Temperature Dependence Of Photosynthesis Research Articles

Seaweed assemblages under a climate change scenario: Functional responses to temperature of eight intertidal seaweeds match recent abundance shifts

Field evidence is essential to assess the consequences of climate change but a solid causal link often requires additional information obtained under controlled laboratory conditions. Additionally, the functional response to temperature may also help to discriminate species potentially more vulnerable to warming. Using a highly resolved temperature gradient, we examined the temperature dependence of photosynthesis and respiration in eight intertidal seaweeds that recently followed opposite abundance trends in NW Iberia. The temperature dependence of photosynthesis was consistently different between the macroalgae that increased and those that decreased their abundance in the last decade and a half, with photosynthesis twice more sensitive in the upward group. Unlike photosynthesis, the temperature dependence of respiration was unrelated to the abundance trend group, implying that the net metabolic scaling with temperature varied between the two groups of seaweeds. Overall, our results provide experimental support to the role of temperate as a likely driver of the changes in abundance recorded by field-monitoring studies. They also suggest that the temperature dependence of photosynthesis and respiration assessed in short-term experiments may serve as a biomarker of the potential vulnerability of some seaweed to the consequences of water warming.

Optimum Temperatures for Net Primary Productivity of Three Tropical Seagrass Species.

Rising sea water temperature will play a significant role in responses of the world's seagrass meadows to climate change. In this study, we investigated seasonal and latitudinal variation (spanning more than 1,500 km) in seagrass productivity, and the optimum temperatures at which maximum photosynthesis and net productivity (for the leaf and the whole plant) occurs, for three seagrass species (Cymodocea serrulata, Halodule uninervis, and Zostera muelleri). To obtain whole plant net production, photosynthesis, and respiration rates of leaves and the root/rhizome complex were measured using oxygen-sensitive optodes in closed incubation chambers at temperatures ranging from 15 to 43°C. The temperature-dependence of photosynthesis and respiration was fitted to empirical models to obtain maximum metabolic rates and thermal optima. The thermal optimum (Topt) for gross photosynthesis of Z. muelleri, which is more commonly distributed in sub-tropical to temperate regions, was 31°C. The Topt for photosynthesis of the tropical species, H. uninervis and C. serrulata, was considerably higher (35°C on average). This suggests that seagrass species are adapted to water temperature within their distributional range; however, when comparing among latitudes and seasons, thermal optima within a species showed limited acclimation to ambient water temperature (Topt varied by 1°C in C. serrulata and 2°C in H. uninervis, and the variation did not follow changes in ambient water temperature). The Topt for gross photosynthesis were higher than Topt calculated from plant net productivity, which includes above- and below-ground respiration for Z. muelleri (24°C) and H. uninervis (33°C), but remained unchanged at 35°C in C. serrulata. Both estimated plant net productivity and Topt are sensitive to the proportion of below-ground biomass, highlighting the need for consideration of below- to above-ground biomass ratios when applying thermal optima to other meadows. The thermal optimum for plant net productivity was lower than ambient summer water temperature in Z. muelleri, indicating likely contemporary heat stress. In contrast, thermal optima of H. uninervis and C. serrulata exceeded ambient water temperature. This study found limited capacity to acclimate: thus the thermal optima can forewarn of both the present and future vulnerability to ocean warming during periods of elevated water temperature.

ビワの光合成と光および温度との関係とその季節変化

ビワの光合成と光および温度との関係とその季節変化

Stream metabolism and the open diel oxygen method: Principles, practice, and perspectives

AbstractGlobal quantitative estimations of ecosystem functions are vital. Among those, ecosystem respiration and photosynthesis contribute to carbon cycling and energy flow to food webs. These can be estimated in streams with the open channel diel oxygen method (single or two stations) essentially relying on a mass balance of oxygen over a defined reach. The method is generally perceived as low cost and easy to apply with new drift free optic sensors. Yet, it remains challenging on several key issues reviewed here: measurements of gas transfer at the air‐water interface, appropriate mixing of tracers, uncertainty propagation in the calculations, spatial heterogeneity in oxygen concentrations, the derivation of net primary production (NPP) or autotrophic respiration, and the temperature dependence of photosynthesis and respiration. An extremely simple modeling tool is presented in an Excel workbook recommended for teaching the basic principles of the method. The only method able to deal with stream spatial heterogeneity is the method by Demars et al. Example data, Excel workbook, and R script are provided to run stream metabolism calculations. Direct gas exchange determination is essential in shallow turbulent streams, but modeling may be more accurate in large (deep) rivers. Lateral inflows should be avoided or well characterized. New methods have recently been developed to estimate NPP using multiple diel oxygen curves. The metabolic estimates should not be systematically temperature corrected to compare streams. Other recent advances have improved significantly the open channel diel oxygen method, notably the estimation of respiration during daylight hours.

