Optimum Temperature For Net Photosynthesis Research Articles

Volatile organic compound emission in tundra shrubs – Dependence on species characteristics and the near-surface environment

Temperature is one of the key abiotic factors during the life of plants, especially in the Arctic region which is currently experiencing rapid climate change. We evaluated plant traits and environmental variables determining leaf temperature in tundra shrubs and volatile organic compound (VOC) emissions with field measurements on deciduous tundra shrubs, Salix myrsinites and Betula nana, and evergreen Cassiope tetragona and Rhododendron lapponicum. Higher leaf-to-air temperature difference was observed in evergreen, compared to deciduous shrubs. Evergreen shrubs also showed continuously increasing photosynthesis with increasing temperature, suggesting high thermal tolerance. For the deciduous species, the optimum temperature for net photosynthesis was between our measurement temperatures of 24 °C and 38 °C. Air temperature and vapor pressure deficit were the most important variables influencing leaf temperature and VOC emissions in all the studied plants, along with stomatal density and specific leaf area in the deciduous shrubs. Using climate data and emission factors from our measurements, we modelled total seasonal tundra shrub VOC emissions of 0.3–2.3 g m−2 over the main growing season. Our results showed higher-than-expected temperature optima for photosynthesis and VOC emission and demonstrated the relative importance of plant traits and local environments in determining leaf temperature and VOC emissions in a subarctic tundra.

Ecophysiology of bryophytes in the changing environment

Studies of the impact of climate change on plants are generally based on vascular plants. Bryophytes basically differ from tracheophytes in having a smaller size and a poikilohydric strategy for water and nutrients. Their survival and growth are highly dependent on their external environment. Bryophytes are able to lose most of their cell water without dying, and resume normal metabolism after rehydration, gaining positive carbon balance over wet-dry cycles. This sort of adaptation is called desiccation tolerance. Desiccation tolerance is a common phenomenon among bryophytes: cells of bryophytes in exposed sunny sites can switch from full turgor to air dryness quickly, while the species of moist habitats dry more slowly, and can suffer stress by even moderate drying. Bryophytes can maintain efficient photosynthesis under low light conditions, have low chl a/b ratios, and their optimum growth is possible within a limited temperature range. When certain bryophytes’ CO2 assimilation is suppressed, photorespiration activity increases and becomes the main electron sink. Bryophytes, as sensitive components of various vegetation types, are capable of effective light absorption during their desiccation, rehydration, freezing and melting, with the help of coexisting alga and vascular plant energy dissipation mechanisms. They have a relatively low optimal temperature for growth (with narrow T range for net photosynthetic gain), with only minor differences between the optimum temperatures for net photosynthesis in polar, temperate and tropical populations of the same species. Bryophytes have a low acclimatization potential for high temperatures, taking into account that they are generally drought avoiders. Temperature acclimatization potential is of high importance for the physiological basis of altitudinal distribution and the likely responses of bryophytes to climate change. Bryophytes with their small and resistant spores are able to disperse over long distances by wind, which might help their survival in a changing environment. Dominant vascular vegetation might change as temperature will increase; however, suitable microhabitats for bryophytes might still persist. The abundance and species composition of bryophytes in plant communities is predicted to be altered, just as the function of the whole ecosystem. Based on recent literature and own data, the author makes an attempt to summarize the physiological mechanisms, morphological features and alternative strategies that make bryophytes successful in a constantly changing terrestrial environment. These plants represent a sophisticated solution to the challenges of life at their scale. Further exploration of bryophyte ecophysiology in the changing and stressful environment will provide new information that will assist bryophyte conservation.

