Plant internal oxygen concentrations can drop well below ambient even when the plant grows under optimal conditions. Using pea (Pisum sativum) roots, we show how amenable respiration adapts to hypoxia to save oxygen when the oxygen availability decreases. The data cannot simply be explained by oxygen being limiting as substrate but indicate the existence of a regulatory mechanism, because the oxygen concentration at which the adaptive response is initiated is independent of the actual respiratory rate. Two phases can be discerned during the adaptive reaction: an initial linear decline of respiration is followed by a nonlinear inhibition in which the respiratory rate decreased progressively faster upon decreasing oxygen availability. In contrast to the cytochrome c pathway, the inhibition of the alternative oxidase pathway shows only the linear component of the adaptive response. Feeding pyruvate to the roots led to an increase of the oxygen consumption rate, which ultimately led to anoxia. The importance of balancing the in vivo pyruvate availability in the tissue was further investigated. Using various alcohol dehydrogenase knockout lines of Arabidopsis (Arabidopsis thaliana), it was shown that even under aerobic conditions, alcohol fermentation plays an important role in the control of the level of pyruvate in the tissue. Interestingly, alcohol fermentation appeared to be primarily induced by a drop in the energy status of the tissue rather than by a low oxygen concentration, indicating that sensing the energy status is an important component of optimizing plant metabolism to changes in the oxygen availability.
A technique has been developed to measure absolute intracellular oxygen concentrations in green plants. Oxygen-sensitive phosphorescent microbeads were injected into the cells and an optical multifrequency phase-modulation technique was used to discriminate the sensor signal from the strong autofluorescence of the plant tissue. The method was established using photosynthesis-competent cells of the giant algae Chara corallina L., and was validated by application to various cell types of other plant species.
SummaryExtremophilic organisms are gaining increasing interest because of their unique metabolic capacities and great biotechnological potential. The unicellular acidophilic and mesothermophilic red alga Galdieria sulphuraria (074G) can grow autotrophically in light as well as heterotrophically in the dark. In this paper, the effects of externally added glucose on primary and secondary photosynthetic reactions are assessed to elucidate mixotrophic capacities of the alga. Photosynthetic O 2 evolution was quantified in an open system with a constant supply of CO 2 to avoid rapid volatilization of dissolved inorganic carbon at low pH levels. In the presence of glucose, O 2 evolution was repressed even in illuminated cells. Ratios of variable to maximum chlorophyll fluorescence (F v /F m ) and 77 K fluorescence spectra indicated a reduced photochemical efficiency of photosystem II. The results were corroborated by strongly reduced levels of the photosystem II reaction centre protein D1. The downregulation of primary photosynthetic reactions was accompanied by reduced levels of the Calvin Cycle enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Both effects depended on functional sugar uptake and are thus initiated by intracellular rather than extracellular glucose. Following glucose depletion, photosynthetic O 2 evolution of illuminated cells commenced after 15 h and Rubisco levels again reached the levels of autotrophic cells. It is concluded that true mixotrophy, involving electron transport across both photosystems, does not occur in G. sulphuraria 074G, and that heterotrophic growth is favoured over autotrophic growth if sufficient organic carbon is available.
A two-dimensional NLO chromophore (7) with three donor-substituted branches and truxenone as the central coupling unit was synthesised from tribromotruxenone by Stille coupling with N,N-di(4-methoxyphenyl)-4'-(tributylstannylethynyl)phenylamine. UV/visible spectroscopy and hyper-Rayleigh scattering measurements prove the truxenone moiety to be a far stronger electron acceptor than, for example, a nitro group. In addition, coupling of excited states leads to an enhanced quadratic hyperpolarisability of 7 compared with one-dimensional reference chromophores. The large redox-potential separation of about 400 mV of the three reductive waves in the cyclic voltammetry also indicates strong electronic coupling of the truxenone unit. Semiempirical computations at AM1-CI level were used to explain the strong coupling; these calculations also suggest a quartet high-spin state for the truxenone trianion. UV/visible spectroelectrochemical investigations of the oxidation and the first reduction of 7 show that both processes weaken the CT excitation so as to modulate the NLO properties.
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