Temperature response of photosynthesis in different drug and fiber varieties of Cannabis sativa L.

The temperature response on gas and water vapour exchange characteristics of three medicinal drug type (HP Mexican, MX and W1) and four industrial fiber type (Felinq 34, Kompolty, Zolo 11 and Zolo 15) varieties of Cannabis sativa, originally from different agro-climatic zones worldwide, were studied. Among the drug type varieties, optimum temperature for photosynthesis (Topt) was observed in the range of 30-35°C in high potency Mexican HPM whereas, it was in the range of 25-30°C in W1. A comparatively lower value (25°C) for Topt was observed in MX. Among fiber type varieties, Topt was around 30°C in Zolo 11 and Zolo 15 whereas, it was near 25°C in Felinq 34 and Kompolty. Varieties having higher maximum photosynthesis (PN max) had higher chlorophyll content as compared to those having lower PN max. Differences in water use efficiency (WUE) were also observed within and among the drug and fiber type plants. However, differences became less pronounced at higher temperatures. Both stomatal and mesophyll components seem to be responsible for the temperature dependence of photosynthesis (PN) in this species, however, their magnitude varied with the variety. In general, a two fold increase in dark respiration with increase in temperature (from 20°C to 40°C) was observed in all the varieties. However, a greater increase was associated with the variety having higher rate of photosynthesis, indicating a strong association between photosynthetic and respiratory rates. The results provide a valuable indication regarding variations in temperature dependence of PN in different varieties of Cannabis sativa L.

Intraspecific variation in temperature dependence of gas exchange characteristics among Plantago asiatica ecotypes from different temperature regimes

There are large inter- and intraspecific differences in the temperature dependence of photosynthesis, but the physiological cause of the variation is poorly understood. Here, the temperature dependence of photosynthesis was examined in three ecotypes of Plantago asiatica transplanted from different latitudes, where the mean annual temperature varies between 7.5 and 16.8 degrees C. Plants were raised at 15 or 30 degrees C, and the CO(2) response of photosynthetic rates was determined at various temperatures. When plants were grown at 30 degrees C, no difference was found in the temperature dependence of photosynthesis among ecotypes. When plants were grown at 15 degrees C, ecotypes from a higher latitude maintained a relatively higher photosynthetic rate at low measurement temperatures. This difference was caused by a difference in the balance between the capacities of two processes, ribulose-1,5-bisphosphate regeneration (J(max)) and carboxylation (V(cmax)), which altered the limiting step of photosynthesis at low temperatures. The organization of photosynthetic proteins also varied among ecotypes. The ecotype from the highest latitude increased the J(max) : V(cmax) ratio with decreasing growth temperature, while that from the lowest latitude did not. It is concluded that nitrogen partitioning in the photosynthetic apparatus and its response to growth temperature were different among ecotypes, which caused an intraspecific variation in temperature dependence of photosynthesis.

Temperature response of photosynthesis and its interaction with light intensity in sweet orange leaf discs under non-photorespiratory condition