Physiological responses of habitat‐forming seaweeds to increasing temperatures

Kelps and fucoids are dominant habitat-forming seaweeds along temperate rocky coastlines. Here, we tested the physiological performance of a dominant kelp (Ecklonia radiata) and two fucoids (Scytothalia dorycarpa and Sargassum fallax), distributed along the southwest coast of Australia. Photosynthesis and respiration were measured against increments in temperature for seaweeds collected along a latitudinal gradient in ocean temperature from Kalbarri (warm) to Hamelin Bay (cool). We found a similar decrease in photosynthetic activity from cooler to warmer latitudes in all three species. Seaweeds collected from warmer locations had significantly lower chlorophyll a concentration compared to cooler locations which could explain the lower levels of photosynthetic activity at warmer latitudes. The Q10 values for photosynthesis and respiration tended to decrease from cooler to warmer locations. For all species, the optimum temperature for net photosynthesis remained similar across the locations. However, within locations, the optimum temperature for S. fallax (25.2°C) was significantly higher than E. radiata (24.0°C) and S. dorycarpa (23.6°C). The reduction rates of net photosynthesis observed after optimum temperature showed the greatest variation among the species within and across locations. A thermal performance breadth analysis revealed a broader performance range for S. fallax (21.5–28.4°C) followed by E. radiata (21.2–26.5°C) and S. dorycarpa (21.4–25.8°C). These results highlight the differences in temperature sensitivity among the three species which help explain their current distributional patterns and have potential implications for future responses to future ocean warming.

Photosynthetic performance of Xanthoria mawsonii C. W. Dodge in coastal habitats, Ross Sea region, continental Antarctica

Xanthoria mawsonii C. W. Dodge was found to perform well physiologically in a variety of habitats at high latitudes in continental Antarctica. The net photosynthetic rate of 7·5 μ mol CO2 kg−1 s−1 is exceptionally high for Antarctic lichens. Field and laboratory measurements proved the photosynthetic apparatus to be highly adapted to strong irradiance. The cold resistance of the photosystem II reaction centres is higher than the photosynthetic CO2 fixation process. Optimum temperature for net photosynthesis was c. 10°C. The lichen grows along water channels where it is frequently inundated and hydrated to maximum water content, although net photosynthesis is strongly depressed by super saturation. In these habitats the lichen is photosynthetically active for long periods of time. Xanthoria mawsonii also grows at sites where it depends entirely on the early spring snow melt and occasional snow fall for moisture. It has an exceptionally short reactivation phase and is able to utilize snow immediately. Recovery of activity by absorbing water vapour from air, though practically possible, seems to be of ecological importance only under snow at subzero temperatures.

Photosynthetic activity of a temperate coral Acropora pruinosa (Scleractinia, Anthozoa) with symbiotic algae in Japan

SUMMARYPhysiological properties of the temperate hermatypic coral Acropora pruinosa Brook with symbiotic algae (zooxanthellae) on the southern coast of the Izu Peninsula, Shizuoka Prefecture, central Japan, were compared between summer and winter. Photosynthesis and respiration rates of the coral with symbiotic zooxanthellae were measured in summer and winter under controlled temperatures and irradiances with a differential gasvolumeter (Productmeter). Net photosynthetic rate under all irradiances was higher in winter than in summer at the lower range of temperature (12–20°C), while lower than in summer at the higher range of temperature (20–30°C). The optimum temperature for net photosynthesis was apt to fall with the decrease of irradiance both in summer and winter, whereas it was higher in summer than in winter under each irradiance. At 25/ 50/100 μmol photons nr2 s−1, it was nearly the sea‐water temperature in each season. Dark respiration rate was higher in winter than in summer, especially in the range from 20–30°C. In both seasons the optimum temperature for gross photosynthesis was 28°C under 400 μmol photons nr2 s−1 and lowered with decreasing irradiance up to 22°C under 25 μmol photons nr2 s−1 in summer, while 20°C under the same irradiance in winter. The optimum temperature for production/respiration (P/R) ratio was higher in summer than in winter under each irradiance. Results indicated that metabolism of coral and zooxanthellae is adapted to ambient temperature condition under nearly natural irradiance in each season.

