Photosynthesis Research

Strawberry (Fragaria × ananassa) plants were grown in field plots at the current ambient [CO2], and at ambient + 300 and ambient + 600 μmol mol−1 [CO2]. Approximately weekly measurements were made of single leaf gas exchange of upper canopy leaves from early spring through fall of two years, in order to determine the temperature dependence of the stimulation of photosynthesis by elevated [CO2], whether growth at elevated [CO2] resulted in acclimation of photosynthesis, and whether any photosynthetic acclimation was reduced when fruiting created additional demand for the products of photosynthesis. Stimulation of photosynthetic CO2 assimilation by short-term increases in [CO2] increased strongly with measurement temperature. The stimulation exceeded that predicted from the kinetic characteristics of ribulose-1,5-bisphosphate carboxylase at all temperatures. Acclimation of photosynthesis to growth at elevated [CO2] was evident from early spring through summer, including the fruiting period in early summer, with lower rates under standard measurement conditions in plants grown at elevated [CO2]. The degree of acclimation increased with growth [CO2]. However, there were no significant differences between [CO2] treatments in total nitrogen per leaf area, and photosynthetic acclimation was reversed one day after switching the [CO2] treatments. Tests showed that acclimation did not result from a limitation of photosynthesis by triose phosphate utilization rate at elevated [CO2]. Photosynthetic acclimation was not evident during dry periods in midsummer, when the elevated [CO2] treatments conserved soil water and photosynthesis declined more at ambient than at elevated [CO2]. Acclimation was also not evident during the fall, when plants were vegetative, despite wet conditions and continued higher leaf starch content at elevated [CO2]. Stomatal conductance responded little to short-term changes in [CO2] except during drought, and changed in parallel with photosynthetic acclimation through the seasons in response to the long-term [CO2] treatments. The data do not support the hypothesis that source-sink balance controls the seasonal occurrence of photosynthetic acclimation to elevated [CO2] in this species.

Fourier transform infrared (FTIR) spectroscopy probes the vibrational properties of amino acids and cofactors, which are sensitive to minute structural changes. The lack of specificity of this technique, on the one hand, permits us to probe directly the vibrational properties of almost all the cofactors, amino acid side chains, and of water molecules. On the other hand, we can use reaction-induced FTIR difference spectroscopy to select vibrations corresponding to single chemical groups involved in a specific reaction. Various strategies are used to identify the IR signatures of each residue of interest in the resulting reaction-induced FTIR difference spectra. (Specific) Isotope labeling, site-directed mutagenesis, hydrogen/deuterium exchange are often used to identify the chemical groups. Studies on model compounds and the increasing use of theoretical chemistry for normal modes calculations allow us to interpret the IR frequencies in terms of specific structural characteristics of the chemical group or molecule of interest. This review presents basics of FTIR spectroscopy technique and provides specific important structural and functional information obtained from the analysis of the data from the photosystems, using this method.

Climate change could impact nutrient bioavailability in aquatic environment. To understand the interaction of nutrient bioavailability and elevated CO2, Chlorella vulgaris cells were grown in ambient air or 5% CO2 in different concentrations of nitrogen and phosphorus in a photobioreactor. The chlorophyll content, photosynthesis and respiration rates increased in 5% CO2 to support higher biomass production. The nutrient limitation in the growth media resulted in reduced photosynthetic rates of the algal cells and their PSI, PSII, and whole chain electron transport rates and biomass production. Conversely, their lipid content increased partly due to upregulation of expression of several lipid biosynthesis genes. The order of downregulation of photosynthesis and upregulation in lipid production due to nutrient limitation was in the order of N > P. The N-50 and 5% CO2 culture had only 10% reduction in biomass and 32% increase in lipids having 85% saturated fat required for efficient biofuel production. This growth condition is ideal for generation of biodiesel required to reduce the consumption of fossil fuel and combat global warming.

