SEASONAL PATTERNS OF CO2 AND WATER VAPOR EXCHANGE OF JUNCUS ROEMERIANUS SCHEELE IN A GEORGIA SALT MARSH

CO₂ and water vapor exchange studies of intact plants of black needle rush (Juncus roemerianus Scheele) were conducted in an undisturbed marsh community on Sapelo Island, Georgia. The seasonal patterns of the light and temperature responses of net photosynthesis, transpiration, leaf diffusive conductance, water-use efficiency and respiration were determined five times over the year. Internal resistances to CO₂ uptake were also evaluated. Net photosynthesis was highest in early spring, but declined only slightly through the year. A distinct and moderate temperature optimum of net photosynthesis was observed with decreasing rates above 30 C. Leaf conductances to water vapor were similar at all seasons and were high at cooler temperatures and decreased with increasing temperature. Transpiration was relatively high and constant during all seasons. The water-use efficiency of photosynthesis was high below 25 C, but decreased sharply above that temperature. Dark respiration was relatively low. Seasonal changes reflected changes in leaf density. Decreasing stomatal conductances and increasing respiration rates reduced net photosynthesis at higher temperatures. The stomatal resistance increased and internal resistances to CO₂ uptake decreased over the year, but the total resistance remained constant. The internal resistance to CO₂ uptake was consistently higher than the stomatal resistance. These seasonal response patterns show that J. roemerianus is well adapted to the seasonal changes in ambient temperature and irradiance and other microenvironmental factors in the high marsh. These physiological characteristics permit this C₃ species to maintain a high productivity in a seasonally hot and stressful environment.     

Variability of photosynthesis-irradiance curves and ecosystem respiration in a small river

• 1 We investigated photosynthesis‐irradiance relationships (P‐I curves; P = oxygen production rate due to photosynthesis, I = light irradiance rate at the water surface) and ecosystem respiration in a 9 km long reach of a river that is characterised by light conditions favouring primary production, high ambient nutrient concentrations, a high re‐aeration rate, and frequent spates. We addressed the question of how disturbances (spates) and season influence photosynthesis and ecosystem respiration.
• 2 We used an oxygen mass‐balance model of the river to identify ecosystem respiration rates and the two parameters of a hyperbolic P‐I function (P_max = maximum oxygen production rate due to photosynthesis, α = the initial slope of the P‐I function). The model was fitted to dissolved oxygen concentrations quasi‐continuously recorded at the end of the reach. We estimated parameters for 137 three‐day periods (during the years 1992–97) and subsequently explored the potential influence of season and disturbances (spates) on P_max, α and ecosystem respiration using stepwise regression analysis.
• 3 Photosynthesis‐irradiance relationships and ecosystem respiration were subject to distinct seasonal variation. Only a minor portion of the variability of P‐I curves could be attributed to disturbance (spates), while ecosystem respiration did not correlate with disturbance related parameters. Regular seasonal variation in photosynthesis and ecosystem respiration apparently prevailed due to the absence of severe disturbances (a lack of significant bedload transport during high flow).

     

One‐year experiment on the physiological response of the Mediterranean crustose coralline alga,Lithophyllum cabiochae,to elevated pCO2 and temperature