Photosynthetic light and temperature responses of Eucalyptus cloeziana and Eucalyptus argophloia

Acclimation of gas exchange to temperature and light was determined in 18-month-old plants of humid coastal (Gympie) and dry inland (Hungry Hills) provenances of Eucalyptus cloeziana F.Muell., and in those of a dry inland provenance of Eucalyptus argophloia Blakely. Plants were acclimated at day/night temperatures of 18/13, 23/18, 28/23 and 33/28�C in controlled-temperature glasshouses for 4 months. Light and temperature response curves were measured at the beginning and end of the acclimation period. There were no significant differences in the shape and quantum-yield parameters among provenances at 23, 28 and 33�C day temperatures. Quantum yield [μmol CO2 μmol–1 photosynthetic photon flux density (PPFD)] ranged from 0.04 to 0.06 and the light response shape parameter ranged from 0.53 to 0.78. Similarly, no consistent trends in the rate of dark respiration for plants of each provenance were identified at the four growth temperatures. Average values of dark respiration for the plants of the three provenances ranged from 0.61 to 1.86 μmol m–2 s–1. The optimum temperatures for net photosynthesis increased from 23 to 32�C for the humid- and from 25 to 33�C for the dry-provenance E. cloeziana and from 21 to 33�C for E. argophloia as daytime temperature of the growth environment increased from 18 to 33�C. These results have implications in predicting survival and productivity of E. cloeziana and E. argophloia in areas outside their natural distribution.

Photosynthesis and respiration in bladelets of Ecklonia cava Kjellman (Laminariales, Phaeophyta) in two localities with different temperature conditions

SUMMARY Characteristics of photosynthesis and respiration of bladelets were compared between Ecklonia cava Kjellman sporophytes growing in a warmer temperate locality (Tei, Kochi Pref., southern Japan) and in a cooler temperate locality (Nabeta, Shizuoka Pref., central Japan). Photosynthesis and respiration were measured with a differential gas-volumeter (Productmeter). In photosynthesis-light curves at 20°C, the rate of net photosynthesis was almost the same at light intensities lower than 25 μmol m−2 s−1 and the light-saturation occurred at 200–400 μmol m−2s−1 in plants of both localities. The light-saturated net photosynthetic rates were higher in winter and spring than in summer and autumn in both plants. The optimum temperature for net photosynthesis at 400 μmol m−2s−1 was 27°C throughout the year in the Tei plant and 25–27°C in the Nabeta plant. The decrease of net photosynthetic rates in the supraoptimal temperature range up to 29°C was sharper in winter and spring than in summer and autumn in both plants, being smaller in the Tei plant than in the Nabeta plant in all seasons. The dark respiration rate always increased with water temperature rise in both plants. No clear differences were found in the dark respiration rate between Tei and Nabeta plants except that when measured against dry weight, the Tei plant showed a slightly lower rate as compared with the Nabeta plant.

Seasonal photosynthetic responses to light and temperature in white spruce (Picea glauca) seedlings planted under an aspen (Populus tremuloides) canopy and in the open.

Photosynthetic light and temperature response curves were measured seasonally in seedlings of white spruce (Picea glauca (Moench.) Voss) grown for two years in the understory of aspen (Populus tremuloides Michx.) or in the open in central Alberta. Light-saturated rate of net photosynthesis, the optimum temperature for net photosynthesis, transpiration rate, photochemical efficiency, and stomatal and mesophyll conductances increased from spring to summer and declined thereafter, whereas dark respiration rate and compensation and saturation points were highest in spring. Depression of photosynthetic parameters was greater in open-grown seedlings than in understory seedlings during the periods in spring and autumn when night frosts were common. Net photosynthetic rates were similar in understory and open-grown seedlings in summer, but they were significantly lower in open-grown seedlings in spring and autumn. Significantly lower transpiration rates and stomatal conductances in open-grown seedlings than in understory seedlings were also observed at 15 and 25 degrees C in the autumn. Shoot and needle growth were less in open-grown seedlings than in understory seedlings. In summer, when irradiances were low in the aspen understory, understory white spruce seedlings maintained a positive carbon balance by decreasing their compensation and saturation points and increasing their photochemical efficiency compared to spring and autumn.

Photosynthetic temperature responses of Eucalyptus globulus and Eucalyptus nitens.

