Photosynthesis In Spring Research Articles

Phenology of Photosynthesis in Winter‐Dormant Temperate and Boreal Forests: Long‐Term Observations From Flux Towers and Quantitative Evaluation of Phenology Models

AbstractWe examined the seasonality of photosynthesis in 46 evergreen needleleaf (evergreen needleleaf forests (ENF)) and deciduous broadleaf (deciduous broadleaf forests (DBF)) forests across North America and Eurasia. We quantified the onset and end (StartGPP and EndGPP) of photosynthesis in spring and autumn based on the response of net ecosystem exchange of CO2 to sunlight. To test the hypothesis that snowmelt is required for photosynthesis to begin, these were compared with end of snowmelt derived from soil temperature. ENF forests achieved 10% of summer photosynthetic capacity ∼3 weeks before end of snowmelt, while DBF forests achieved that capacity ∼4 weeks afterward. DBF forests increased photosynthetic capacity in spring faster (1.95% d−1) than ENF (1.10% d−1), and their active season length (EndGPP–StartGPP) was ∼50 days shorter. We hypothesized that warming has influenced timing of the photosynthesis season. We found minimal evidence for long‐term change in StartGPP, EndGPP, or air temperature, but their interannual anomalies were significantly correlated. Warmer weather was associated with earlier StartGPP (1.3–2.5 days °C−1) or later EndGPP (1.5–1.8 days °C−1, depending on forest type and month). Finally, we tested whether existing phenological models could predict StartGPP and EndGPP. For ENF forests, air temperature‐ and daylength‐based models provided best predictions for StartGPP, while a chilling‐degree‐day model was best for EndGPP. The root mean square errors (RMSE) between predicted and observed StartGPP and EndGPP were 11.7 and 11.3 days, respectively. For DBF forests, temperature‐ and daylength‐based models yielded the best results (RMSE 6.3 and 10.5 days).

Combining NDVI, PRI and the quantum yield of solar-induced fluorescence improves estimations of carbon fluxes in deciduous and evergreen forests

We used automated spectroradiometers to continuously monitor changes in the optical parameters of phenological and photosynthetic traits in beech and spruce forests. We examined seasonal variations in the normalized difference vegetation index (NDVI), photochemical reflectance index (PRI), and solar-induced fluorescence in the oxygen A band (SIFA) that was estimated using a 3-FLD discrimination method from radiance data. The optical parameters tracked the activation and cessation of photosynthesis in spring and autumn. Data at photon fluxes >1200 μmol m−2 s−1 during extended noon hours were used to link the seasonal PRI and SIFA variations to the dynamics of photosynthesis. Seasonal PRI was significantly correlated with photosynthetic light-use efficiency (LUE) with R2 values of 0.66 and 0.48 for the measurements in beech and spruce forests, respectively. SIFA emissions were significantly correlated with the gross primary production (GPP) of the evergreen spruce forest (R2 = 0.47), but R2 was only 0.13 when measured in the beech forest. The correlations between the optical parameters and GPP or LUE, however, tended to be lower when using a dataset with constant NDVI. Introducing an equation combining NDVI, PRI, and the quantum yield of SIFA emission increased R2 for LUE estimation to 0.77 in the spruce forest and 0.63 in the beech forest. GPP was estimated from the parametric equation with improved accuracy reaching R2 = 0.53 and RMSE = 5.95 μmol CO2 m−2 s−1 in spruce forest and R2 = 0.58 and RMSE = 5.23 μmol CO2 m−2 s−1 in beech forest. Parametric equations were more efficient in estimating photosynthesis in datasets that consisted of an entire season's data. By combining NDVI, PRI and the quantum yield of SIFA, we could thus substantially improve estimations of carbon fluxes in diverse deciduous and evergreen canopies.

Seasonal variation in the canopy color of temperate evergreen conifer forests.

