Does greater leaf-level photosynthesis explain the larger solar energy conversion efficiency of Miscanthus relative to switchgrass?

C(4) perennial grasses are being considered for bioenergy because of their high productivity and low inputs. In side-by-side replicated trials, Miscanthus (Miscanthus x giganteus) has previously been found more than twice as productive as switchgrass (Panicum virgatum). The hypothesis that this difference is attributable to higher leaf photosynthetic rates was tested on established plots of switchgrass and Miscanthus in central Illinois with >3300 individual measurements on 20 dates across the 2005 and 2006 growing seasons. Seasonally integrated leaf-level photosynthesis was 33% higher in Miscanthus than switchgrass (P < 0.0001). This increase in carbon assimilation comes at the expense of additional transpiration since stomatal conductance was on average 25% higher in Miscanthus (P < 0.0001). Whole-chain electron transport rate, measured simultaneously by modulated chlorophyll fluorescence, was similarly 23% higher in Miscanthus (P < 0.0001). Efficiencies of light energy transduction into whole chain photosynthetic electron transport, leaf nitrogen use and leaf water use were all significantly higher in Miscanthus. These may all contribute to its higher photosynthetic rates, and in turn, productivity. Systematic measurement of photosynthesis over two complete growing seasons in the field provides a unique dataset explaining why the productivity of these two species differs and for validating mechanistic production models for these emerging bioenergy crops.

Effects of a cosmetic 'anti-ageing' product improves photoaged skin [corrected].

BACKGROUND: Very few over-the-counter cosmetic 'anti-ageing' products have been subjected to a rigorous double-blind, vehicle-controlled trial of efficacy. Previously we have shown that application of a cosmetic 'anti-ageing' product to photoaged skin under occlusion for 12 days can stimulate the deposition of fibrillin-1. This observation infers potential to repair and perhaps clinically improve photoaged skin. OBJECTIVE: We examined another similar over-the-counter cosmetic 'anti-ageing' product using both the patch test assay and a 6-month double-blind, randomized controlled trial (RCT), with a further 6-month open phase to assess clinical efficacy in photoaged skin. METHODS: For the patch test, commercially [corrected] available test product and its vehicle were applied occluded for 12-days to photoaged forearm skin (n = 10) prior to biopsy and immunohistochemical assessment of fibrillin-1; all-transretinoic acid (RA) [corrected] was used as a positive control. Sixty photoaged subjects were recruited to the RCT (test product, n = 30 vs. vehicle, n = 30; once daily for 6-months; face & hands) [corrected] with clinical assessments performed at recruitment and following 1-, 3- & 6-months of use [corrected]. Twenty-eight subjects had skin biopsies (dorsal wrist) at baseline and at 6 months of treatment for immunohistochemical assessment of fibrillin-1 (test product, n = 15; vehicle, n = 13). All subjects [corrected] received test product for a further 6-months. Final clinical assessments were performed at the end of this open period; 27 subjects received test product for 12-months [corrected]. RESULTS: In the 12-day patch test assay, we observed significant immunohistological deposition of fibrillin-1 in skin treated by test product and RA as compared to untreated baseline (P = 0.005 and 0.015 respectively). In the clinical RCT, at 6 months, compared to baseline assessment, 43% of subjects on test product had an improvement in facial wrinkles (P = 0.013), whereas only 22% of subjects using vehicle had clinical improvement (P = ns). Between group comparison of test product and vehicle was non-significant (P = 0.10). After 12 months, there was a significant benefit of test product over that projected for vehicle (70% vs. 33% of subjects improving; combined Wilcoxon rank tests, P = 0.026). There was significant deposition of fibrillin-1 in skin treated for 6 months with test product (mean +/- SE; vehicle, 1.84 +/- 0.23; test product, 2.57 +/- 0.19; P = 0.019). CONCLUSION: An over-the-counter cosmetic 'anti-ageing' product demonstrated clear benefit over vehicle in fibrillin-1 deposition over a 6-month trial period. There was a corresponding but non-significant trend towards clinical improvement in facial wrinkles. Clinical improvements in the treated group were increased after a further 6-months of use. This study demonstrates that a cosmetic may improve the appearance of wrinkles and further supports the use of fibrillin-1 as a robust biomarker for repair of photoaged dermis.