Steady-state photosynthetic responses to leaf temperature of 4-year-old Eucalyptus globulus Labill. and E. nitens (Deane and Maiden) Maiden trees were measured between 10 and 35 degrees C at approximately monthly intervals from early spring to midwinter. The photosynthetic temperature optimum of recently expanded leaves in the sun canopy was linearly related to the average temperature of the preceding week during the 9-month measurement period. The optimum temperature for net photosynthesis of E. globulus increased from 17 to 23 degrees C as the mean daily temperature increased from 7 to 16 degrees C. Similarly, the optimum temperature for net photosynthesis of E. nitens increased from 14 to 20 degrees C as the mean daily temperature increased from 7 to 19 degrees C. The temperature for maximum photosynthetic response of E. globulus and E. nitens was similar at each measurement time, but the photosynthetic performance of E. nitens was less sensitive to temperatures above and below this optimum than that of E. globulus. In December, the apical shoots of branches of E. globulus had a net photosynthetic temperature optimum of between 10 and 15 degrees C. The corresponding values for expanding leaves, fully expanded leaves from the current year's growth, and fully expanded leaves from the previous year's growth were 15, 20 and 20-25 degrees C, respectively. In a second experiment, E. globulus clones taken from four mother plants originating from climatically dissimilar locations within Tasmania were acclimated at day/night temperatures of 10/15, 18/23 and 25/30 degrees C in temperature-controlled greenhouses. Another set of clones was acclimated in a shadehouse where temperatures ranged between 10 and 25 degrees C and with a mean daily temperature of approximately 15 degrees C. Plants grown at 25/30 degrees C had significantly lower net photosynthetic rates when measured at 10 and 20 degrees C than plants grown at lower temperatures. Plants grown at 10/15 degrees C had significantly lower net photosynthetic rates when measured at 30 degrees C than plants grown at higher temperatures. Plants grown at the ambient conditions prevailing in midautumn in Hobart had significantly higher net photosynthetic rates at 20 degrees C than plants raised in the greenhouses and were equal best performers at 10 and 30 degrees C. A comparison of the light response curves of the plants showed that the maximum rate of net photosynthesis was affected by the growth temperature, whereas the apparent quantum efficiency remained unchanged. There were no significant differences in the photosynthetic temperature responses of the four genotypes derived from climatically dissimilar locations within Tasmania. A comparison of temperature response models for E. globulus indicated that incomplete acclimation (defined by a slope value of less than 1 for the linear relationship between the temperature optimum for photosynthesis and the growth temperature) generally resulted in a greater daily carbon uptake than complete acclimation (slope value of 1).

Cold resistance and metabolic activity of lichens below 0°C

Laboratory measurements show that lichens are extremely tolerant of freezing stress and of low-temperature exposure. Metabolic activity recovered quickly after severe and extended cold treatment. Experimental results demonstrate also that CO 2 exchange is already active at around −20°C. The psychrophilic character of polar lichen species is demonstrated by optimum temperatures for net photosynthesis between 0 and 15°C. In situ measurements show that lichens begin photosynthesizing below 0°C if the dry thalli receive fresh snow. The lowest temperature measured in active lichens was −17°C at a continental Antarctic site. The fine structure and the hydration state of photobiont and mycobiont cells were studied by low-temperature scanning electron microscopy (LTSEM) of frozen hydrated specimens. Water potentials of the frozen system are in the range of or even higher than those allowing dry lichens to start photosynthesis by water vapor uptake at +10°C. The great success of lichens in polar and high alpine regions gives evidence of their physiological adaptation to low temperatures. In general lichens are able to persist through glacial periods, but extended snow cover and glaciation are limiting factors.

Acclimation of Photosynthesis and Dark Respiration of a Submersed Angiosperm beneath Ice in a Temperate Lake.