Summary Evergreen conifer forests are the most prevalent land cover type in North America. Seasonal changes in the color of evergreen forest canopies have been documented with near‐surface remote sensing, but the physiological mechanisms underlying these changes, and the implications for photosynthetic uptake, have not been fully elucidated.Here, we integrate on‐the‐ground phenological observations, leaf‐level physiological measurements, near surface hyperspectral remote sensing and digital camera imagery, tower‐based CO2 flux measurements, and a predictive model to simulate seasonal canopy color dynamics.We show that seasonal changes in canopy color occur independently of new leaf production, but track changes in chlorophyll fluorescence, the photochemical reflectance index, and leaf pigmentation. We demonstrate that at winter‐dormant sites, seasonal changes in canopy color can be used to predict the onset of canopy‐level photosynthesis in spring, and its cessation in autumn. Finally, we parameterize a simple temperature‐based model to predict the seasonal cycle of canopy greenness, and we show that the model successfully simulates interannual variation in the timing of changes in canopy color.These results provide mechanistic insight into the factors driving seasonal changes in evergreen canopy color and provide opportunities to monitor and model seasonal variation in photosynthetic activity using color‐based vegetation indices.

Seasonal Photoprotective Responses in Needles of Korean Fir (Abies koreana) Over an Altitudinal Gradient on Mount Halla, Jeju Island, Korea

Chloroplast pigments and chlorophyll fluorescence were characterized in needles of Korean fir (Abies koreana) in summer, winter, and spring at three altitudes on Jeju Island, Korea. High light-harvesting efficiency (intrinsic photosystem II efficiency) and indirect evidence for high photosynthetic rates (high levels ofβ-carotene and chlorophyll b) during the growing season contrasted with mid-winter downregulation of light-harvesting efficiency involving retention of high zeaxanthin levels and locked-in photoprotective thermal dissipation (from low chlorophyll fluorescence emission). Neoxanthin levels were inversely correlated with sustained photoprotection in the winter, and lutein to xanthophyll cycle carotenoid levels decreased from summer to winter, suggesting that zeaxanthin plays the prominent role in winter photoprotection of Korean fir needles. Summer was apparently most conducive to photosynthesis, consistent with high levels of summer precipitation on Jeju Island, and in contrast to fir and other conifers in a climate with dry summers at high altitudes in Colorado, U.S.A., where studies have shown that the wet spring is the season most favorable for photosynthesis. Lastly, despite there being only 300 m difference in altitude among the three sites, there were discernible differences in (i) accumulation of zeaxanthin in winter (as an indicator for the severity of conditions, with highest levels at the highest altitude), (ii) apparent photosynthesis rates in summer (from β-carotene levels, with highest levels at the highest altitude), and (iii) transition to increased photosynthesis in spring (from fluorescence emission levels, slowest at the highest altitude).

Freezing induced leaf movements and their potential implications to early spring carbon gain: Rhododendron maximum as exemplar

Summary 1 Thermonastic leaf movements (TLM) are induced by freezing and are correlated with freezing tolerance, and our goal was to determine the significance of TLM to photosynthesis during the winter and early spring. 2 We performed field experiments in which we prevented TLM of Rhododendron maximum leaves and determined the consequences of that prevention to photoinhibition (measured by chlorophyll fluorescence) from fall to spring, photosynthesis (measured by gas exchange) in the winter, and recovery of photosynthesis in the spring. 3 TLM significantly reduced photoinhibition in the winter for leaves on branches in the outer canopy of R. maximum plants, but not for leaves on inner canopy branches. 4 During warm periods in the winter, TLM were associated with significantly lower photoinhibition, but TLM did not have a significant effect on photosynthesis during these times. 5 In early spring, leaves with TLM recovered from photoinhibition more quickly than for leaves prevented from TLM. 6 Photosynthesis in the early spring was higher at any stomatal conductance for leaves with TLM than for leaves prevented from TLM for outer canopy leaves only. 7 Our results demonstrate that TLM during the winter prevent excessive photoinhibition and promote rapid recovery of photosynthesis in the early spring.

Freezing induced leaf movements and their potential implications to early spring carbon gain:Rhododendron maximumas exemplar

1 Thermonastic leaf movements (TLM) are induced by freezing and are correlated with freezing tolerance, and our goal was to determine the significance of TLM to photosynthesis during the winter and early spring. 2 We performed field experiments in which we prevented TLM of Rhododendron maximum leaves and determined the consequences of that prevention to photoinhibition (measured by chlorophyll fluorescence) from fall to spring, photosynthesis (measured by gas exchange) in the winter, and recovery of photosynthesis in the spring. 3 TLM significantly reduced photoinhibition in the winter for leaves on branches in the outer canopy of R. maximum plants, but not for leaves on inner canopy branches. 4 During warm periods in the winter, TLM were associated with significantly lower photoinhibition, but TLM did not have a significant effect on photosynthesis during these times. 5 In early spring, leaves with TLM recovered from photoinhibition more quickly than for leaves prevented from TLM. 6 Photosynthesis in the early spring was higher at any stomatal conductance for leaves with TLM than for leaves prevented from TLM for outer canopy leaves only. 7 Our results demonstrate that TLM during the winter prevent excessive photoinhibition and promote rapid recovery of photosynthesis in the early spring.