Does Long-Term Elevation of CO2 Concentration Increase Photosynthesis in Forest Floor Vegetation? (Indiana Strawberry in a Maryland Forest).

As the partial pressure of CO2 (pCO2) in the atmosphere rises, photorespiratory loss of carbon in C3 photosynthesis will diminish and the net efficiency of light-limited photosynthetic carbon uptake should rise. We tested this expectation for Indiana strawberry (Duchesnea indica) growing on a Maryland forest floor. Open-top chambers were used to elevate the pCO2 of a forest floor habitat to 67 Pa and were paired with control chambers providing an ambient pCO2 of 38 Pa. After 3.5 years, D. indica leaves grown and measured in the elevated pCO2 showed a significantly greater maximum quantum efficiency of net photosynthesis (by 22%) and a lower light compensation point (by 42%) than leaves grown and measured in the control chambers. The quantum efficiency to minimize photorespiration, measured in 1% O2, was the same for controls and plants grown at elevated pCO2. This showed that the maximum efficiency of light-energy transduction into assimilated carbon was not altered by acclimation and that the increase in light-limited photosynthesis at elevated pCO2 was simply a function of the decrease in photorespiration. Acclimation did decrease the ribulose-1,5-bisphosphate carboxylase/oxygenase and light-harvesting chlorophyll protein content of the leaf by more than 30%. These changes were associated with a decreased capacity for light-saturated, but not light-limited, photosynthesis. Even so, leaves of D. indica grown and measured at elevated pCO2 showed greater light-saturated photosynthetic rates than leaves grown and measured at the current atmospheric pCO2. In situ measurements under natural forest floor lighting showed large increases in leaf photosynthesis at elevated pCO2, relative to controls, in both summer and fall. The increase in efficiency of light-limited photosynthesis with elevated pCO2 allowed positive net photosynthetic carbon uptake on days and at locations on the forest floor that light fluxes were insufficient for positive net photosynthesis in the current atmospheric pCO2.

Increased Accumulation of Carbohydrates and Decreased Photosynthetic Gene Transcript Levels in Wheat Grown at an Elevated CO2 Concentration in the Field.

Repression of photosynthetic genes by increased soluble carbohydrate concentrations may explain acclimation of photosynthesis to elevated CO2 concentration. This hypothesis was examined in a field crop of spring wheat (Triticum aestivum L.) grown at both ambient (approximately 360 [mu]mol mol-1) and elevated (550 [mu]mol mol-1) atmospheric CO2 concentrations using free-air CO2 enrichment at Maricopa, Arizona. The correspondence of steady-state levels of mRNA transcripts (coding for the 83-kD photosystem I apoprotein, sedoheptulose-1,7-bisphosphatase, phosphoribulokinase, phosphoglycerokinase, and the large and small subunits of ribulose-1,5-bisphosphate carboxylase/oxygenase) with leaf carbohydrate concentrations (glucose-6-phosphate, glucose, fructose, sucrose, fructans, and starch) was examined at different stages of crop and leaf development and through the diurnal cycle. Overall only a weak correspondence between increased soluble carbohydrate concentrations and decreased levels for nuclear gene transcripts was found. The difference in soluble carbohydrate concentration between leaves grown at elevated and current ambient CO2 concentrations diminished with crop development, whereas the difference in transcript levels increased. In the flag leaf, soluble carbohydrate concentrations declined markedly with the onset of grain filling; yet transcript levels also declined. The results suggest that, whereas the hypothesis may hold well in model laboratory systems, many other factors modified its significance in this field wheat crop.