The response of respiration, photosynthesis, and calcification to elevated pCO₂ and temperature was investigated in isolation and in combination in the Mediterranean crustose coralline alga Lithophyllum cabiochae. Algae were maintained in aquaria during 1 year at near‐ambient conditions of irradiance, at ambient or elevated temperature (+3°C), and at ambient (ca. 400 μatm) or elevated pCO₂ (ca. 700 μatm). Respiration, photosynthesis, and net calcification showed a strong seasonal pattern following the seasonal variations of temperature and irradiance, with higher rates in summer than in winter. Respiration was unaffected by pCO₂ but showed a general trend of increase at elevated temperature at all seasons, except in summer under elevated pCO₂. Conversely, photosynthesis was strongly affected by pCO₂ with a decline under elevated pCO₂ in summer, autumn, and winter. In particular, photosynthetic efficiency was reduced under elevated pCO₂. Net calcification showed different responses depending on the season. In summer, net calcification increased with rising temperature under ambient pCO₂ but decreased with rising temperature under elevated pCO₂. Surprisingly, the highest rates in summer were found under elevated pCO₂ and ambient temperature. In autumn, winter, and spring, net calcification exhibited a positive or no response at elevated temperature but was unaffected by pCO₂. The rate of calcification of L. cabiochae was thus maintained or even enhanced under increased pCO₂. However, there is likely a trade‐off with other physiological processes. For example, photosynthesis declines in response to increased pCO₂ under ambient irradiance. The present study reports only on the physiological response of healthy specimens to ocean warming and acidification, however, these environmental changes may affect the vulnerability of coralline algae to other stresses such as pathogens and necroses that can cause major dissolution, which would have critical consequence for the sustainability of coralligenous habitats and the budgets of carbon and calcium carbonate in coastal Mediterranean ecosystems.     

Seasonal patterns of CO2 and water vapor exchange of three salt marsh succulents

Summary Diurnal carbon dioxide exchange patterns of three salt marsh succulents, Borrichia frutescens, Batis maritima and Salicornia virginica, were determined on a seasonal basis in the marsh at Sapelo Island, Georgia. Year-round photosynthetic activity was observed in these species though winter rates of CO₂ exchange were reduced. Net primary productivity, estimated using gas exchange techniques, agreed with previously reported harvest data. The role of light and temperature in the control of seasonal photosynthetic changes was investigated. A similar variation in light utilization with season was found in all three species, while seasonal temperature acclimation was species dependent. Less than 20% of fixed CO₂ was lost through dark respiration in any of the species.Water use in the salt marsh succulents was found to be relatively inefficient. High rates of transpiration were observed both summer and winter in the succulents.The succulents were judged to be C₃ plants on the basis of several criteria.Contribution No. 391 from the University of Georgia Marine Institute     

Photosynthetic and respiratory responses of Gracilaria vermiculophylla (Ohmi) Papenfuss collected from Kumamoto, Shizuoka and Iwate, Japan

The photosynthetic and respiratory responses to irradiance, salinity and temperature of the red alga, Gracilaria vermiculophylla, collected from Kumamoto, Shizuoka and Iwate in Japan were studied using an electronic Dissolved Oxygen sensor. The parameters derived from the photosynthesis versus irradiance relationship indicated the potential to acclimate to broad irradiance variations in all of the populations of G. vermiculophylla collected from these three sites. In addition, the light-saturated photosynthesis rate (P _max) and the dark respiration rate of all populations increased with increasing temperature up to 20–30°C, while the P _max decreased at 35°C. All populations also showed a broad variation of photosynthetic responses to salinity changes in the range from 10 to 30 psu. On the other hand, the population from Iwate showed high photosynthetic efficiency, especially in the temperature range of 5–10°C, and showed low values of saturation irradiance compared to the populations from Shizuoka and Kumamoto. These results suggest that there is greater potential to acclimate to low irradiance and low temperature in the population from Iwate compared to those from the Shizuoka and Kumamoto populations. However, the P _max of the populations from Iwate and Shizuoka was reached at 20°C and 25°C, respectively, while the Kumamoto population reached P _max at 30°C. This implies that the latter population has greater potential to tolerate higher temperatures than the former. Such characteristics in photosynthesis and respiration of G. vermiculophylla collected from the three locations probably indicate an acclimation to prevailing environmental conditions in their respective habitats.     

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

Physiological 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.     

