Photosynthesis Research (v.105, #1)

Very little is known about the physiological interactions between plants and endophytic bacteria. We investigated the impact of three endophytic bacteria, Bacillus pumilus 2-1, Chryseobacterium indologene 2-2, and Acinetobacter johnsonii 3-1, on the photosynthetic capacity and growth of sugar beet. Endophyte-free plants were obtained first and infected with the bacteria. Measurements of total chlorophyll content revealed very significant differences between endophyte-free beet plants and some infected by endophytic bacteria. The maximum photochemical yield (Fv/Fm) was used to determine any photosynthetic effect on plants caused by biotic or abiotic factors. After 30 days of growth, there was significantly higher Fv/Fm for endophyte-infected than endophyte-free plants. The light response curves of beet showed that photosynthetic capacity was significantly increased in endophyte-infected plants. Photosynthesis of endophyte-free plants was saturated at 1,300 μmol m−2 s−1, whereas endophyte-infected plants were not saturated at the irradiance used. The effect seemed to be due to promotion of electron transport in the thylakoid membranes. Promotion of photosynthetic capacity in sugar beet was due to increased chlorophyll content, leading to a consequent increased carbohydrate synthesis. It is possible that the increased maximum yield of photosynthesis in sugar beet was promoted by phytohormones and produced by the bacteria.
Keywords: Bacteria endophytes; Sugar beet; Photosynthesis; Bacillus pumilus ; Chryseobacterium indologene ; Acinetobacter johnsonii

Altered physiological function, not structure, drives increased radiation-use efficiency of soybean grown at elevated CO2 by Uwe Rascher; Bernhard Biskup; Andrew D. B. Leakey; Justin M. McGrath; Elizabeth A. Ainsworth (15-25).
Previous studies of elevated carbon dioxide concentration ([CO2]) on crop canopies have found that radiation-use efficiency is increased more than radiation-interception efficiency. It is assumed that increased radiation-use efficiency is due to changes in leaf-level physiology; however, canopy structure can affect radiation-use efficiency if leaves are displayed in a manner that optimizes their physiological capacity, even though the canopy intercepts the same amount of light. In order to determine the contributions of physiology and canopy structure to radiation-use and radiation-interception efficiency, this study relates leaf-level physiology and leaf display to photosynthetic rate of the outer canopy. We used a new imaging approach that delivers three-dimensional maps of the outer canopy during the growing season. The 3D data were used to model leaf orientation and mean photosynthetic electron transport of the outer canopy to show that leaf orientation changes did not contribute to increased radiation-use; i.e. leaves of the outer canopy showed similar diurnal leaf movements and leaf orientation in both treatments. Elevated [CO2] resulted in an increased maximum electron transport rate (ETRmax) of light reactions of photosynthesis. Modeling of canopy light interception showed that stimulated leaf-level electron transport at elevated [CO2], and not alterations in leaf orientation, was associated with stimulated radiation-use efficiency and biomass production in elevated [CO2]. This study provides proof of concept of methodology to quantify structure–function relationships in combination, allowing a quantitative estimate of the contribution of both effects to canopy energy conversion under elevated [CO2].
Keywords: Three-dimensional canopy surface; Photosynthesis; Light reactions; Chlorophyll fluorescence; Elevated CO2 ; Glycine max ; Structure–function relations

Selenium-induced protection of photosynthesis activity in rape (Brassica napus) seedlings subjected to cadmium stress. Fluorescence and EPR measurements by Maria Filek; Janusz Kościelniak; Maria Łabanowska; Elżbieta Bednarska; Ewa Bidzińska (27-37).
Fluorescence and electron paramagnetic resonance measurements were used to study selenium influence on photosystem activity in rape seedlings affected by Cd stress. Water cultures containing Hoagland nutrients were supplemented with 400 μM of CdCl2, 2 μM of Na2SeO4 and a mixture of both CdCl2 and Na2SeO4. The seedlings were cultured till the first leaf reached about 1 cm in length. Cadmium-induced changes in the activity of both photosystems were partly diminished by Se presence in the nutrient medium. Electron microscopy photographs confirmed less degradation in chloroplasts of plants cultured on media containing Se. It is suggested that sucrose groups of starch, which is deposited in greater amounts in Cd-stressed plants, may act as traps for free radicals produced under those conditions.
Keywords: Fluorescence; EPR; Photosystems; Cadmium; Selenium; Rape plants