Net carbon storage in a poplar plantation (POPFACE) after three years of free-air CO2 enrichment.

A high-density plantation of three genotypes of Populus was exposed to an elevated concentration of carbon dioxide ([CO(2)]; 550 micromol mol(-1)) from planting through canopy closure using a free-air CO(2) enrichment (FACE) technique. The FACE treatment stimulated gross primary productivity by 22 and 11% in the second and third years, respectively. Partitioning of extra carbon (C) among C pools of different turnover rates is of critical interest; thus, we calculated net ecosystem productivity (NEP) to determine whether elevated atmospheric [CO(2)] will enhance net plantation C storage capacity. Free-air CO(2) enrichment increased net primary productivity (NPP) of all genotypes by 21% in the second year and by 26% in the third year, mainly because of an increase in the size of C pools with relatively slow turnover rates (i.e., wood). In all genotypes in the FACE treatment, more new soil C was added to the total soil C pool compared with the control treatment. However, more old soil C loss was observed in the FACE treatment compared with the control treatment, possibly due to a priming effect from newly incorporated root litter. FACE did not significantly increase NEP, probably as a result of this priming effect.

Plant growth regulators control ozone damage to wheat yield.

• Tropospheric ozone (O3 ) is damaging to plants and decreases crop yields. This study investigated the role of two plant growth regulators in affecting wheat-yield responses to elevated [O3 ]. In a controlled factorial experiment, wheat plants were treated with combinations of Ethephon, which releases ethene, Chlormequat, which blocks gibberellin synthesis, and elevated [O3 ]. • Spring-wheat plants were subjected to lifelong exposures to ambient or moderately elevated [O3 ]. At flag-leaf emergence, the plants were treated with Ethephon and/or Chlormequat, or untreated (controls). Gas-exchange measurements were made at anthesis; morphology, biomass, and yield components were recorded at harvest. • Elevated [O3 ] accelerated development and decreased the number of grains per ear and ears per plant. Chlormequat abolished these O3 effects, protecting against yield reduction though not biomass loss. Ethephon treatment partially protected against O3 -induced biomass loss though not yield reduction. • This study suggests that the effects of elevated [O3 ] on development and allocation are more important in determining the yield response of wheat than the accompanying decline in photosynthesis and biomass accumulation.

Photosynthesis and stomatal conductance responses of poplars to free-air CO2 enrichment (PopFACE) during the first growth cycle and immediately following coppice.

• Using the Poplar Free Air CO2 Enrichement (PopFACE) facility we investigated the effects of elevated [CO2 ] on the diurnal and growth cycle responses of photosynthesis and conductance in three poplar species. • In situ diurnal measurements of photosynthesis were made on Populus alba, P. nigra and P. ×euramericana and, in parallel, in vivo maximum capacity for carboxylation (Vc,max ) and maximum rates of electron transport (Jmax ) were determined by gas exchange measurement. • Light saturated (Asat ) and daily integrated (A') photosynthesis increased at elevated [CO2 ] in all species. Elevated [CO2 ] decreased Vc,max and Jmax for P. nigra and Jmax for P.¥euramericana but had no effect on stomatal conductance in any of the species throughout the first growth cycle. During post-coppice re-growth, elevated [CO2 ] did not increase Asat in P. nigra and P.×euramericana due to large decreases in Vc,max and Jmax . • A 50% increase in [CO2 ] under these open-air field conditions resulted in a large and sustained increase in Asat . Although there were some differences between the species, these had little effect on photosynthetic rates at the growth [CO2 ]. Nevertheless the results show that even fast growing trees grown without rooting volume restriction in the open may still show some down-regulation of photosynthetic potential at elevated [CO2 ].

Macro-invertebrate populations and production on a salt-marsh in east England dominated by Spartina anglica.