Seasonal patterns of CO2 and water vapor exchange of the tall and short height forms of Spartina alterniflora Loisel in a Georgia salt marsh

Summary Seasonal patterns of the responses of net photosynthesis, transpiration, leaf diffusive conductance, water-use efficiency and respiration to temperature, light and CO₂ concentration were determined on intact plants of the short and tall height forms of Spartina alterniflora. The studies were conducted on in situ plants in an undisturbed marsh community on Sapelo Island, Ga. Net photosynthesis of the tall form at full sunlight was significantly higher than the short form except during the winter months. Tall S. alterniflora did not light saturate during any season, whereas the short form tended to saturate during all seasons except the summer. The temperature optima of photosynthesis of both forms were similar and showed acclimation to prevailing seasonal temperatures. Leaf conductances to water vapor decreased with increasing temperature and were significantly different between the height forms only at higher temperatures. Dark respiration was relatively low at seasonal temperatures, but increased with temperature. Dark respiration and the respiratory Q₁₀ of the short form tended to be slightly higher than those of the tall form during all seasons. Transpiration rates and water-use efficiency of the tall form were generally higher than the short form.The seasonal response patterns showed intrinsic differences in the capacities of the height forms to metabolize CO₂ and respond to prevailing environmental parameters. Analyses of the components of the CO₂ diffusion pathway suggested that metabolic or internal components were more important than stomatal factors in determining the photosynthetic patterns of the short height form. It is suggested that the observed differences in the physiological responses of the height forms of the C₄ species are due to micro-habitat differences between the low and high marsh. Higher salinity, lower nitrogen availability and other soil factors may limit the CO₂ and water vapor exchange capacity of the short form compared to the tall.Contribution No. 401 from the University of Georgia Marine Institute     

Photosynthesis and calcification in the articulated coralline alga Ellisolandia elongata (Corallinales,Rhodophyta) from intertidal rock pools

Calcifying coralline algae are functionally important in many ecosystems but their existence is now threatened by global climate change. The aim of this study is to improve our understanding of coralline algal metabolic functions and their interactions by assessing the respiration, photosynthesis and calcification rates in an articulated (geniculate) coralline alga, Ellisolandia elongata. Algal samples selected for this case study were collected from an intertidal rock-pool on the coast of Brittany (France). Physiological rates were assessed in summer and winter by measuring the concentration of oxygen, dissolved inorganic carbon and total alkalinity fluxes at five irradiance levels and in the dark using incubation chambers.

Respiration, photosynthetic and calcification rates were strongly affected by seasonal changes. Respiration increased with temperature, being ten-fold higher in summer than in winter. Photosynthetic parameters of the photosynthesis-irradiance (P-E) curve, P_g^max, P_n^max and E_k, were two- to three-fold higher in summer relative to winter. Photoinhibition was observed under high irradiance indicating an acclimation of E. elongata to low irradiance levels. Parameters of the calcification-irradiance (G-E) curve, G^max and E_k, were approximately two-fold higher in summer compared with winter. In summer, calcification rates were more strongly inhibited under high irradiance than photosynthetic rates, suggesting a dynamic relationship between these metabolic processes. By inhabiting intertidal rock pools, E. elongata exhibits tolerance to a dynamic physico-chemical environment. Information on respiration, photosynthesis and calcification rates in a calcifying coralline alga inhabiting such dynamic environments in terms of pH and temperature is important in order to better understand how ocean acidification and warming will affect coralline algae in the future.     

Phenology,irradiance and temperature characteristics of a freshwater red alga,Nemalionopsis tortuosa (Thoreales), from Kagoshima,southern Japan