Trophic status of Chlamydomonas reinhardtii influences the impact of iron deficiency on photosynthesis by Aimee M. Terauchi; Graham Peers; Marilyn C. Kobayashi; Krishna K. Niyogi; Sabeeha S. Merchant (39-49).
To investigate the impact of iron deficiency on bioenergetic pathways in Chlamydomonas, we compared growth rates, iron content, and photosynthetic parameters systematically in acetate versus CO2-grown cells. Acetate-grown cells have, predictably (2-fold) greater abundance of respiration components but also, counter-intuitively, more chlorophyll on a per cell basis. We found that phototrophic cells are less impacted by iron deficiency and this correlates with their higher iron content on a per cell basis, suggesting a greater capacity/ability for iron assimilation in this metabolic state. Phototrophic cells maintain both photosynthetic and respiratory function and their associated Fe-containing proteins in conditions where heterotrophic cells lose photosynthetic capacity and have reduced oxygen evolution activity. Maintenance of NPQ capacity might contribute to protection of the photosynthetic apparatus in iron-limited phototrophic cells. Acetate-grown iron-limited cells maintain high growth rates by suppressing photosynthesis but increasing instead respiration. These cells are also able to maintain a reduced plastoquinone pool.
Keywords: Acetate; Algae; LhcSR; Non-photochemical quenching; Respiration

The contribution to reduction of oxygen by ferredoxin (Fd) to the overall reduction of oxygen in isolated pea thylakoids was studied in the presence of Fd versus Fd + NADP+. The overall rate of electron transport was measured using a determination of Photosystem II quantum yield from chlorophyll fluorescence parameters, and the rate of oxidation of Fd was measured from the light-induced redox changes of Fd. At low light intensity, increasing Fd concentration from 5 to 30 μM in the absence of NADP+ increased the proportion of oxygen reduction by Fd from 25–35 to 40–60% in different experiments. This proportion decreased with increasing light intensity. When NADP+ was added in the presence of 15 μM Fd, which was optimal for the NADP+ reduction rate, the participation of Fd in the reduction of oxygen was low, no more than 10%, and it also decreased with increasing light intensity. At high light intensity, the overall oxygen reduction rates in the presence of Fd + NADP+ and in the presence of Fd alone were comparable. The significance of reduction of dioxygen either by water-soluble Fd or by the membrane-bound carriers of the photosynthetic electron transport chain for redox signaling under different light intensities is discussed.
Keywords: Electron transport; Ferredoxin; Oxygen reduction; Photosynthesis; Thylakoids

The regulatory effect of light quality on the photosynthetic apparatus in attached leaves of rice plants was investigated by keeping rice plants under natural light, in complete darkness, or under illumination with light of different colors. When leaves were left in darkness and far-red (FR)-light conditions for 6 days at 30°C, there was an initial lag in chlorophyll (Chl) content, Chl a/b ratio, and maximum photosystem (PS) II photochemistry that lasted until the second day; these then rapidly decreased on the fourth day. In contrast, ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) rapidly disappeared with no lag under low or zero light conditions. By using spectrophotometric quantitation, it was determined that the PSII and PSI reaction centers were regulated by light quality, but cytochrome (Cyt) f was regulated by light intensity. However, the PSII heterogeneity was also strongly modified by the light intensity; PSIIα with the large antenna decreased markedly both in content and in antenna size. Consequently, the PSIIα/PSI ratio declined under FR-light because the low intensity of FR-light dominated over its quality in the modulation of the PSIIα/PSI ratio. An imbalance between them induced the generation of reactive oxygen species (ROS), although the ROS were scavenged by stromal enzymes such as superoxide dismutase (SOD), ascorbate peroxidase (APX), and glutathione reductase (GR). The activities of these stromal enzymes are also regulated by light quality. Thus, although the photosynthetic apparatus is regulated differently depending on light quality, light quality may play an important role in the regulation of the photosynthetic apparatus.
Keywords: Light quality; Light absorption balance; Photosystem stoichiometry; PSII heterogeneity; Reactive oxygen species; Rice (Oryza sativa L.)

Kinetic bacteriochlorophyll fluorometer by Péter Kocsis; Emese Asztalos; Zoltán Gingl; Péter Maróti (73-82).
A pump and probe fluorometer with a laser diode as single light source has been constructed for measurement of fast induction and relaxation of the fluorescence yield in intact cells, chromatophores and isolated reaction centers of photosynthetic bacteria. The time resolution of the fluorometer is limited by the repetition time of the probing flashes to 20 μs. The apparatus offers high sensitivity, excellent performance and can become a versatile device for a range of demanding applications. Some of them are demonstrated here including fast and easy investigation of the (1) organization and redox state of the photosynthetic apparatus of the intact cells of different bacterial strains and mutants and (2) electron transfer reactions on donor and acceptor sides of isolated reaction centers. The compact design of the mechanics, optics, electronics, and data processing makes the device easy to use as outdoor instrument or to integrate into larger measuring systems.
Keywords: Bacterial photosynthesis; Intact cells; Reaction center; Fluorescence induction; Fluorescence relaxation; Portable fluorometer