Spring Thaw and Its Effect on Terrestrial Vegetation Productivity in the Western Arctic Observed from Satellite Microwave and Optical Remote Sensing

Abstract Global satellite remote sensing records show evidence of recent vegetation greening and an advancing growing season at high latitudes. Satellite remote sensing–derived measures of photosynthetic leaf area index (LAI) and vegetation gross and net primary productivity (GPP and NPP) from the NOAA Advanced Very High Resolution Radiometer (AVHRR) Pathfinder record are utilized to assess annual variability in vegetation productivity for Alaska and northwest Canada in association with the Western Arctic Linkage Experiment (WALE). These results are compared with satellite microwave remote sensing measurements of springtime thaw from the Special Sensor Microwave Imager (SSM/I). The SSM/I-derived timing of the primary springtime thaw event was well correlated with annual anomalies in maximum LAI in spring and summer (P ≤ 0.009; n = 13), and GPP and NPP (P ≤ 0.0002) for the region. Mean annual variability in springtime thaw was on the order of ±7 days, with corresponding impacts to annual productivity of approximately 1% day−1. Years with relatively early seasonal thawing showed generally greater LAI and annual productivity, while years with delayed seasonal thawing showed corresponding reductions in canopy cover and productivity. The apparent sensitivity of LAI and vegetation productivity to springtime thaw indicates that a recent advance in the seasonal thaw cycle and associated lengthening of the potential period of photosynthesis in spring is sufficient to account for the sign and magnitude of an estimated positive vegetation productivity trend for the western Arctic from 1982 to 2000.

The winter Arctic Oscillation, the timing of spring, and carbon fluxes in the Northern Hemisphere

Various observations show trends toward warmer and earlier springs in the Northern Hemisphere. We hypothesize that the positive trend in the winter Arctic Oscillation (AO) has led to higher winter temperatures, advanced spring, and increased seasonal amplitudes in atmospheric CO2. To test this hypothesis, we modeled leaf‐out and terrestrial carbon fluxes using the Simple Biosphere model, Version 2 (SiB2) and the National Centers for Environmental Prediction (NCEP) reanalysis for 1958–2002. We found that our modeled leaf‐out trends were consistent with observed phenology and that the winter AO trend can statistically explain 20–70% of the modeled leaf‐out trends in the eastern United States and northern Europe. We also found that warmer winter temperatures associated with the positive trend in the winter AO increased winter respiration. At the same time, these warmer winter temperatures advanced the date of leaf‐out, increasing the total spring uptake of atmospheric CO2 by plants. These seasonally asymmetric trends toward increased respiration in winter and increased photosynthesis in spring can help explain the trend toward increased seasonal amplitudes in observed atmospheric CO2 concentration.

The productivity of irrigated legumes in northern Victoria. 2. Effect of grazing management