This study aimed to evaluate the response of photosynthesis (A), given by photosynthetic O2 evolution, to increasing temperature from 25 to 50ºC in sweet orange (Citrus sinensis (L.) Osbeck) leaf discs under non-photorespiring conditions. In order to evaluate the response of gross photosynthesis to temperature and the balance between photosynthetic and respiratory activities, respiration (Rd) rates were also measured, i.e. the O2 uptake in each temperature. In addition, light response curves of photosynthesis were performed by varying the photosynthetic photon flux density (PPFD) from 0 to 1160 µmol m-2 s-1 at 25 and 40ºC. The highest A values were observed at 35 and 40ºC, whereas the highest Rd values were noticed at 50ºC. A higher relationship A/Rd was found at 30 and 35ºC, suggesting an optimum temperature of 35ºC when considering the balance between photosynthesis and respiration under non-photorespiring condition. Overall, heat effects on plant metabolism were more evident when evaluating the relationship A/Rd. In light response curves, higher A values were also found at 40ºC under PPFD higher than 300 µmol m-2 s-1. Light saturation point of photosynthesis was increased at 40ºC, without significant change of quantum efficiency under low PPFD. Respiration was also enhanced at 40ºC, and as a consequence, the light compensation point increased. The better photosynthetic performance at 35-40ºC was supported by higher photochemical efficiency in both light and temperature response curves. The temperature-dependence of photosynthesis was affected by growth temperature, i.e. a high air temperature during plant growth is a probable factor leading to a higher photosynthetic tolerance to heat stress.

Seasonal changes in temperature dependence of photosynthetic rate in rice under a free-air CO(2) enrichment.

Influences of rising global CO(2) concentration and temperature on plant growth and ecosystem function have become major concerns, but how photosynthesis changes with CO(2) and temperature in the field is poorly understood. Therefore, studies were made of the effect of elevated CO(2) on temperature dependence of photosynthetic rates in rice (Oryza sativa) grown in a paddy field, in relation to seasons in two years. Photosynthetic rates were determined monthly for rice grown under free-air CO(2) enrichment (FACE) compared to the normal atmosphere (570 vs 370 micromol mol(-1)). Temperature dependence of the maximum rate of RuBP (ribulose-1,5-bisphosphate) carboxylation (V(cmax)) and the maximum rate of electron transport (J(max)) were analysed with the Arrhenius equation. The photosynthesis-temperature response was reconstructed to determine the optimal temperature (T(opt)) that maximizes the photosynthetic rate. There was both an increase in the absolute value of the light-saturated photosynthetic rate at growth CO(2) (P(growth)) and an increase in T(opt) for P(growth) caused by elevated CO(2) in FACE conditions. Seasonal decrease in P(growth) was associated with a decrease in nitrogen content per unit leaf area (N(area)) and thus in the maximum rate of electron transport (J(max)) and the maximum rate of RuBP carboxylation (V(cmax)). At ambient CO(2), T(opt) increased with increasing growth temperature due mainly to increasing activation energy of V(cmax). At elevated CO(2), T(opt) did not show a clear seasonal trend. Temperature dependence of photosynthesis was changed by seasonal climate and plant nitrogen status, which differed between ambient and elevated CO(2).

Temperature Dependence of Photosynthesis in Arabidopsis Plants with Modifications in Rubisco Activase and Membrane Fluidity

Net photosynthesis (Pn) is reversibly inhibited at moderately high temperature. To investigate this further, we examined the effects of heat stress on Arabidopsis plants in which Rubisco activase or thylakoid membrane fluidity has been modified. During heating leaves from 25 to 40 degrees C at 250 ppm CO2 and 1% O2, the wild-type (WT), plants expressing the 43 kDa isoform only (rwt43), and plants accumulating activase 40% of WT (R100) exhibited similar inhibitions in the Pn and Rubisco activation state. Despite better membrane integrity than WT, plants having less polyunsaturation of thylakoid lipids (fad7/8 double mutant) failed to maintain greater Pn than the WT. Plants expressing the 46 kDa isoform only (rwt46) exhibited the most inhibition, but plants expressing a 46 kDa isoform incapable of redox regulation (C411A) were similar to the WT. The null mutant (rca) exhibited a continuous decline in Pn. As measured by fluorescence, electron transport activity decreased concomitantly with Pn but PSII was not damaged. Following a quick recovery to 25 from 40 degrees C, whereas most lines recovered 90% Pn, the rwt46 and rca lines recovered only to 59 and <10%, respectively. As measured by NADP-malate dehydrogenase activation, after an initial increase at 30 degrees C, stromal oxidation in the WT and rwt46 plants did not increase further as Pn decreased. These results provide additional insight into the role of Rubisco activation and activase in the reversible heat inhibition of Pn.

