Photosynthesis

About: Photosynthesis is a research topic. Over the lifetime, 19789 publications have been published within this topic receiving 895197 citations.

Specific reduction of chloroplast carbonic anhydrase activity by antisense RNA in transgenic tobacco plants has a minor effect on photosynthetic CO2 assimilation

Abstract: As an approach to understanding the physiological role of chloroplast carbonic anhydrase (CA), this study reports on the production and preliminary physiological characterisation of transgenic tobacco (Nicotiana tabacum L.) plants where chloroplast CA levels have been specifically suppressed with an antisense construct directed against chloroplast CA mRNA. Primary transformants with CA levels as low as 2% of wild-type levels were recovered, together with intermediate plants with CA activities of about 20–50% of wild-type levels. Plants with even the lowest CA levels were not morphologically distinct from the wild-type plants. Segregation analysis of the low-CA character in plants grown from T1 selfed seed indicated that at least one of the low-CA plants appears to have two active inserts and that at least two of the intermediate-CA plants have one active insert. Analysis of CO2 gas exchange of a group of low-CA plants with around 2% levels of CA indicated that this large reduction in chloroplastic CA did not appear to cause a measurable alteration in net CO2 fixation at 350 μbar CO2 and an irradiance of 1000 μmol quanta·m−2·s−1. In addition, no significant differences in Rubisco activity, chlorophyll content, dry weight per unit leaf area, stomatal conductance or the ratio of intercellular to ambient CO2 partial pressure could be detected. However, the carbon isotope compositions of leaf dry matter were significantly lower (0.85%o) for low-CA plants than for wildtype plants. This corresponds to a 15-μbar reduction in the CO2 partial pressure at the sites of carboxylation. The difference, which was confirmed by concurrent measurement of discrimination with gas exchange, would reduce the CO2 assimilation rate by 4.4%, a difference that could not be readily determined by gas-exchange techniques given the inherent variability found in tobacco. A 98% reduction in CA activity dramatically reduced the 18O discrimination in CO2 passing over the leaf, consistent with a marked reduction in the ratio of hydrations to carboxylations. We conclude that a reduction in chloroplastic CA activity of two orders of magnitude does not produce a major limitation on photosynthesis at atmospheric CO2 levels, but that normal activities of the enzyme appear to play a role in facilitated transfer of CO2 within the chloroplast, producing a marginal improvement in the efficiency of photosynthesis in C3 plants.

Leaf anatomy enables more equal access to light and CO2 between chloroplasts

Abstract: The function of a leaf is photosynthesis, which requires the interception of light and access to atmospheric CO2 while controlling water loss. This paper examines the influence of leaf anatomy on both light capture and CO2 diffusion. As photosynthetic metabolism is spread between many chloroplasts, a leaf faces the challenge of matching light capture by a given chloroplast with the metabolic capacity of that chloroplast. Chloroplasts nearest the leaf surface receive the greatest irradiance and therefore absorb more light per unit chlorophyll than chloroplasts in the centre of a leaf. Electron transport and carbon fixation capacities per unit of chlorophyll decline with increasing depth in the leaf, to compensate for the decline in light absorbed per unit chlorophyll. Many key photosynthetic protein complexes in chloroplasts have nuclear encoded genetic information. Consequently, all chloroplasts within a given cell have a similar metabolic complement, which limits the potential gradient of photosynthetic capacity per unit chlorophyll across the leaf. A simple model couples light absorption through the leaf (based on the Beer–Lambert law) with the profile of chlorophyll through a leaf and the gradient in photosynthetic capacity. It is validated by comparison with 14CO2 fixation profiles through spinach leaves obtained in various studies. The model can account for published 14C fixation profiles obtained with blue, red and green light of different irradiances and white light applied in different combinations to the adaxial and abaxial surfaces of spinach leaves. The model confirms that spongy mesophyll increases the apparent extinction coefficient of chlorophyll compared to palisade tissue. The palisade tissue nearest the surface which receives light facilitates the penetration of light to a greater depth, while spongy mesophyll promotes scattering to enhance light absorption, thus reducing the gradient in light absorbed per unit chlorophyll through a leaf. CO2 fixation faces a diffusional limitation, which necessitates Rubisco to be spread evenly across the cell walls exposed to intercellular airspace. Mesophyll cell structure reflects the need to have a large cell surface per unit volume exposed to airspaces. The regular array of columnar cells in palisade tissue, or cell lobing in monocot leaves, results in greater exposed surface per unit tissue volume than spongy mesophyll. The exposed surface area per unit leaf area scales with photosynthetic capacity such that the difference in CO2 partial pressure between substomatal cavities and the sites of carboxylation within chloroplasts is, on average, independent of photosynthetic capacity of the leaf. However, Rubisco specific activity declines as the Rubisco content per unit leaf area increases due to greater internal diffusional limitations.

Phytoplankton photoacclimation and photoadaptation in response to environmental gradients in a shelf sea

Abstract: Variability in the photosynthetic performance of natural phytoplankton communities, due to both taxonomic composition and the physiological acclimation of these taxa to environmental conditions, was assessed at contrasting sites within a temperate shelf sea region. Physiological parameters relating to the structure of the photosystem II (PSII) antenna and processes downstream from PSII were evaluated using a combination of fast repetition rate fluorescence, oxygen flash yields, spectral fluorescence, and 14 C photosynthesis versus irradiance measurements. Parameters relating to PSII antenna structure, specifically the functional absorption cross-section (sPSII) and the chlorophyll to PSII reaction center ratio, varied principally as a result of spatial (horizontal) taxonomic differences. Phenotypic plasticity in the size of the PSII light-harvesting antenna appeared to be limited. In contrast, parameters related to electron transport rates (ETRs) downstream of PSII, including the maximum ETR (1/tPSII), the chlorophyll-specific maximum rate of carbon fixation (P ), and the light-saturation intensity ( Ek), all decreased from the surface to the subsurface chlorophyll * maximum (SCM) in stratified waters. The primary photoacclimation response to the vertical light gradient thus resulted in decreasing light-saturated carbon fixation per reaction center with increasing depth. Increases in the ratio of PSII reaction centers to carbon fixation capacity thus dominated the phenotypic response to decreased irradiance within the SCM. Perhaps counterintuitively, phytoplankton populations within fully mixed water columns, characterized by low mean irradiance, were acclimated or adapted to relatively high irradiance. Photoacclimation describes the phenotypic response of algae to changes in irradiance at the organism level (Falkowski and LaRoche 1991) and can be assessed by measuring dif

Carbonic Anhydrase Activity in Leaves and Its Role in the First Step of C4 Photosynthesis

Abstract: In C4 plants carbonic anhydrase catalyzes the critical first step of C4 photosynthesis, the hydration of CO2 to bicarbonate. The maximum activity of this enzyme in C4 leaf extracts, measured by H+ production with saturating CO2 and extrapolated to 25°C, was found to be 3,000 to 10,000 times the maximum photosynthesis rate for these leaves. Similar activities were found in C3 leaf extracts. However, the calculated effective activity of this enzyme at in vivo CO2 concentrations was apparently just sufficient to prevent the rate of conversion of CO2 to HCO3− from limiting C4 photosynthesis. This conclusion was supported by the mass spectrometric determination of leaf carbonic anhydrase activities.