Ceratophyllum demersum L. remained physiologically active beneath ice of a southeastern Michigan lake. The effect of seasonally low photosynthetic photon flux density (PPFD) and cold but nonfreezing temperature on whole-plant physiology was studied. Net photosynthesis was measured at six temperatures and 12 PPFDs. Net photosynthesis, soluble protein concentration, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) protein concentration, and Rubisco activity of winter plants were 32, 31, 33, and 70% lower, respectively, than those of plants collected in the summer. Optimum temperatures for net photosynthesis of winter and summer plants were 5 and 30[deg]C, respectively. Dark respiration of winter plants was up to 313% greater than that of summer plants. Reduced Rubisco activity and increased dark respiration interacted to reduce net photosynthesis. Interaction of reduced net photosynthesis and increased dark respiration increased CO2 and light compensation points and the light saturation point of winter plants. Growth of C. demersum was limited by the ambient phosphorus concentration of lake water during summer. Apical stem segments of winter-collected plants had 54 and 35% more phosphorus and nitrogen, respectively, than summer-collected plants. Physiologically active perennation beneath ice enabled C. demersum to accumulate phosphorus during the winter when it was most abundant. Partial uncoupling of phosphorus acquisition from utilization may reduce phosphorus limitation upon growth during the summer when phosphorus concentration is seasonally the lowest.

Thermal acclimation of photosynthesis by the arctic plant Saxifraga cernua

The thermal acclimations of net photosynthesis, dark respiration, and photorespiration have been studied in the arctic plant Saxifraga cernua. The gas exchange of whole plants grown to maturity under different temperature regimes was analysed for individual plants transferred from (i) 10 to 20 (referred to as high-temperature acclimation) and (ii) 20 to 5 °C (low-temperature acclimation). High- and low-temperature acclimation resulted in shifts of the leaf temperature optimum for net photosynthesis of whole plants in the direction of the new growth temperature. That the acclimating temperature directly affected the photosynthetic apparatus was indicated by (i) changes in the optimum temperature for gross photosynthesis of whole plants and (ii) a change in the oxygen sensitivity of net photosynthesis after acclimation to a new growth temperature. The change in the optimum temperature for net photosynthesis was also due, in part, to altered dark respiration rates which increased during acclimation to low growth temperatures. These results suggest that such acclimation in arctic species like S. cernua arose as a result of the selective pressure of fluctuating temperatures which are experienced during the growth season to maximize annual growth under arctic and subarctic conditions.

Vegetation and ecology of ice-free areas of Northern Victoria Land, Antarctica

At Birthday Ridge, a small ice free area in northern Victoria Land (70°48′S, 167°00′E), cryptogamic vegetation is mostly confined to gaps between granitc rocks. The sheltering effect on lichens and mosses was analyzed by continuous measurements of the microclimate at various levels between the rocks. Although warming by solar radiation was favourable for the existence of cryptogams, rocks strongly insolated were mostly devoid of lichens and mosses. Lichens in the soaked active state were heated to above air temperature but did not reach more than 10°C. The presence of lichens was dependent on the moisture conditions of the habitat. It was observed that snow, the only source of moisture, accumulated in summer only in deeper levels between rocks, and that the snow rapidly melted on contact with the lichens. After a snow shower,Usnea sulphurea gained 67% andUmbilicaria decussata 94% of their maximum water capacity. During one quarter of the time period of 7 days the lichens were soaked and therefore capable of carrying out photosynthesis. The lichens in soaked state had always less than optimum temperatures for net photosynthesis. The rock gaps at Birthday Ridge form oases, the only localities where moisture is provided, and temperature is high enough to enable growth of lichens and mosses.Bryum is also able to exist in the upper 3 cm of soil.

Ecophysiology of lichens in the dry valleys of Southern Victoria Land, Antarctica

CO2 exchange was measured in three cryptoendolithic lichen samples from the ice-free mountains of Southern Victoria Land. Optimum temperature for net photosynthesis at different light intensities ranged from below 2°C to about 7°C; the upper compensation point ranged from 5°C to 15.7°C, and the lower compensation point could be estimated in two samples as being between-6°C and-8°C. Dark respiration rates were higher than those of net photosynthesis. The results indicate that these cryptoendolithic lichens are not more adapted to low temperatures than are crustose lichens from coastal Antarctica. Calculations show that optimum temperatures for photosynthesis may not be reached in the natural environment. Photosynthetic rates were low, but they increased when samples were split into smaller pieces, permitting gas exchange through broken rock surfaces. In nature, when gas exchange is possible only through the intact rock crust, these apparent rates are probably even lower.