The effects of exposure of a field-grown winter cultivar of barley (Hordeum vulgare L.) to Photosystem I (PS I) photoinhibitory conditions in the form of bright day-light combined with chilling conditions were investigated. PS I photoinhibition was manifested by damage to the Fe-S centers of PS I and to the PS I-A/B polypeptides. Up to 20% of the PS I complexes were photoinactivated. Upon transfer to room temperature, the plants partially recovered from PS I photoinhibition, although damage was still detectable after one week. These results demonstrate that PS I photoinhibition is a physiologically relevant phenomenon in chilling-tolerant plants grown under field conditions. In order to study the induction of cyclic electron transport around PS I by PS I photoinhibitory conditions, antibodies raised against the NDH-I subunit of the NDH complex (a component of cyclic electron transport) were used to measure NDH levels in the exposed plants. A marked increase in the amount of NDH complex and a corresponding increase in NADPH dehydrogenase activity in the thylakoids were observed. The data indicate that the response to PS I-photoinhibitory conditions may involve regulated changes in cyclic electron transport around PS I.

During their life cycle, plants must be able to adapt to wide variations in the supply of soil nitrogen (N). Changes in N availability, and in the relative concentrations of NO3 −and NH4 +, are known to have profound regulatory effects on the N uptake systems in the root, on C and N metabolism throughout the plant, and on root and shoot morphology. Optimising the plant’s responses to fluctuations in the N supply requires co-ordination of the pathways of C and N assimilation, as well as establishment of the appropriate allocation of resources between root and shoot growth. Achieving this integration of responses at the whole plant level implies long-distance signaling mechanisms that can communicate information about the current availability of N from root-to-shoot, and information about the C/N status of the shoot in the reverse direction. In this review we will discuss recent advances which have contributed to our understanding of these long-range signaling pathways.

The following question is addressed here: do healthy leaves absorb, as the spectra published over the last 50 years indicate, some 5–20% of incident radiation in the 780–900 nm region? The answer is found to be negative, and previous findings result from incomplete collection of the transmitted light by the detection system (even when the leaf is placed next to, but outside, the entrance port of an integrating sphere). A simple remedy for this inherent flaw in the experimental arrangement is applied successfully to leaves (of 10 unrelated species) differing in thickness, age and pigment content. The study has shown that, from an optical standpoint, a leaf tissue is a highly scattering material, and the infinite reflectance of a leaf is exceedingly sensitive to trace amounts of absorbing components. It is shown that water contributes, in a thick leaf (Kalanchoe blossfeldiana), an easily detectable signal even in the 780–900 nm region. The practical benefits resulting from improved measurements of leaf spectra are pointed out.

In oxygenic photosynthetic systems, the cytochrome b6f (Cytb6f) complex (plastoquinol:plastocyanin oxidoreductase) is a heart of the hub that provides connectivity between photosystems (PS) II and I. In this review, the structure and function of the Cytb6f complex are briefly outlined, being focused on the mechanisms of a bifurcated (two-electron) oxidation of plastoquinol (PQH2). In plant chloroplasts, under a wide range of experimental conditions (pH and temperature), a diffusion of PQH2 from PSII to the Cytb6f does not limit the intersystem electron transport. The overall rate of PQH2 turnover is determined mainly by the first step of the bifurcated oxidation of PQH2 at the catalytic site Qo, i.e., the reaction of electron transfer from PQH2 to the Fe2S2 cluster of the high-potential Rieske iron–sulfur protein (ISP). This point has been supported by the quantum chemical analysis of PQH2 oxidation within the framework of a model system including the Fe2S2 cluster of the ISP and surrounding amino acids, the low-potential heme b6L, Glu78 and 2,3,5-trimethylbenzoquinol (the tail-less analog of PQH2). Other structure–function relationships and mechanisms of electron transport regulation of oxygenic photosynthesis associated with the Cytb6f complex are briefly outlined: pH-dependent control of the intersystem electron transport and the regulatory balance between the operation of linear and cyclic electron transfer chains.