A nonparametric method for separating photosynthesis and respiration components in CO2 flux measurements

Future climate change is expected to affect ecosystem‐atmosphere CO2 exchange, particularly through the influence of temperature. To date, however, few studies have shown that differences in the response of net ecosystem CO2 exchange (NEE) to temperature among ecosystems can be explained by differences in the photosynthetic and respiratory processes that compose NEE. Using a new nonparametric statistical model, we analyzed data from four forest ecosystems. We observed that differences among forests in their ability to assimilate CO2 as a function of temperature were attributable to consistent differences in the temperature dependence of photosynthesis and respiration. This observation provides empirical validation of efforts to develop models of NEE from the first‐principle relationships between photosynthetic and respiratory processes and climate. Our results also showed that models of seasonal dynamics in NEE that lack specific consideration of the temperature dependence of respiration and photosynthesis are likely to carry significant uncertainties.

Seasonal and temperature dependence of photosynthesis and respiration for two co-occurring broad-leaved tree species with contrasting leaf phenology.

We measured the seasonal and temperature responses of leaf photosynthesis and respiration of two co-occurring native New Zealand tree species with contrasting leaf phenology: winter-deciduous fuchsia (Fuchsia excorticata J. R. Forst & G. Forst) and annual evergreen wineberry (Aristotelia serrata J. R. Forst & G. Forst). There was no difference in the amount of nitrogen per unit leaf area (Narea, range 40-160 mmol m-2, P = 0.18) or specific leaf area (S, range 8-27 m2 kg-1, P = 0.87) in summer leaves of wineberry or fuchsia. The amount of nitrogen per unit leaf area and S varied significantly with height of leaves in the canopy for both species (r2 range 0.61-0.87). Parameters describing the maximum rates of rubisco carboxylation (Vcmax) and electron transport (Jmax) were related significantly to Narea, and were 60% higher on average in spring and summer leaves than in autumn and winter leaves for both species. The seasonal effect remained significant (P < 0.001) when Narea was included in a regression model, indicating that seasonal changes were not only due to changes in Narea. Values for Vcmax and Jmax were 30% lower in wineberry leaves than in fuchsia leaves on average, although the difference ranged from 15% in summer leaves to 39% in autumn leaves. Activation energies describing the temperature dependence of Vcmax and Jmax in wineberry were 111 and 114% of corresponding values for fuchsia (Ea (Vcmax) = 39.1 kJ mol-1, Ea (Jmax) = 32.9 kJ mol-1). Respiration at night was the same (P = 0.34) at 10 degrees C for both species (R10 = 0.7 micromol m-2 s-1), although activation energies (E0) were higher in wineberry than in fuchsia (47.4 and 32.9 kJ mol-1 K-1, respectively). These results show that rates of photosynthesis are higher in winter-deciduous fuchsia than in annual evergreen wineberry.

Affixing the O to Rubisco: discovering the source of photorespiratory glycolate and its regulation.

The source of glycolate in photorespiration and its control, a particularly active and controversial research topic in the 1970s, was resolved in large part by several discoveries and observations described here. George Bowes discovered that the key carboxylation enzyme Rubisco (ribulosebisphosphate carboxylase/oxygenase) is competitively inhibited by O(2) and that O(2) substitutes for CO(2) in the initial 'dark' reaction of photosynthesis to yield glycolate-P, the substrate for photorespiration. William Laing derived an equation from basic enzyme kinetics that describes the CO(2), O(2), and temperature dependence of photosynthesis, photorespiration, and the CO(2) compensation point in C(3) plants. Jerome Servaites established that photosynthesis cannot be increased by inhibiting the photorespiratory pathway prior to the release of photorespiratory CO(2), andDouglas Jordan discovered substantial natural variation in the Rubisco oxygenase/carboxylase ratio. A mutant Arabidopsis plant with defective glycolate-P phosphatase, isolated by Chris Somerville, definitively established the role of O(2) and Rubisco in providing photorespiratory glycolate. Selection techniques to isolate photorespiration-deficient plants were devised by Jack Widholm and by Somerville, but no plants with reduced photorespiration were found. Somerville's approach, directed mutagenesis of Arabidopsis plants, was subsequently successful in the isolation of numerous other classes of mutants and revolutionized the science of plant biology.