Phenology, irradiance and temperature characteristics of a freshwater benthic red alga, Nemalionopsis tortuosa Yoneda et Yagi (Thoreales), were examined from Kagoshima Prefecture, southern Japan for the conservation of this endemic and endangered species. Field surveys confirmed that algae occurred in shaded habitats from winter to early summer, and disappeared during August through November. A net photosynthesis–irradiance (P–E) model revealed that net photosynthetic rate quickly increased and saturated at low irradiances, where the saturating irradiance (E_k) and compensation irradiance (E_c) were 10 (8–12, 95% credible interval (CRI)) and 8 (6–10, 95% CRI) μmol photon m^?2 s^?1, respectively. Gross photosynthesis and dark respiration was determined over a range of temperatures (8–36°C) by dissolved oxygen measurements, and revealed that the maximum gross photosynthetic rate was highest at 29.5 (27.4–32.0, 95%CRI) °C. Dark respiration also increased linearly when temperature increased from 8°C to 36°C, indicating that the increase in dark respiration at higher temperature most likely caused decreases in net photosynthesis. The maximum quantum yield (Fv/Fm) that was determined using a pulse amplitude modulated‐chlorophyll fluorometer (Imaging‐PAM) was estimated to be 0.51 (0.50–0.52, 95%CRI) and occurred at an optimal temperature of 21.7 (20.1–23.4, 95%CRI) °C. This species can be considered well‐adapted to the relatively low natural irradiance and temperature conditions of the shaded habitat examined in this study. Our findings can be applied to aid in the creation of a nature‐reserve to protect this species.     

Effect of fruiting on carbon budgets of apple tree canopies

Summary Carbon budgets were calculated from net photosynthesis and dark respiration measurements for canopies of field-grown, 3-year-old apple trees (Malus domestica Borkh.) with maximum leaf areas of 5.4 m² in a temperature-controlled Perspex tree chamber, measured in situ over 2 years (July 1988 to October 1990) by computerized infrared gas analysis using a dedicated interface and software. Net photosynthesis (Pn) and carbon assimilation per leaf area peaked at respectively 8.3 and 7.7 mol CO₂ m^–2 s^–1 in April. Net photosynthesis (Pn) and dark respiration (Rd) per tree peaked at 3.6 g CO₂ tree^–1 h^–1 (Pn) and 1.2 g CO₂ tree^–1 h^–1 (Rd), equivalent to 4.2 mol CO₂ (Pn) and 1.4 mol CO₂ (Rd) m^–2 s^–1 with maximum carbon gain per tree in August and maximum dark respiration per tree in October 1988 and 1989. In May 1990, a tree was deblossomed. Pn (per tree) of the fruiting apple tree canopy exceeded that of the non-fruiting tree by 2–2.5 fold from June to August 1990, attributed to reduced photorespiration (RI), and resulting in a 2-fold carbon gain of the fruiting over the non-fruiting tree. Dark respiration of the fruiting tree canopy progressively exceeded, with increasing sink strength of the fruit, by 51% (June–August), 1.4-fold (September) and 2-fold (October) that of the non-fruiting tree due to leaf (i. e. not fruit) respiration to provide energy (a) to produce and maintain the fruit on the tree and (b) thereafter to facilitate the later carbohydrate translocation into the woody perennial parts of the tree. The fruiting tree reached its optium carbon budget 2–4 weeks earlier (August) then the non-fruiting tree (September 1990). In the winter, the trunk respired 2–100 g CO₂ month^–1 tree^–1. These data represent the first long-term examination of the effect of fruiting without fruit removal which shows increased dark respiration and with the increase progressing as the fruit developed.     

LIGHT AND TEMPERATURE AS FACTORS REGULATING SEASONAL GROWTH AND DISTRIBUTION OF ULOTHRIX ZONATA (ULVOPHYCEAE)1

Ulothrix zonata (Weber and Mohr) Kütz. is an unbranched filamentous green alga found in rocky littoral areas of many northern lakes. Field observations of its seasonal and spatial distribution indicated that it should have a low temperature and a high irradiance optimum for net photosynthesis, and at temperatures above 10°C it should show an increasingly unfavorable energy balance. Measurements of net photosynthesis and respiration were made at 56 combinations of light and temperature. Optimum conditions were 5°C and 1100 μE·m^?2·s^?1 at which net photosynthesis was 16.8 mg O₂·g^?1·h^?1. As temperature increased above 5° C optimum irradiance decreased to 125 μE·m^?2·s^?1 at 30°C. Respiration rates increased with both temperature and prior irradiance. Light-enhanced respiration rates were significantly greater than dark respiration rates following irradiance exposures of 125 μE·m^?2·s^?1 or greater. Polynomials were fitted to the data to generate response surfaces. Polynomial equations represent statistical models which can accurately predict photosynthesis and respiration for inclusion in ecosystem models.     

