

“…Seemann and Sharkey 1986;von Caemmerer and Edmondson 1986;Sharkey 1989;Sage et al 1990). However, exceptions have been reported (Seemann 1989;Grub and Machler 1990). Anyway, full activation and high RuBP would still be consistent with shared control if Rubisco were nevertheless not fully RuBP-saturated in vivo (see Woodrow and Berry 1988;Sharkey 1989 for a discussion of this problem), or if additional factors affect the in-vivo activity of Rubisco (see Woodrow and Mott 1988), or if control is exerted by factors which control the access of CO2 to the active site (see below).…”