The populations and production of the macroinvertebrates of a Spartina anglica salt-marsh in eastern England were studied over two years. A total of fifteen species were recorded in the sediments, of which twelve species were of regular occurrence, and the total population density recorded ranged from 3,481 m-2 to 11,444m-2 over the twenty-four sampling occasions.The four most abundant species were Nereis diversicolor, Tubifex costatus, Corophium volutator and Hydrobia ulvae. Thirteen further taxa were associated with the canopy of Spartina, with a total population density ranging from 0 to 1,149 m-2. Total monthly standing crop ranged from 1.8 to 8.5 g C m-2 with peaks in July/August in both years. Nereis diversicolor contributed 55% to 86% of total biomass in each month.Production and respiration for each species was determined and annual assimilation calculated. The total annual production was ∼16 g C m-2 a-1 in both 1979 and 1980, with a corresponding assimilation of ∼60 g C m-2 a-1 Nereis diversicolar accounted for >80% of production and assimilation in both years, and the species is clearly of considerable potential importance in the dissipation of Spartina material. The canopy dwelling species accounted for about 1% of the total annual production and assimilation.

Leaf and canopy photosynthetic CO2 uptake of a stand of Echinochloa polystachya on the Central Amazon floodplain : Are the high potential rates associated with the C4 syndrome realized under the near-optimal conditions provided by this exceptional natural habitat?

The C4 grass Echinochloa polystachya, which forms dense and extensive monotypic stands on the Varzea floodplains of the Amazon region, provides the most productive natural higher plant communities known. The seasonal cycle of growth of this plant is closely linked to the annual rise and fall of water level over the floodplain surface. Diurnal cycles of leaf photosynthesis and transpiration were measured at monthly intervals, in parallel with measurements of leaf area index, canopy light interception and biomass. By artificial manipulation of the light flux incident on leaves in the field light-response curves of photosynthesis at the top and near to the base of the canopy were generated. Fitted light-response curves of CO2 uptake were combined with information of leaf area index, incident light and light penetration of the canopy to estimate canopy rates of photosynthesis. Throughout the period in which the floodplains were submerged photosynthetic rates of CO2 uptake (A) for the emergent leaves were high with a mean of c. 30 µmol m-2 s-1 at mid-day and occasional values of 40 µmol m-2 s-1. During the brief dry phase, when the floodplain surface is uncovered, there was a significant depression of A, with mid-day mean values of c. 17 µmol m-2 s-1. This corresponded with a c. 50% decrease in stomatal conductance, and a c. 35% depression in the ratio of the leaf inter-cellular to external CO2 concentration (c i/c a). During the dry phase, a midday depression of rates of CO2 assimilation was observed. The lowest leaf area index (F) was c. 2 in November-December, when the flood plain was dry, and again in May, when the rising floodwaters were submerging leaves faster than they were replaced. The maximum F of c. 5 was in August when the floodwaters were receding rapidly. Canopy light interception efficiency varied from 0.90 to 0.98. Calculated rates of canopy photosynthesis exceeded 18 mol C m-2 mo-1 throughout the period of flooding, with a peak of 37 mol C m-2 mo-1 in August, but declined to 13 mol C m-2 mo-1 in November during the dry phase. Estimated uptake of carbon by the canopy from the atmosphere, over 12 months, was 3.57 kg C m-2. This was insufficient to account for the 3.99 kg C m-2 of net primary production, measured simultaneously by destructive harvesting. It is postulated that this discrepancy might be accounted for by internal diffusion of CO2 from the CO2-rich waters and sediments via the roots and stems to the sites of assimilation in the leaves.

Very high productivity of the C4 aquatic grass Echinochloa polystachya in the Amazon floodplain confirmed by net ecosystem CO2 flux measurements.