The productivity of irrigated white (Trifolium repens L.) and red (Trifolium pratense L.) clover swards was compared in an experiment of more than 3 years duration. It was hypothesised that white clover would be more productive than red clover when defoliation was frequent and intense, and less productive when defoliation was infrequent and lax. The experiment was a factorial design involving 2 species of clover [white clover (cv. Haifa) and red clover (cv. Redquin)], 2 grazing frequencies and 2 grazing intensities (with the criteria for both being based on quantities of herbage present before/after grazing). There were 4 extra treatments sown: perennial ryegrass (Lolium perenne L. cv. Grasslands Nui) and white clover (cv. Haifa), lucerne (Medicago sativa L. cv. Validor), Persian clover (Trifolium resupinatum L. cv. Maral) or subterranean clover (Trifolium subterraneum L. cv. Trikkala), but only 1 defoliation treatment was used for each of these treatemnts. There were 4 replicated blocks of all treatments. Apparent growth rates [calculated from measurements of dry matter (DM) removed by grazing] of white clover ranged from a low of 10 kg DM/ha.day in winter to a high of 70 kg DM/ha.day in summer. The growth rates of white clover swards were superior to those of ryegrass and white clover swards over summer, but were generally lower from May to October. In 2 of the 4 years, frequent grazing of white clover resulted in greater (P<0.05) production than infrequent grazing (average of 12.8 v. 10.7 t DM/ha) whereas intensity of grazing only affected DM net accumulation in the first year (P<0.05). The data show no evidence of a decline in productivity over time. Sward structure of white clover was influenced by grazing treatment with the numerically highest yielding treatment (frequent and hard) having the highest density of stolon tips (vegetative buds). In relation to days of regrowth, the frequently grazed treatment had higher levels of net photosynthesis in spring and summer compared with the infrequently grazed treatment. The frequently grazed treatment achieved positive carbon balance immediately after grazing and reached maximum levels of photosynthesis at 8–10 days, whereas the infrequent treatment showed negative carbon balance for the first 2–3 days after grazing with maximum photosynthesis being achieved later than in the frequently grazed treatment. When net photosynthesis was related to leaf area, there were fewer differences between the 2 treatments. The exception was in spring when photosynthesis was lowest where the initial leaf area was highest in the infrequent and hard treatment. Maximum photosynthesis was achieved at diminishing leaf area index from spring through to winter. Red clover was the most productive legume in the first year after establishment, but it did not persist beyond the second year and its DM net accumulation was reduced by more frequent grazing (12.4 v. 15.3 t DM/ha in the first year and 6.1 v. 9.1 t DM/ha in the second year; P<0.05). The DM net accumulation of lucerne was greater than that in any other treatment (an average of 16.7 t DM/ha in the 2 completed years), whereas the annual legumes, subterranean clover and Persian clover, averaged 6.6 and 10.7 t DM/ha.year, respectively. The seasonal growth rate data showed that lucerne had very good summer production whereas the annuals tended to be at least as good as the perennials from May to October.

Intermittent low temperatures constrain spring recovery of photosynthesis in boreal Scots pine forests

AbstractDuring winter and early spring, evergreen boreal conifers are severely stressed because light energy cannot be used when photosynthesis is pre‐empted by low ambient temperatures. To study photosynthetic performance dynamics in a severe boreal climate, seasonal changes in photosynthetic pigments, chloroplast proteins and photochemical efficiency were studied in a Scots pine forest near Zotino, Central Siberia. In winter, downregulation of photosynthesis involved loss of chlorophylls, a twofold increase in xanthophyll cycle pigments and sustained high levels of the light stress‐induced zeaxanthin pigment. The highest levels of xanthophylls and zeaxanthin did not occur during the coldest winter period, but rather in April when light was increasing, indicating an increased capacity for thermal dissipation of excitation energy at that time. Concomitantly, in early spring the D1 protein of the photosystem II (PSII) reaction centre and the light‐harvesting complex of PSII dropped to their lowest annual levels. In April and May, recovery of PSII activity, chloroplast protein synthesis and rearrangements of pigments were observed as air temperatures increased above 0°C. Nevertheless, severe intermittent low‐temperature episodes during this period not only halted but actually reversed the physiological recovery. During these spring low‐temperature episodes, protective processes involved a complementary function of the PsbS and early light‐induced protein thylakoid proteins. Full recovery of photosynthesis did not occur until the end of May. Our results show that even after winter cold hardening, photosynthetic activity in evergreens responds opportunistically to environmental change throughout the cold season. Therefore, climate change effects potentially improve the sink capacity of boreal forests for atmospheric carbon. However, earlier photosynthesis in spring in response to warmer temperatures is strongly constrained by environmental variation, counteracting the positive effects of an early recovery process.

Air temperature triggers the recovery of evergreen boreal forest photosynthesis in spring

AbstractThe timing of the commencement of photosynthesis (P*) in spring is an important determinant of growing‐season length and thus of the productivity of boreal forests. Although controlled experiments have shed light on environmental mechanisms triggering release from photoinhibition after winter, quantitative research for trees growing naturally in the field is scarce. In this study, we investigated the environmental cues initiating the spring recovery of boreal coniferous forest ecosystems under field conditions. We used meteorological data and above‐canopy eddy covariance measurements of the net ecosystem CO2 exchange (NEE) from five field stations located in northern and southern Finland, northern and southern Sweden, and central Siberia. The within‐ and intersite variability for P* was large, 30–60 days. Of the different climate variables examined, air temperature emerged as the best predictor for P* in spring. We also found that ‘soil thaw’, defined as the time when near‐surface soil temperature rapidly increases above 0°C, is not a useful criterion for P*. In one case, photosynthesis commenced 1.5 months before soil temperatures increased significantly above 0°C. At most sites, we were able to determine a threshold for air‐temperature‐related variables, the exceeding of which was required for P*. A 5‐day running‐average temperature (T5) produced the best predictions, but a developmental‐stage model (S) utilizing a modified temperature sum concept also worked well. But for both T5 and S, the threshold values varied from site to site, perhaps reflecting genetic differences among the stands or climate‐induced differences in the physiological state of trees in late winter/early spring. Only at the warmest site, in southern Sweden, could we obtain no threshold values for T5 or S that could predict P* reliably. This suggests that although air temperature appears to be a good predictor for P* at high latitudes, there may be no unifying ecophysiological relationship applicable across the entire boreal zone.