Seasonal photosynthetic performance of the endemic antarctic red algaPalmaria decipiens (Reinsch) Ricker

 A male gametophyte of the endemic Antarctic red macroalga Palmaria decipiens (Reinsch) Ricker was cultivated under fluctuating daylengths, simulating the seasonal changes at the site of collection (King George Island, Antarctica). The plant was maintained at 0±1°C, an irradiance of 25 μmol m^-2 s^-1 and under growth-saturating nutrient conditions. Samples were taken at intervals of 3–6 weeks to measure growth, photosynthesis, dark respiration and pigment content. The growth optimum in spring coincided with a higher photosynthetic activity. Whereas dark respiration was constantly low over the year, there was a rapid increase in maximum photosynthetic rate (P_max) in conditions corresponding to September and October. This was correlated with a change in the initial slope (α) of the photosynthesis versus irradiance (P vs I) curve. Higher activity in photosynthesis mainly resulted from higher Chl a and phycobilin concentrations during Antarctic spring, an indication of an increase in absorption cross-section areas of photosynthetic reaction centres. These changes in physiology are discussed in relation to the seasonal growth “strategy” of the species, which is controlled by seasonal variation in daylength. Received: 27 February 1995/Accepted 3 October 1995     

Temporal changes in biomass specific photosynthesis during the summer: regulation by environmental factors and the importance of phytoplankton succession

Measurements of phytoplankton photosynthesis vs. irradiance relationships have been made at 3–7 day intervals in Lake Erken (central Sweden) for three years during summer stratification. Both the rate of light-limited (^B) and light-saturated (P_max ^B) photosynthesis per unit chlorophyll a showed distinct and similar temporal trends in each year. Seasonal trends were especially evident for P_max ^B, which increased in value for several weeks following the onset of thermal stratification, and then declined in the presence of the large colonial blue-green alga, Gloeotrichia echinulata. By late summer, when the biomass of G. echinulata had decreased, P_max ^B again rose to its early summer value. The covariation of biomass-specific photosynthesis with the blooming of G. echinulata was the one clear seasonal (week-month) pattern which emerged in each of 3 years. Over short (day-week) time scales, changes in ^B were related to changes in irradiance exposure on the day of sampling. However, the relationship between these two parameters was variable in time, since it was superimposed upon longer term trends controlled by changes in phytoplankton species composition. Increases in G. echinulata biomass corresponded with a deepening of the thermocline, which both increased internal phosphorus loading and the transport of resting G. echinulata colonies into the epilimnion. The timing and magnitude of the yearly G. echinulata bloom was as a result related to the seasonal development of thermal stratification. These results illustrate the importance of seasonal changes in the phytoplankton community as a factor regulating rates of biomass specific photosynthesis, particularly when the successional changes involve species with very different life strategies.     

Respiration acclimation contributes to high carbon-use efficiency in a seasonally dry pine forest

