Rate-Limiting Steps of Electron Transport in Chloroplasts during Ontogeny and Senescence of Barley

Abstract

Partial photochemical activities and concentrations of electron carriers were measured relative to chlorophyll in barley (Hordeum vulgare L.) thylakoids, isolated from primary leaves during ontogeny and senescence. Thylakoids from mature leaves generated somewhat higher quantum efficiencies than thylakoids from premature or senescing leaves; this phenomenon did not appear to be caused by any deficiency of water-splitting enzyme. Under conditions of saturating light, the noncycic electron flux from water to the reducing side of photosystem I increased during leaf ontogeny, peaked at maturity, and declined during senescence. However, electron fluxes appeared to be Umited at different steps before and after leaf maturity. Before leaf maturity, the rate-limiting step was located prior to the reoxidation of plastohydroquinone. After leaf maturity, the decline in noncyclic electron flux correlated with a decrease in the concentration of cytochromesf and b6. This correlation, together with a consideration of mechanisms of entry and exit of electrons in 3-(3,4-dichlorophenyl)-1,1-dimethylurea-treated thylakoids, suggests that the cytochrome f/b6-containing complex, and not plastocyanin or P700, is the site of entry of electrons from the reduced forms of 2,6-dichlorophenolindophenol and diaminodurene. It is therefore proposed that in senescing leaves the cytochrome f/bs-containing complex limited electron transport by constraining the rate of reduction of cytochromefby plastohydroquinone.It has been well established that maximum rates of net photosynthesis, as measured by 02 evolution or CO2 assimilation, are achieved at or just before leaf maturity, followed by an extended period of decline (7,24,26). Previous research on changes in chloroplast electron transport during foliar ontogeny and senescence (for a review, see Sestak [25]), have shown that rates of chloroplast electron transport in isolated thylakoids, expressed on a Chl basis, generally follow similar patterns of rise and decline to net photosynthesis in whole leaves.Most methods for measuring rates of electron transport in vitro rely on artificial electron donors, acceptors and inhibitors. Recently, the sites of action of many of these compounds have been clarified (13,(29)(30)(31) Stromal enzymes appeared to be absent from thylakoid preparations, as evidenced by the absence of bands corresponding to the mol wt of both large and small subunits of ribulose bisP carboxylase-oxygenase when thylakoid preparations were subjected to SDS-polyacrylamide gel electrophoresis (6). Estimations of Chl a and b (28) were compared with total Chl (5), to show that degradation products were not significant (8).Photochemical Activities. Relationships between electron donors, carriers, and inhibitors used during this investigation are shown in Figure 1.Photochemical Activities Measured Spectrophotometrically. The light-driven reduction of ferricyanide or DCPIP in both reactions a and b (below) was followed by an Aminco-DW2a

Ethylene Insensitivity Results in Down-Regulation of Rubisco Expression and Photosynthetic Capacity in Tobacco

“…The electron transport capacity ( J max ) calculated from the data presented in Figure 5 was also significantly lower in ethylene-insensitive tobacco plants. Cytochrome f content is generally strongly correlated with electron transport capacity and is thought to be an important rate-limiting step in this process (Holloway et al, 1983). Our finding that Cytochrome f content was also lower in the ethylene-insensitive tobacco plants (Table I) supports this view.…”

Photosynthetic Characteristics of Rice Leaves Aged under Different Irradiances from Full Expansion through Senescence

“…Although loss of Chl is a commonly observed symptom of senescence, Chl frequently disappears most slowly during senescence among the photosynthetic components (11,12,14,21). As shown in Figure 4, low irradiance strongly retarded the decline in the Chl content and it remained almost constant until late senescence.…”

“…Second, we determined the amounts of Rubisco, CF,, and Cyt f proteins in relation to the gas-exchange data. The limitation of CFI(ATP-ase) and Cyt f contents for electron transport capacities has been repeatly reported (6,12,17,35). In addition, we analyzed their relationships to leaf-nitrogen content, and characterized the photosynthetic system of the leaves aged under low irradiance in relation to acclimation of shade plants.…”

“…Several quantitative analyses indicated that decrease in photosynthesis under ambient air conditions can be predicted from changes in the amounts and kinetics of Rubisco (5,23). Electron transport activities and the reaction-center complexes also decreased during leaf senescence, and their positive correlations with photosynthesis have been frequently reported (3,(12)(13)(14). Thus, it can be expected that a decline in photosyn- 'This work was supported in part by Grants-in-Aid for International Scientific Research (63044018) from the Ministry ofEducation, Science and Culture of Japan.…”

Stay‐green genotypes of Phaseolus vulgaris L.: chloroplast proteins and chlorophyll catabolites during foliar senescence

“…Figure 2 shows pronounced retention of three non-chlorophyll-binding thylakoid polypeptides in mutant Phaseolus. Water-splitting, the cytochrome bf complex and photosystem I have all been shown previously to be labile (though not necessarily equally so) during normal leaf senescence (Holloway, Maclean & Scott, 1983;Woolhouse & Jenkins, 1983;HUditch, Thomas & Rogers, 1986;Grover, 1993). The genetic lesion in Phaseolus has a generally stabilizing effect on thylakoid membrane integrity, a conclusion supported by the ultrastructural observations of Ronning et al (1991).…”