Four-Component Structure of Plant Leaf Metabolism: Specific Features of Temperature Dependence of the Components

Metabolism in living systems is an integrated network of interacting processes directed toward an increase or conservation of the nonequilibrium. This is achieved by generating gradients of concentration, biosynthesis, or resynthesis of various compounds. In addition to integrity, metabolic processes are characterized by specialization, thereby giving rise to the multicomponent structure of metabolism. It was shown in the preceding work [1] that the structure of metabolic reactions in plant leaves consists of four components. It was also shown that one to four components of integral metabolism ( R s ) can be observed in plant leaves in accordance with their physiological state. The scheme of the four-component structure of plant metabolism is shown in Fig. 1. In this scheme, R b is the metabolic component maintaining the basic (functionally inactive) state of a native leaf. Because the leaf energy in this state is spent only to maintain its intactness, the structure of the leaf metabolism contains only the R b component. The ability to fulfil other functions is achieved during leaf activation. The activation is a structural rearrangement causing an increase in the membrane permeability, a decrease in the thermal stability of leaf structures, and other nonequilibrium processes. Therefore, an additional increase in the efficiency of sustaining metabolism and the corresponding energy expenditure are required to maintain activated leaves in the native state [1]. This additional increase in the efficiency of sustaining metabolism corresponds to the component R a . Therefore, R a is the metabolic component maintaining the leaf in the functionally active state. Therefore, supportive metabolism ( R m ) may contain one ( R b ) or two ( R b + R a ) components. R f and R g are the functional and growth components of leaf metabolism, respectively. R g is a component of R s only during plant growth. This component of leaf metabolism corresponds to the de novo biosynthesis. The other growthsupporting processes (primarily, transport processes) are regarded as auxiliary. Therefore, they were included in R f . Thus, R g and R f are coupled with one another, and R f is also coupled with R a . Indeed, as noted above, R a is the metabolic component maintaining the leaf in the functionally active state (Fig. 1). The temperature dependence of leaf metabolism has not yet been analyzed in terms of the four-component structure of plant leaf metabolism. The literature describes mainly the components R s , R f , and R g . The component R a has been isolated only recently, and there are few literature data on this component thus far. The temperature coefficient ( Q ) of the basic metabolism ( R b ) was calculated in the preceding work [2] only within the temperature range from 10 to 30 ie . Therefore, experimental study of temperature dependence of components R b and R a and comparative analysis of temperature dependences of the four components of leaf metabolism were the goals of this work. The literature data on the temperature dependence of photosynthesis [3], transport, and growth [4] were used to analyze the temperature dependence of the R f and R g components of leaf metabolism. Leaves of two plant species with contrasting thermal resistance were used: the thermophilous maize ( Zea mays L.) and the cold-resistant cabbage ( Brassica oleracia L.). Plants were grown in the field. Apparently healthy, separated, and mature leaves (i.e., leaves that did not grow any longer) were studied. After thick leaf ribs had been removed, leaf plates were cut into segments. The segment areas in cabbage and maize leaves were about 4 and 12 cm 2 , respectively. The leaf segments were mixed to obtain the total population, from which elementary samples (repetitions) were randomly taken. The total populations for the cabbage and maize leaves were 80 and 30 segments, respectively. The samples were incubated for 20 h at 25ie in a dark thermostat at an air humidity of 100%. Because metabolic reactions of mature leaves in situ do not contain the component R g , whereas the components R a and R f were eliminated during incubation [2, 5], metabolism of incubated leaves was characterized by a single component, R b . When incubation was over, 50% of the samples were activated by 20-min exposure to an ambient illuminance of 30 klx in a humid atmosphere containing no CO 2 . The lack of CO 2 inhibited biosynthesis of GENERAL BIOLOGY