Inter-annual and seasonal variations of energy and water vapour fluxes above a Pinus sylvestris forest in the Siberian middle taiga

Long-term eddy covariance measurements of energy and water fluxes and associated climatic parameters were carried out above a Scots pine (Pinus sylvestris) forest in the middle taiga zone of Central Siberia. Data from June 1998 through October 2000 are presented. With the exception of winter 1998/1999, data collection over this period were more or less continuous. A distinct seasonality in surface energy exchange characteristics was observed in all years. In early spring in the absence of physiological activity by the vegetation, about 80% of the net radiation was partitioned for sensible heat, resulting in Bowen ratios, b, as high as 8. In the 1‐2 wk period associated with onset of photosynthesis in spring, evaporation rates increased rapidly and b rapidly dropped. However, even during summer months, sensible heat fluxes typically exceeded latent heat fluxes and b remained above 2.0. Observed daily evaporation rates varied between 0.5‐1.0 mm d−1 in spring and autumn and 1.5‐2 mm d−1 in midsummer. The overall average for the three growing seasons examined was 1.25 mm d−1. Precipitation was on average 230 mm for the growing period, with evaporation over the same time being about 190 mm for both 1999 and 2000. This represented only about 35% of the equilibrium evaporation rate. There was typically a positive hydrological balance of 40 mm for the growing season as a whole. However, in all three years examined, evaporation exceeded precipitation totals by 20‐40 mm in at least one calendar month during summer. During the growing season, daily averaged surface conductances varied between 0.15 and 0.20 mol m−2 s−1 (3‐4.5 mm s−1) in dry or cool months and 0.30‐0.35 mol m−2 s−1 (6.5‐8 mm s−1) in moist and warm months. Despite a negative hydrological balance during midsummer, there was little evidence for reduced canopy conductances in response to soil water deficits. This may have been the consequence of roots accessing water from within or just above a perched water table, located at about 2 m depth.

Inter-annual and seasonal variations of energy and water vapour fluxes above a Pinus sylvestris forest in the Siberian middle taiga

Long-term eddy covariance measurements of energy and water fluxes and associated climatic parameters were carried out above a Scots pine (Pinus sylvestris) forest in the middle taiga zone of Central Siberia. Data from June 1998 through October 2000 are presented. With the exception of winter 1998/1999, data collection over this period were more or less continuous. A distinct seasonality in surface energy exchange characteristics was observed in all years. In early spring in the absence of physiological activity by the vegetation, about 80% of the net radiation was partitioned for sensible heat, resulting in Bowen ratios, β, as high as 8. In the 1–2 wk period associated with onset of photosynthesis in spring, evaporation rates increased rapidly and β rapidly dropped. However, even during summer months, sensible heat fluxes typically exceeded latent heat fluxes and β remained above 2.0. Observed daily evaporation rates varied between 0.5–1.0 mm d−1 in spring and autumn and 1.5–2 mm d−1 in midsummer. The overall average for the three growing seasons examined was 1.25 mm d−1. Precipitation was on average 230 mm for the growing period, with evaporation over the same time being about 190 mm for both 1999 and 2000. This represented only about 35% of the equilibrium evaporation rate. There was typically a positive hydrological balance of 40 mm for the growing season as a whole. However, in all three years examined, evaporation exceeded precipitation totals by 20–40 mm in at least one calendar month during summer.During the growing season, daily averaged surface conductances varied between 0.15 and 0.20 mol m−2 s−1 (3–4.5 mm s−1) in dry or cool months and 0.30–0.35 mol m−2 s−1 (6.5–8 mm s−1) in moist and warm months. Despite a negative hydrological balance during midsummer, there was little evidence for reduced canopy conductances in response to soil water deficits. This may have been the consequence of roots accessing water from within or just above a perched water table, located at about 2 m depth.