Fluxes of CO2 and H2O vapour from dense stands of the C4 emergent macrophyte grass Echinochloa polystachya were measured by eddy covariance in both the low water (LW) and high water (HW, flooded) phases of the annual Amazon river cycle at Manaus, Brazil. Typical clear-sky midday CO2 uptake rates by the vegetation stand (including detritus, sediment or water surface) were 30 and 35 µmol CO2 (ground) m-2 s-1 in the LW and HW periods, respectively. A rectangular hyperbola model fitted the responses of "instantaneous" (20- or 30-min average) net CO2 exchange rates to incident photosynthetic photon flux densities (PFD) well. Stand evaporation rates were linearly related to PFD. The major difference in CO2 uptake rates between the two periods was the larger respiration flux during LW due to the CO2 efflux from sediment, roots and litter. Integrated 20- or 30-min fluxes were used to derive relationships between daily CO2 and H2O vapour fluxes and incident radiation. The daily CO2 fluxes were almost linearly related to incident radiation, but there was evidence of saturation at the highest daily radiation totals. Annual productivity estimated from the daily model in 1996-1997 agreed closely with that previously estimated for 1985-1986 from a leaf-scale photosynthetic model, but were some 15% less than those derived at that time from biomass harvests. Both CO2 uptake and water use efficiency were comparable with those found in fertilised maize fields in warm temperate conditions.

The role of herbicides in the erosion of salt marshes in eastern England.

Laboratory studies and field trials were conducted to investigate the role of herbicides on saltmarsh vegetation, and their possible significance to saltmarsh erosion. Herbicide concentrations within the ranges present in the aquatic environment were found to reduce the photosynthetic efficiency and growth of both epipelic diatoms and higher saltmarsh plants in the laboratory and in situ. The addition of sublethal concentrations of herbicides resulted in decreased growth rates and photosynthetic efficiency of diatoms and photosynthetic efficiency of higher plants. Sediment stability also decreased due to a reduction in diatom EPS production. There was qualitative evidence that diatoms migrated deeper into the sediment when the surface was exposed to simazine, reducing surface sediment stability by the absence of a cohesive biofilm. Sediment loads on leaves severely reduced photosynthesis in Limonium vulgare. This, coupled with reduced carbon assimilation from the effects of herbicides, could have large negative consequences for plant productivity and over winter survival of saltmarsh plants. The data support the hypothesis that sublethal herbicide concentrations could be playing a role in the increased erosion of salt marshes that has occurred over the past 40 years.

The use of spinal opioids in cancer pain.

Approximately two-thirds of cancer patients suffer from significant pain. Until recently, less expensive and less costly conservative treatments have been utilized to treat these patients' pain, but the outcomes have been poor, with many experiencing inadequate pain relief. Although intraspinal opioid therapy is relatively new and more invasive than traditional treatments, it can provide most cancer patients better pain relief with less side effects. Intraspinal opioids can be used successfully to treat intractable malignant pain states.

Repair of photoaged dermal matrix by topical application of a cosmetic 'antiageing' product.

BACKGROUND: Photoaged skin is characterized by coarse and fine wrinkles. The mechanism of wrinkle formation appears to involve changes to components of the dermal extracellular matrix. Topical treatment with all-trans retinoic acid (RA) can repair photoaged dermal matrix; this is regarded as the 'gold standard' against which repair agents are judged. To date, little is known regarding the ability of over-the-counter 'antiageing' products to repair photoaged skin. OBJECTIVES: We used a modified occluded patch test to ascertain whether topical applications of cosmetic 'antiageing' products are able to repair photoaged human skin. METHODS: Commercially available test products [basic moisturizer, 'antiageing' cream containing different active complex levels (6% active: lipopentapeptide, white lupin peptides, antioxidants, retinyl palmitate; 2% active: lipopentapeptide, white lupin peptides, antioxidants)] were applied under occlusion for 12 days prior to biopsy and histological assessment in photoaged volunteers (n=9). RA was used as a positive control. RESULTS: In agreement with previous studies, the patch-test study revealed that RA produced significant fibrillin-1 deposition in the papillary dermis (P<0.01) but had little effect on procollagen I or matrix metalloproteinase-1 expression. The 6% total active complex formulation, however, increased the deposition of fibrillin-1 and procollagen I (P<0.01, P<0.05, respectively). CONCLUSIONS: This study indicates that in an in vivo 12-day patch test an over-the-counter cosmetic product can induce changes in photoaged dermal extracellular matrix, which are indicative of repair.

