Cotyledonary leaves of Cucumis sativus cv. Poinsette exhibited senescence-induced losses in chlorophyll (Chl) and protein contents within three weeks since germination. Chl and protein concentrations in cotyledonary leaves approached maximum on 6th d after germination and they declined to 50 and 41 %, respectively, by the 20th day of growth. Activities of both photosystem (PS) 2 and PS1 decreased by 33 and 31 %, respectively, on the 20th day, compared to the control 6th day. Changes in sensitivity of PS2 to inhibitors like atrazine and dibromothymoquinone and sensitivity of PS1 to KCN accompanied the changes in PS2 and PS1 activities. Hence both the acceptor side of PS2 and the donor side of PS1 are affected by senescence-induced changes in cucumber cotyledonary leaves. and J. S. S. Prakash, M. A. Baig, Prasanna Mohanty.
Influence of respiration on photosynthesis in Synechocystis PCC6803 was studied by measuring the redox transients of cytochrome f (cyt f) upon excitation of the cells with repetitive single turnover flashes. Upon the addition of KCN the flash-induced oxidation of cyt f was increased and the rereduction of cyt f+ was accelerated. Dependence of these effects on the concentration of KCN clearly demonstrated the existence of two cyanide-sensitive oxidases interacting with photosynthesis: cyt aa3, which was sensitive to low concentrations of cyanide, and an alternative oxidase, which could be suppressed by using ≥1 mM KCN. The interaction between the photosynthetic and the respiratory electron transport chains was regulated mainly by the activity of the alternative cyanide-sensitive oxidase. The oxidative pathway involving the alternative cyanide-sensitive oxidase was insensitive to salicyl hydroxamic acid and azide. The close resemblance of the inhibition pattern reported here and that described for chlororespiration in algae and higher plants strongly suggest that an oxidase of the same type as the alternative cyanide-sensitive oxidase of cyanobacteria functions as a terminal oxidase in chloroplasts. and C. Büchel, O. Zsíros, G. Garab.
Photosynthetic and respirátory electron transfers in cyanobacteria not only serve the bioenergetic needs of these prokaryotes during day and night time. The common use of the plastoquinone pool and the cytochrome (cyt) complex also establishes possibilities for sharing photosystem (PS) 2 plus dehydrogenases at the donor side and cyt c oxidase plus PSI at the acceptor side. Given metabolic conditions and radiant energy supply, the available choices may give rise to unusual combinations of connected electron transfer activities, for example PS2 and cyt c oxidase. In vivo measurements of energy storage in PSI cyclic photophosphorylation via photoacoustic spectroscopy, and of the P700 redox statě via absorbance changes at 820 nm detected with the pulse amplitudě modulation technique (PAM), as well as of PS2 fluorescence yield, all in the absence or presence of the cyt c oxidase inhibitor KCN were combined to demonstrate that the two pathways at the acceptor side communicate in vivo. This type of regulation serves proper poising of electron flow through and around PSI. The impaired cyt c oxidase activity (in this study achieved by addition of KCN) prevents a truly oxidized statě of P700 to be reached, which hampers electron passage from PS2. The relative overreduction of PS 1 in the KCN intoxicated samples reduces the electron flow directed to biosynthesis. The results illustrate the versatility of the P700 redox statě measurements at 820 nm as a means to study in vivo electron fluxes in cyanobacteria.
The decrease in inorganic phosphate (Pi) content of 10-d-old Phaseolus vulgaris L. plants did not affect rates of photosynthesis (PN) and respiration (RD), leaf growth, and adenylate concentration. Two weeks of phosphate starvation influenced the ATP content and leaf growth more than PN and RD. The ATP concentration in the leaves of 15- and 18-d-old phosphate deficient (-P) plants after a light or dark period was at least half of that in phosphate sufficient (+P, control) plants. Similar differences were found in fresh and dry matter of leaves. However, PN declined to 50 % of control in 18-d-old plants only. Though the RD of -P plants (determined as both CO2 evolution and O2 uptake) did not change, an increased resistance of respiration to KCN and higher inhibition by SHAM (salicylhydroxamic acid) suggested a higher engagement of alternative pathway in respiration and a lower ATP production. The lower demand for ATP connected with inhibition of leaf growth may influence the ATP producing processes and ATP concentration. Thus, the ATP concentration in the leaves depends stronger on Pi content than on PN and RD. and M. Mikulska, J.-L. Bomsel, A. M. Rychter.