Seasonal Patterns of CO2Exchange in the Shoot and Root of Loblolly Pine Seedlings

Seedlings of six full-sib families of loblolly pine were grown outdoors in clay pots for two growing seasons. Dark respiration of shoot and root and CO2 exchange of the shoot in the light were measured periodically over a temperature range bracketing ambient conditions. Both shoot and root showed different physiological responses as seasonal temperature changed, but differences among crosses were not apparent. Optimum temperatures for net photosynthesis shifted with seasonal temperatures from a high of nearly 25 C in midsummer to a low of 10 C in midwinter. Root respiration was higher at 30 C in the summer than at 30 C any time after August and higher at 0 C in the winter than during summer. Dark shoot respiration was high during initial shoot elongation and second-year bud break but fell thereafter. Rates of net photosynthesis remained high for a longer period of time before declining. A peak in root respiration occurred in October of the second year following a steady decline throughout the growing season. A period of rapid root growth in late autumn appeared to follow summer shoot growth.

Carbon dioxide exchange in Cladina lichens from subarctic and temperate habitats.

The survival potential of lichens in a given habitat is determined by the response of CO2 exchange to photosynthetically active radiation (PhAR), thallus temperature, and thallus relative water content (RWC). Therefore morphologically similar lichens from contrasting climatic environments 1) should differ in their CO2 exchange responses, and 2) these differences should reflect adaptations to their climatic regimes. The CO2 exchange responses of a subarctic (55°N, 67°W) Cladina stellaris (Opiz) Brodo population and a temperate (29°N, 82°W) Cladina evansii (Abb.) Hale and W. Culb, population were used to test these two related hypotheses.Infrared gas analysis with lichens collected in September-October 1975 established that the two populations differed in their responses to incident PhAR, thallus temperature, and thallus RWC. Net photosynthesis in C. stellaris had an optimum at a lower temperature and a greater relative photosynthetic capacity at low temperatures than did C. evansii. Cladina evansii maintained net photosynthesis above 35°C thallus temperature; C. stellaris did not. In both species the optimum temperature for net photosynthesis increased with increasing irradiance. The C. stellaris light saturation point was consistently lower than that of C. evansii. Both species had maximal rates of net photosynthesis at 70-80% relative water content. In C. evansii the CO2 exchange rates, expressed as percentages of the maximum rate, declined more rapidly under suboptimal conditions. The absolute CO2 exchange rates of C. evansii were greater than those of C. stellaris. At 20°C and 90-95% RWC, resaturation respiration occurred in both species and continued until 6-7 h after wetting.Contrasts in the temporal patterns of thallus condition at each collection site suggest that not all differences in the two response surfaces reflect climatic adaptation. The two populations appear well adapted to incident PhAR and thallus temperature regimes but the 70-80% RWC optimum for net photosynthesis common to both species is puzzling since their water regimes differ markedly. The overall adaptedness of the CO2 exchange responses in the two species cannot be judged without a comprehensive quantitative analysis of carbon balance under differing climatic regimes.

PHYSIOLOGICAL‐ENVIRONMENTAL INTERACTIONS IN LICHENS

SUMMARYThe control of the net photosynthetic rate in Peltigera canina var. praetextata and P. polydactyla by temperature, light and moisture has been examined on a seasonal basis. Mean optimum values of 6 mg CO2 g−1·h−1 are achieved by young lobes of either species during late spring, summer and early winter. However, under full winter conditions optimum net photo‐synthetic rates fall to 3 mg CO2 g−1·h−1. Light saturation is approximately 500 μE m−2 s−1 at all times of the year, but the temperature at which maximum rates of net photosynthesis are achieved changes markedly throughout the year in both species. In winter, maximum rates occur at an experimental thallus temperature of 5°C but by mid‐summer, the optimum temperature for net photosynthesis is c. 30°C, which corresponds to equivalent ambient temperature levels in the field. This temperature acclimation of net photosynthetic optima is achieved without any significant change in respiration rate and shows a remarkable degree of homeostasis during spring, summer and early winter. The results strongly emphasize the unsatisfactory nature of the univariate experimental design which has often been used in lichen physiological ecology. The extreme importance of a controlled storage temperature for experimental lichen material is also clearly evident. The low net photosynthetic rates in winter show the same pattern in both Peltigera species during these low temperature and short day conditions.