Predictions of warming and drying in the Mediterranean and other regions require quantifying of such effects on ecosystem carbon dynamics and respiration. Long‐term effects can only be obtained from forests in which seasonal drought is a regular feature. We carried out measurements in a semiarid Pinus halepensis (Aleppo pine) forest of aboveground respiration rates of foliage, R_f, and stem, R_t over 3 years. Component respiration combined with ongoing biometric, net CO₂ flux [net ecosystem productivity (NEP)] and soil respiration measurements were scaled to the ecosystem level to estimate gross and net primary productivity (GPP, NPP) and carbon‐use efficiency (CUE=NPP/GPP) using 6 years data. GPP, NPP and NEP were, on average, 880, 350 and 211 g C m^?2 yr^?1, respectively. The above ground respiration made up half of total ecosystem respiration but CUE remained high at 0.4. Large seasonal variations in both R_f and R_t were not consistently correlated with seasonal temperature trends. Seasonal adjustments of respiration were observed in both the normalized rate (R₂₀) and short‐term temperature sensitivity (Q₁₀), resulting in low respiration rates during the hot, dry period. R_f in fully developed needles was highest over winter–spring, and foliage R₂₀ was correlated with photosynthesis over the year. Needle growth occurred over summer, with respiration rates in developing needles higher than the fully developed foliage at most times. R_t showed a distinct seasonal maximum in May irrespective of year, which was not correlated to the winter stem growth, but could be associated with phenological drivers such as carbohydrate re‐mobilization and cambial activity. We show that in a semiarid pine forest photosynthesis and stem growth peak in (wet) winter and leaf growth in (dry) summer, and associated adjustments of component respiration, dominated by those in R₂₀, minimize annual respiratory losses. This is likely a key for maintaining high CUE and ecosystem productivity similar to much wetter sites, and could lead to different predictions of the effect of warming and drying climate on productivity of pine forests than based on short‐term droughts.     

Photosynthetic acclimation in relation to nitrogen allocation in cucumber leaves in response to changes in irradiance

Leaves deep in canopies can suddenly be exposed to increased irradiances following e.g. gap formation in forests or pruning in crops. Studies on the acclimation of photosynthesis to increased irradiance have mainly focused on the changes in photosynthetic capacity (A_max), although actual irradiance often remains below saturating level. We investigated the effect of changes in irradiance on the photosynthesis irradiance response and on nitrogen allocation in fully grown leaves of Cucumis sativus. Leaves that fully developed under low (50 µmol m^?2 s^?1) or moderate (200 µmol m^?2 s^?1) irradiance were subsequently exposed to, respectively, moderate (LM‐leaves) or low (ML‐leaves) irradiance or kept at constant irradiance level (LL‐ and MM‐leaves). Acclimation of photosynthesis occurred within 7 days with final A_max highest in MM‐leaves, lowest in LL‐leaves and intermediate in ML‐ and LM‐leaves, whereas full acclimation of thylakoid processes underlying photosystem II (PSII) efficiency and non‐photochemical quenching occurred in ML‐ and LM‐leaves. Dark respiration correlated with irradiance level, but not with A_max. Light‐limited quantum efficiency was similar in all leaves. The increase in photosynthesis at moderate irradiance in LM‐leaves was primarily driven by nitrogen import, and nitrogen remained allocated in a similar ratio to Rubisco and bioenergetics, while allocation to light harvesting relatively decreased. A contrary response of nitrogen was associated with the decrease in photosynthesis in ML‐leaves. Net assimilation of LM‐leaves under moderate irradiance remained lower than in MM‐leaves, revealing the importance of photosynthetic acclimation during the leaf developmental phase for crop productivity in scenarios with realistic, moderate fluctuations in irradiance that leaves can be exposed to.     

Nitrogen nutrition,photosynthesis and carbon allocation in ectomycorrhizal pine

Summary Studies examined net photosynthesis (Pn) and dry matter production of mycorrhizal and nonmycorrhizalPinus taeda at 6 intervals over a 10-month period. Pn rates of mycorrhizal plants were consistently greater than nonmycorrhizal plants, and at 10 months were 2.1-fold greater. Partitioning of current photosynthate was examined by pulse-labelling with¹⁴CO₂ at each of the six time intervals. Mycorrhizal plants assimilated more¹⁴CO₂, allocated a greater percentage of assimilated¹⁴C to the root systems, and lost a greater percentage of¹⁴C by root respiration than did nonmycorrhizal plants. At 10 months, the quantity of¹⁴CO₂ respired by roots per unit root weight was 3.6-fold greater by mycorrhizal than nonmycorrhizal plants. Although the stimulation of photosynthesis and translocation of current photosynthate to the root system by mycorrhiza formation was consistent with the source-sink concept of sink demand, foliar N and P concentrations were also greater in mycorrhizal plants.Further studies examined Pn and dry matter production ofPinus contorta in response to various combinations of N fertilization (3, 62, 248 ppm), irradiance and mycorrhizal fungi inoculation. At 16 weeks of age, 6 weeks following inoculation with eitherPisolithus tinctorius orSuillus granulatus, Pn rates and biomass were significantly greater in mycorrhizal than nonmycorrhizal plants. Mycorrhizal plants had significantly greater foliar %P, but not %N, than did nonmycorrhizal plants. Fertilization with 62 ppm N resulted in greater mycorrhiza formation than either 3 or 248 ppm. Increased irradiance resulted in increased mycorrhiza formation.     