Can fast-growing plantation trees escape biochemical down-regulation of photosynthesis when grown throughout their complete production cycle in the open air under elevated carbon dioxide?

Poplar trees sustain close to the predicted increase in leaf photosynthesis when grown under long-term elevated CO2 concentration ([CO2]). To investigate the mechanisms underlying this response, carbohydrate accumulation and protein expression were determined over four seasons of growth. No increase in the levels of soluble carbohydrates was observed in the young expanding or mature sun leaves of the three poplar genotypes during this period. However, substantial increases in starch levels were observed in the mature leaves of all three poplar genotypes grown in elevated [CO2]. Despite the very high starch levels, no changes in the expression of photosynthetic Calvin cycle proteins, or in the starch biosynthetic enzyme ADP-glucose pyrophosphorylase (AGPase), were observed. This suggested that no long-term photosynthetic acclimation to CO2 occurred in these plants. Our data indicate that poplar trees are able to 'escape' from long-term, acclimatory down-regulation of photosynthesis through a high capacity for starch synthesis and carbon export. These findings show that these poplar genotypes are well suited to the elevated [CO2] conditions forecast for the middle of this century and may be particularly suited for planting for the long-term carbon sequestration into wood.

WIMOVAC: a software package for modelling the dynamics of plant leaf and canopy photosynthesis.

The ability to predict net carbon exchange and production of vegetation in response to predicted atmospheric and climate change is critical to assessing the potential impacts of these changes. Mathematical models provide an important tool in the study of whole plant, canopy and ecosystem responses to global environmental change. Because this requires prediction beyond experience, mechanistic rather than empirical models are needed. The uniformity and strong understanding of the photosynthetic process, which is the primary point of response of plant production to global atmospheric change, provides a basis for such an approach. Existing modelling systems have been developed primarily for expert modellers and have not been easily accessible to experimentalists, managers and students. Here we describe a modular modelling system operating within Windows to provide this access. WIMOVAC (Windows Intuitive Model of Vegetation response to Atmosphere and Climate Change) is designed to facilitate the modelling of various aspects of plant photosynthesis with particular emphasis on the effects of global climate change. WIMOVAC has been designed to run on IBM PC-compatible computers running Microsoft Windows. The package allows the sophisticated control of the simulation processes for photosynthesis through a standardized Windows user interface and provides automatically formatted results as either tabulated data or as a range of customizable graphs. WIMOVAC has been written in Microsoft Visual Basic, to facilitate the rapid development of user-friendly modules within the familiar Windows framework, while allowing a structured development. The highly interactive nature of controls adopted by WIMOVAC makes it suitable for research, management and educational purposes.

An in vivo analysis of photosynthesis during short-term O3 exposure in three contrasting species.

The depressions of photosynthetic CO2 uptake following O3 exposures of 200 and 400 nmol mol(-1) for between 4 and 16 h were compared between Pisum sativum, Quercus robur and Triticum aestivum, and the potential causes of change identified in vivo. Photosynthetic change was examined by analysis of CO2, O2, O3 and water vapour exchanges together with chlorophyll fluorescence in controlled environments. Under identical fumigation conditions, each species showed very similar rates of O3 consumption. The light-saturated rate of CO2 uptake showed a statistically significant decrease in each species with increasing O3 dose. Although stomatal conductance declined in parallel with CO2 uptake this did not account for the observed decrease in photosynthesis. The decrease in mesophyll conductance resulted primarily from a decrease in the apparent carboxylation capacity, implying in decreased activity of ribulose 1,5-bisphosphate carboxylase/oxygenase. The maximum capacity of carboxylation was consequently reduced by over 30% and 50% after 16 h fumigation with 200 and 400 nmol mol(-1) O3 respectively. Additionally, in Q. robur, a statistically significant inhibition of the CO2 saturated rate of photosynthesis occurred after 16 h with 400 nmol mol(-1) O3, suggesting that the ability to regenerate ribulose 1,5-bisphosphate was also impaired. None of the species showed any significant decrease in the efficiency of light-limited photosynthesis following fumigation at 200 nmol mol(-1) O3, but effects were apparent at 400 nmol mol(-1) O3. The common feature in all three species was a decline in carboxylation capacity which preceded any other change in the photosynthetic apparatus.