Effects of light and temperature on leaf anatomy and photosynthesis in Fragaria vesca.

Fragaria vesca, the woodland strawberry, was grown under a series of controlled environments including variations in light intensity, average temperatures, and temperature amplitude around a constant mean. Observations on CO2 exchange capacities, leaf anatomy, and cell ultrastructure were made for each treatment to determine relationships between these variables. With increasing light intensity, leaf thickness, leaf density, and mesophyll cell surface area and volume per leaf surface area increased. Net photosynthesis (NPS) per leaf weight decreased with increasing light pretreatment while NPS per area increased from low to medium intensity, then decreased at the highest intensity. Depression of photosynthesis at the highest light pretreatment may have been due to massive starch accumulation in the chloroplasts associated with the sodium vapor lamps used. Correlation of all anatomical variables was highly significant with dark respiration and NPS per dry weight but insignificant for NPS per leaf area. In the variable temperature treatments, photosynthetic acclimation occurred with a shift in optimum temperature for NPS in the direction of prevailing growth temperature. Absolute rates were highest at moderate pretreatment temperatures and were reduced by extreme growth temperatures. Thick leaves with low density mesophyll became thinner and more dense with increasing growth temperature corresponding to an increase in maximum net photosynthetic rates. Leaves became thicker and more dense at the highest temperatures, but with an increase in cell damage and indications of changes in metabolic pathways. Highest correlations for gas exchange rates were with specific leaf weight (weight per area). Correlation with other anatomical variables were scattered or insignificant. It was concluded that adaptation to a range of environmental conditions cannot be consistently attributed to changes in mesophyll cell volume or surface area.

Comparative CO2 exchange patterns in mosses from two tundra habitats at Barrow, Alaska

The effects of variation in light intensity, temperature, and water content on rates of net photosynthesis and dark respiration have been investigated in two common tundra mosses, Polytrichum alpinum from drier habitats and Calliergon sarmentosum from wetter habitats at Barrow, Alaska. Optimum temperatures for net photosynthesis of 10–15 °C for both species and saturating light intensities (photosynthetically active radiation (PhAR), 400–700 nm) of about 0.12 cal cm−2 min−1 for P. alpinum and 0.15 cal cm−2 min−1 for C. sarmentosum correlate well with measurements of light intensity and moss tissue temperatures made over the season at the collection site. It is suggested that depressions in net photosynthetic rates around midday might be caused by supraoptimal temperatures and possibly supraoptimal light intensities. Calliergon sarmentosum, a semiaquatic species required a higher water content (about 450% dry weight) than P. alpinum (about 200%) to reach maximal rates of net photosynthesis. Mean maximal rates of net photosynthesis ranged from about 2.6 to 4.4 mg CO2 g−1 dry weight h−1 for P. alpinum and from about 1.5 to 3.0 mg CO2 g−1 dry weight h−1 for C. sarmentosum. Predictions of net annual production have been made for both species. Predicted levels of 171 g C m−2 per 50-day season for C. sarmentosum compare well with results obtained for species of similar growth form elsewhere in polar regions. For P. alpinum the predicted level of 38.5 g C m−2 per 50-day season compares with observed dry matter production at the same site of 43 g m−2 per season.

C4Photosynthesis in Smooth Pigweed

The response of net photosynthetic rate to temperature and light, the rate of photorespiration, the compensation concentration for CO2, and the pathway of CO2fixation in smooth pigweed (Amaranthus hybridusL.) were investigated using infrared gas analysis and14CO2. Maximum net photosynthetic rates of whole shoots were 45 mg CO2dm-2hr-1at 30 C and 7.8 klux. Intact whole shoots were not light saturated at 10 klux. The optimum temperature for net photosynthesis was between 30 C and 40 C. Photorespiration was low and the CO2compensation concentration was 7 ppm. After a 5-sec exposure to14CO2in the light, label was detected in oxaloacetate, malate, and aspartate. It was concluded that smooth pigweed has the gross photosynthetic characteristics and CO2fixation pathway typical of C4or Hatch-Slack plants.