Inter-specific differences in photosynthetic carbon uptake, photosynthate partitioning and extracellular organic carbon release by deep-water characean algae

1. The effect of light intensity on photosynthesis and the fate of newly fixed organic carbon was compared for three characean algae collected at the same depth (10 m) but differing in their depth distributions. For each species we determined photosynthesis–irradiance (P–E) responses, the partitioning of newly fixed carbon into four intracellular pools (low molecular‐weight compounds, polysaccharides, lipids and proteins) and the extracellular organic carbon (EOC) release at a range of photon flux densities (PFD) 0–60 μmol m^–2 s^–1. 2. The P–E responses differed between the three species, with the light compensation point (E_c) and dark respiration rate highest in the shallowest species (Chara fibrosa), intermediate in the mid‐range species (C. globularis) and lowest in the deepest species (C. corallina). Photosynthetic efficiency (α) and photosynthesis: respiration ratios were lowest in C. fibrosa and highest in C. corallina. 3. In all three species, the low molecular weight pool was the principal photosynthetic product (>60% of fixed C) at 3 μmol m^–2 s^–1 PFD, but its proportional contribution decreased rapidly with increasing irradiance. Polysaccharide rose to become the major product (>35% of fixed C) at saturating PFD (35 μmol m^–2 s^–1). 4. Protein synthesis was saturated at 5 μmol m^–2 s^–1 in all species and was consistently a lower proportion of the fixed carbon in C. corallina than the other species. The fraction incorporated in the lipid pool increased slightly with irradiance but was always less than 10% of fixed C, while the proportion lost as EOC was unaffected by light, being significantly higher in C. fibrosa than the other species. 5. A kinetic experiment with C. fibrosa at 35 μmol m^–2 s^–1 PFD revealed a continued increase in net polysaccharide, protein and lipid synthesis during a 22.5‐h light period, whereas the net size of the low molecular weight pool remained constant. In a subsequent dark period, protein and lipid synthesis continued at the expense of the polysaccharide and low‐molecular‐weight pools. The EOC release rose to a constant low release in the light, then peaked slightly immediately after the dark–light transition before returning to the same rate as in the light. Extrapolating these data over 24 h suggests that the proportion of fixed carbon lost as EOC may be as high as 10% in this species. 6. The interspecific differences in carbon acquisition between the three species reflected their depth distributions, with the deeper species having more efficient photosynthetic metabolism, lower P:R ratios and less EOC release, although no apparent differences in internal partitioning of photosynthate.     

STEM AND LEAF GAS EXCHANGE OF TWO ARID LAND SHRUBS

Gas exchange studies were conducted on two shrub species found in cool shrub-steppe communities of the American West, big sagebrush (Artemisia tridentata subsp. tridentata Nutt.) and broom snakeweed (Gutierrezia sarothrae [Pursh] Britt. and Rusby), with a goal of evaluating characteristics and relative contributions of green stem and leaf material to total shoot CO₂ exchange at different temperatures. Variations in tissue temperature exerted a pronounced effect on CO₂ exchange—net photosynthesis and dark respiration—of green stems and leaves of both species. Definite temperature optima of net photosynthesis were noted, and dark respiration rates consistently increased with increases in temperature. Green stems of both species exhibited sizable dark respiration rates, although stem rates at all temperatures were lower than corresponding leaf rates. Artemisia tridentata did not exhibit sizeable green stem net photosynthesis even under conditions of optimal temperature and water availability, and leaf net photosynthesis rates were much lower than those of G. sarothrae. However, A. tridentata in general possessed a greater leaf biomass than G. sarothrae. Green stems of G. sarothrae exhibited considerable rates of net photosynthesis under both optimal and sub-optimal temperature and water availability conditions. A higher optimum temperature of net photosynthesis was noted for stems than for leaves of G. sarothrae. The adaptive significance of these interspecific differences in CO₂ exchange characteristics is discussed.     