Effect of the Long-Term Elevation of CO(2) Concentration in the Field on the Quantum Yield of Photosynthesis of the C(3) Sedge, Scirpus olneyi.

CO(2) concentration was elevated throughout 3 years around stands of the C(3) sedge Scirpus olneyi on a tidal marsh of the Chesapeake Bay. The hypothesis that tissues developed in an elevated CO(2) atmosphere will show an acclimatory decrease in photosynthetic capacity under light-limiting conditions was examined. The absorbed light quantum yield of CO(2) uptake (ø(abs) and the efficiency of photosystem II photochemistry were determined for plants which had developed in open top chambers with CO(2) concentrations in air of 680 micromoles per mole, and of 351 micromoles per mole as controls. An Ulbricht sphere cuvette incorporated into an open gas exchange system was used to determine ø(abs) and a portable chlorophyll fluorimeter was used to estimate the photochemical efficiency of photosystem II. When measured in an atmosphere with 10 millimoles per mole O(2) to suppress photorespiration, shoots showed a ø(abs) of 0.093 +/- 0.003, with no statistically significant difference between shoots grown in elevated or control CO(2) concentrations. Efficiency of photosystem II photochemistry was also unchanged by development in an elevated CO(2) atmosphere. Shoots grown and measured in 680 micromoles per mole of CO(2) in air showed a ø(abs) of 0.078 +/- 0.004 compared with 0.065 +/- 0.003 for leaves grown and measured in 351 micromoles per mole CO(2) in air; a highly significant increase. In accordance with the change in ø(abs), the light compensation point of photosynthesis decreased from 51 +/- 3 to 31 +/- 3 micro-moles per square meter per second for stems grown and measured in 351 and 680 micromoles per mole of CO(2) in air, respectively. The results suggest that even after 3 years of growth in elevated CO(2), there is no evidence of acclimation in capacity for photosynthesis under light-limited conditions which would counteract the stimulation of photosynthetic CO(2) uptake otherwise expected through decreased photorespiration.

The occurrence of photoinhibition in an over-wintering crop of oil-seed rape (Brassica napus L.) and its correlation with changes in crop growth.

The maximum quantum yield of CO2 uptake (Φ), as a measure of light-limited photosynthetic efficiency, of a Brassica napus crop was measured on most days from mid-October until mid-April. During the winter, Φ was decreased by up to 50%. From January to March, leaves exposed to direct sunlight on days with minimum air temperatures near or below 0° C showed significant reductions in Φ. However, control leaves, artificially shaded from direct sunlight on these days, did not show any decrease. This provides statistical evidence for a light-dependent inhibition of CO2 uptake in the field, termed here photoinhibition. Recovery of Φ during warmer interludes was slow, requiring approx. 2-3 d. Concurrent measurements of light interception by the crop canopy and dry-matter accumulation showed that the efficiency with which intercepted light was converted into dry matter varied, declin between January and March to 33% of the value recorded in the warmer autumn months. Conversion efficiency was significantly and positively correlated with quantum yield. In a closed crop canopy during winter, light will be limiting for photosynthesis for much of the time. Under these conditions depression of Φ at the leaf level may contribute significantly to decreased dry-matter accumulation at the crop level, since the light-limited rate of CO2 uptake is likely to govern canopy photosynthetic rate.