Clonal variation in temperature response of photosynthesis in tea

The effect of temperature on the photosynthetic characteristics of six tea clones (viz., 6017, B/5/63, B/6/61, B/6/62, CCS-26 and T-78), originally from different agro-climatic zones in India, was studied to determine the clonal variation in photosynthesis, if any. The results clearly indicated significant clonal differences in relation to temperature. Of the six clones, B/5/63 and B/6/61 were found to be relatively thermotolerant. Both stomatal and mesophyll components seemed to be responsible for the differences in temperature dependence of photosynthesis, however, their magnitude varied with the tea clones. Differences in water use efficiency were also observed between clones. However, differences became less pronounced at the higher temperature. Clone B/5/63 showed higher water use efficiency and lower values for stomatal conductance and transpiration. Thus this clone may be suitable for relatively dry and exposed sites. A 2–10-fold increase in dark respiration with increase in temperature was also observed. However, higher increase was associated with clones having higher photosynthetic rates, indicating an association between photosynthetic and respiratory rates. The results provide a valuable indication regarding clonal variation in temperature responses of photosynthesis and may be used to offer useful suggestions to tea growers in the initial selection of tea clones.

SEASONAL VARIATION OF PHOTOSYNTHETIC PERFORMANCE AND LIGHT ATTENUATION IN ULVA CANOPIES FROM PALMONES RIVER ESTUARY1

The primary production of Ulva populations relies on their photosynthetic performance, which is dependent on the light availability under natural conditions. This study concerns the light attenuation characteristics in Ulva canopies and the seasonal photosynthetic performance of two different species (Ulva rotundata Blid., Ulva curvata (Kutz.) De Toni) blooming in the Palmones river estuary. Light within canopies differed from that reaching the surface. Light availability was reduced through the water column (at high tide) and Ulva canopies. In addition, light was spectrally filtered. As a result, the photosynthetically usable radiation (PUR) was further attenuated through Ulva canopies, increasing the photosynthetically active radiation/PUR ratio. The muddy sediment deposited on and between the Ulva thalli also drastically restricted the light availability. Thick Ulva mats are frequently found covering the intertidal mudflats, and therefore, thalli within these mats may be subjected to steep light gradients. As a consequence, individual Ulva growth rates cannot be extrapolated to estimate the primary production of Ulva canopies. Interspecific differences were observed for light-saturated photosynthetic rates (Pmax) and light compensation points (LCP), with Ulva curvata generally displaying higher values than did U. rotundata. For both species, maxima were recorded in winter for Pmax, quantum yield, chlorophyll content, and absorptance, whereas minima were found in summer. Dark respiration (Rd) was not seasonally affected, and a maximum LCP was found in summer. To extrapolate these data to field situations, the temperature dependence of photosynthesis should be considered. The Q10 values were 2.44 for Rd and 1.79 for Pmax, whereas the photosynthesis rate at subsaturating light levels was unaffected. The Q10 values showed an enhanced respiratory rate in summer and a minimum in winter, whereas the seasonal differences on Pmax were damped.

Low-temperature limitations of photosynthesis in three tropical Vigna species: A chlorophyll fluorescence study

The temperature-dependence of photosynthesis and chlorophyll (Chl) fluorescence quenching components was studied between 0 and 45°C in three tropical, chilling-sensitive Vigna species and in chilling-tolerant pea. Photosynthesis of the Vigna spp. was approx. 20% more reduced by temperatures between 7 and 30°C than in pea. The latter revealed significant changes in Chl fluorescence parameters at much lower temperature than the Vigna spp. Below 15°C, the reduction state of QA increased quickly in pea, while in Vigna already below 30°C, an increase of reduced QA was obtained. The analysis of different components of non-photochemical Chl fluorescence quenching (qN) revealed, that in pea photoinhibitory quenching (qI) occurred below 13°C. Below ca. 7°C, a sudden breakdown of both qP and the fast relaxing component of qN was observed in pea.In Vigna, susceptibility of LHC II phosphorylation or limitation of electron flow by damage to PS I, the PS II reaction centre or the water-splitting system were not responsible for the chilling-sensitivity of photosynthesis between 5 and 30°C. Instead, photosynthesis was gradually limited by an inefficient use of reduction equivalents. This, in turn may increase susceptibilty to photoinhibition, which occurred below 20°C in Vigna. The combined study of qP and of the different components of qN allowed the demonstration of the subsequent occurrence of different limiting processes with decreasing temperature in the chilling-sensitive Vigna species.

A model of the seasonal pattern of carbon acquisition in two woodland herbs, Mercurialis perennis L. and Geum urbanum L.