Protection and storage of chlorophyll in overwintering evergreens.

How evergreen species store and protect chlorophyll during exposure to high light in winter remains unexplained. This study reveals that the evergreen snow gum (Eucalyptus pauciflora Sieb. ex Spreng.) stores and protects its chlorophylls by forming special complexes that are unique to the winter-acclimated state. Our in vivo spectral and kinetic characterizations reveal a prominent component of the chlorophyll fluorescence spectrum around 715 nm at 77 K. This band coincides structurally with a loss of chlorophyll and an increase in energy-dissipating carotenoids. Functionally, the band coincides with an increased capacity to dissipate excess light energy, absorbed by the chlorophylls, as heat without intrathylakoid acidification. The increased heat dissipation helps protect the chlorophylls from photo-oxidative bleaching and thereby facilitates rapid recovery of photosynthesis in spring.

Seasonal changes in photosystem II organisation and pigment composition in Pinus sylvestris

Conifers of the boreal zone encounter considerable combined stress of low temperature and high light during winter, when photosynthetic consumption of excitation energy is blocked. In the evergreen Pinus sylvestris L. these stresses coincided with major seasonal changes in photosystem II (PSII) organisation and pigment composition. The earliest changes occurred in September, before any freezing stress, with initial losses of chlorophyll, the D1-protein of the PSII reaction centre and of PSII light-harvesting-complex (LHC II) proteins. In October there was a transient increase in F0, resulting from detachment of the light-harvesting antennae as reaction centres lost D1. The D1-protein content eventually decreased to 90%, reaching a minimum by December, but PSII photochemical efficiency [variable fluorescence (Fv)/maximum fluorescence (Fm)] did not reach the winter minimum until mid-February. The carotenoid composition varied seasonally with a twofold increase in lutein and the carotenoids of the xanthophyll cycle during winter, while the epoxidation state of the xanthophylls decreased from 0.9 to 0.1 from October to January. The loss of chlorophyll was complete by October and during winter much of the remaining chlorophyll was reorganised in aggregates of specific polypeptide composition, which apparently efficiently quench excitation energy through non-radiative dissipation. The timing of the autumn and winter changes indicated that xanthophyll de-epoxidation correlates with winter quenching of chlorophyll fluorescence while the drop in photochemical efficiency relates more to loss of D1-protein. In April and May recovery of the photochemistry of PSII, protein synthesis, pigment rearrangements and zeaxanthin epoxidation occurred concomitantly. Indoor recovery of photosynthesis in winter-stressed branches under favourable conditions was completed within 3 d, with rapid increases in F0, the epoxidation state of the xanthophylls and in light-harvesting polypeptides, followed by recovery of D1-protein content and Fv/Fm, all without net increase in chlorophyll. The fall and winter reorganisation allow Pinus sylvestris to maintain a large stock of chlorophyll in a quenched, photoprotected state, allowing rapid recovery of photosynthesis in spring.

Low Growth Temperature Effects a Differential Inhibition of Photosynthesis in Spring and Winter Wheat

In vivo room temperature chlorophyll a fluorescence coupled with CO(2) and O(2) exchange was measured to determine photosynthetic limitation(s) for spring and winter wheat (Triticum aestivum L.) grown at cold-hardening temperatures (5 degrees C/5 degrees C, day/night). Plants of comparable physiological stage, but grown at nonhardening temperatures (20 degrees C/16 degrees C, day/night) were used in comparison. Winter wheat cultivars grown at 5 degrees C had light-saturated rates of CO(2) exchange and apparent photon yields for CO(2) exchange and O(2) evolution that were equal to or greater than those of winter cultivars grown at 20 degrees C. In contrast, spring wheat cultivars grown at 5 degrees C showed 35% lower apparent photon yields for CO(2) exchange and 25% lower light-saturated rates of CO(2) exchange compared to 20 degrees C grown controls. The lower CO(2) exchange capacity is not associated with a lower efficiency of photosystem II activity measured as either the apparent photon yield for O(2) evolution, the ratio of variable to maximal fluorescence, or the level of reduced primary quinone electron acceptor maintained at steady-state photosynthesis, and is most likely associated with carbon metabolism. The lower CO(2) exchange capacity of the spring cultivars developed following long-term exposure to low temperature and did not occur following over-night exposure of nonhardened plants to 5 degrees C.