Temperature and salinity regulation of growth and gas exchange of Salicornia fruticosa (L.) L.

Summary Salicornia fruticosa was collected from a salt marsh on the Mediterranean sea coast in Libya. Growth and gas exchange of this C₃ species were monitered in plants pretreated at various NaCl concentrations (0, 171, 342, 513 and 855 mM). Maximum growth was at 171 mM NaCl under cool growth conditions (20/10° C) and at 342 mM NaCl under warm growth conditions (30/15° C) with minimum growth at 0 mM NaCl (control). Net photosynthesis (Pn) was greatest in plants grown in 171 mM NaCl with plants grown at 513 and 855 mM having lowest rates. Maximum Pn was at 20–25° C shoot temperatures with statistically significant reductions at 30° C in control plants while salt treated plants showed such reductions at 35° C. Salt treatments increased dark respiration over the control at 171 and 342 mM but reduced it at higher concentrations. Photorespiration was reduced by salt treatment and increased by increasing shoot temperature. Greatest transpiration was in 171 mM NaCl treated plants and increasing shoot temperature increased transpiration in all treatments. Stomatal resistance to CO₂ influx was influenced only moderately by temperature while increasing salinity resulted in increased stomatal resistance. In general both temperature and salinity increased the mesophyll resistance to CO₂ influx. The species seems adapted to the warm saline habitat along the Mediterranean sea coast, at least partially, by its ability to maintain relatively high Pn at moderate NaCl concentrations over a broad range of shoot temperatures.     

Seasonal interactive effects of pCO2 and irradiance on the ecophysiology of brown macroalga Fucus vesiculosus L.

ABSTRACT

Stochastic upwelling of seawater in the Baltic Sea from the deep, anoxic bottoms may bring low-pH water rich in CO₂ close to the surface. Such events may become more frequent with climate change and ongoing ocean acidification (OA). Photoautotrophs, such as macroalgae, which are important foundation species, have been proposed to benefit from increased carbon availability due to reduced energetic cost in carbon acquisition. However, the exact effects of CO₂ fertilization may depend on the ambient light environment, as photosynthesis rates depend on available irradiance. In this experimental study, interacting effects of CO₂ addition and irradiance on the habitat-forming macroalga Fucus vesiculosus were investigated during two seasons – winter and summer – in the northern Baltic Sea. Growth rates remained unaffected by CO₂ or irradiance during both seasons, suggesting that direct effects of elevated CO₂ on mature F. vesiculosus are small. Increases in CO₂ affected algal elemental ratios by increasing carbon and decreasing nitrogen content, with resulting changes in the C:N ratio, but only in winter. In summer, chlorophyll a content increased under low irradiance. Increases in CO₂ caused a decline in light-harvesting efficiency (decrease in F_v/F_m and α) under high irradiance in summer, and conversely increased α under low irradiance. High irradiance caused increases in the maximum relative electron transport rate (rETR_max) in summer, but not in winter. Differences between winter and summer indicate that F. vesiculosus responses to CO₂ and irradiance are season-specific. Increases in carbon content during winter could indicate slightly positive effects of CO₂ addition in the long run if the extra carbon gained may be capitalized in growth. The results of this study suggest that increases in CO₂, either through upwelling or OA, may have positive effects on F. vesiculosus, but these effects are probably small.