Seasonal changes in the light and temperature dependence of photosynthesis were investigated in field grown plants of Mercurialis perennis and Geum urbanum. In both species changes in photosynthetic capacity were closely related to the development of the overstorey canopy. In G. urbanum there was a marked shift in the temperature dependence of photosynthesis through the season whereas no such pattern was found in M. perennis. Model predictions of field rates of photosynthesis were made using the measurements of light and temperature dependence in the laboratory and validated against field observations. Long term continuous records of light and temperature in the field were used in conjunction with the model to make predictions of carbon acquisition in shoots of the two species through the season. These calculations indicated that G. urbanum was able to take advantage of high light levels just prior to canopy closure through a combination of high photosynthetic capacity, the ability to maintain photosynthesis at relatively low temperatures and the presence of overwintering leaves. In M. perennis leaf development was early enough to utilise the high spring light period. After canopy closure M. perennis maintained a higher average rate of CO2 flux due to a combination of high apparent quantum efficiency and low rates of respiration.

The light and temperature dependence of photosynthesis and respiration in Potamogeton pectinatus L.

The photosynthesis and dark respiration of healthy shoots of Potamogeton pectinatus L. collected from a stream were examined using infrared gas analysis (IRGA) across a range of temperature (10–35°C) and light (0–2000 μmol m −2 s −1) conditions. The maximum net photosynthetic rate was estimated to be 1.51 mg C g −1 ash free dry weight (AFDW) h −1, with an optimal temperature of 30°C. The maximum observed net photosynthesis was 1.46 mg C g −1 AFDW h −1 at 25°C. Maximum estimated and observed net photosynthesis at 10°C was 63% of net photosynthesis at optimum temperature, showing an unusually broad photosynthetic response across temperature. The photosynthetic characteristics of P. pectinatus indicate a species generalized to a broad range of environmental conditions.

The Temperature-Dependence of Photosynthesis of Some Australian Temperate Rainforest Trees and its Biogeographical Significance

The effect of temperature on photosynthesis was examined in several species of Australian temperate rainforest trees to test three hypotheses regarding the distribution and evolution of the species. The results suggest that changing temperatures in south-eastern and eastern Australia during the Tertiary may have affected species distribution and influenced evolution. For Nothofagus, N. moorei is closest to the ancestral species and has a higher optimum temperature for photosynthesis than the derived species N. cunninghamii. It is hypothesized that this difference is the result of the evolution occurring with declining temperatures in south- eastern Australia during the Tertiary, when the ancestral species evolved into N. cunninghamii. The three Eucryphia species tested do not exhibit such a marked trend, but vary considerably in their relative rates of photosynthesis above and below their optimum temperature for photo- synthesis. The species restricted to northern temperate rainforests which have no close relatives in the southern temperate forests (Ceratopetalum apetalum and Doryphora sassafras) have a substantially higher optimum temperature for photosynthesis than the southern species. These results are consistent with the computer-predicted distributions of the species based on the climate profiles of their known distributions.

A comparative study of Geum rivale L. and G. urbanum L. to determine those factors controlling their altitudinal distribution II. Photosynthesis and respiration: II. Photosynthesis and respiration.

Geum urbanum and G. rivale are a sympatric interfertile pair of species with contrasting distributions. Geum rivale occurs at higher altitudes and more northerly latitudes than G. urbanum. As part of a study to determine the factors responsible for this difference in distribution, a comparison of the carbon economy of the two species was made. The light and temperature dependence of photosynthesis was assessed for plants of both species grown in contrasting light and temperature regimes. Acclimation to the prevailing environmental conditions was more pronounced in G. urbanum. Amongst populations of the two species originating from different altitudes the temperature optimum for photosynthesis was similar. When grown at 25/16 °C, higher rates of respiration were measured he roots of G. rivale at all temperatures, when pretreated at 8/5 °C there was a smaller increase in the rate of respiration in the roots of G. rivale. It is proposed that these differences in root respiration account for observed higher root growth rates at low temperatures in G. rivale. It is concluded that whilst carbon assimilation is not a critical factor determining the upper altitudinal limit of G. urbanum, the pattern of carbon utilization may be important. The greater phenotypic plasticity observed in G. urbanum, may be of value for survival in the herb layer of deciduous woodland where it is most commonly found.