Changes in leaf nitrogen and carbohydrates underlie temperature and CO2 acclimation of dark respiration in five boreal tree species

We tested the hypothesis that acclimation of foliar dark respiration to CO₂ concentration and temperature is associated with adjustments in leaf structure and chemistry. Populus tremuloides Michx. , Betula papyrifera Marsh. , Larix laricina (Du Roi) K. Koch , Pinus banksiana Lamb., and Picea mariana (Mill.) B.S.P. were grown from seed in combined CO₂ (370 or 580 μ mol mol^–1) and temperature treatments (18/12, 24/18, or 30/24 °C). Temperature and CO₂ effects were predominately independent. Specific respiration rates partially acclimated to warmer thermal environments through downward adjustment in the intercept, but not Q ₁₀ of the temperature–response functions. Temperature acclimation of respiration was larger for conifers than broad-leaved species and was associated with pronounced reductions in leaf nitrogen concentrations in conifers at higher growth temperatures. Short-term increases in CO₂ concentration did not inhibit respiration. Growth in the elevated CO₂ concentration reduced leaf nitrogen and increased non-structural carbohydrate concentrations. However, for a given nitrogen concentration, respiration was higher in leaves grown in the elevated CO₂ concentration, as rates increased with increasing carbohydrates. Across species and treatments, respiration rates were a function of both leaf nitrogen and carbohydrate concentrations ( R ² = 0·71, P < 0·0001). Long-term acclimation of foliar dark respiration to temperature and CO₂ concentration is largely associated with changes in nitrogen and carbohydrate concentrations.     

Direct and indirect inhibition of single leaf respiration by elevated CO2 concentrations: Interaction with temperature

Two herbaceous perennials, alfalfa (Medicago sativa L. cv. Arc) and orchard grass (Dactylus glomerata L. cv. Potomac), were grown at ambient (367 μmol mol⁻¹) and elevated (729 μmol mol⁻¹) CO₂ concentrations at constant temperatures of 15, 20, 25 and 30°C in order to examine direct and indirect changes in nighttime CO₂ efflux rate (respiration) of single leaves. Direct (biochemical) effects of CO₂ on nighttime respiration were determined for each growth condition by brief (<30 min) exposure to each CO₂ concentration. If no direct inhibition of respiration was observed, then long-term reductions in CO₂ efflux between CO₂ treatments were presumed to be due to indirect inhibition, probably related to long-term changes in leaf composition. By this criterion, indirect effects of CO₂ on leaf respiration were observed at 15 and 20°C for M. sativa on a weight basis, but not on a leaf area or protein basis. Direct effects however, were observed at 15, 20 and 25°C in D. glomerata; therefore the observed reductions in respiration for leaves grown and measured at elevated relative to ambient CO₂ concentrations could not be distinguished as indirect inhibition. No inhibition of respiration at elevated CO₂ was observed at the highest growth temperature (30°C) in either species. CO₂ efflux increased with measurement and growth temperature for M. sativa at both CO₂ concentrations; however, CO₂ efflux in D. glomerata showed complete acclimation to growth temperature. Stimulation of leaf area and weight by elevated CO₂ levels declined with growth temperature in both species. Data from the present study suggest that both direct and indirect inhibition of respiration are possible with future increases in atmospheric CO₂, and that the degree of each type of respiratory inhibition is a function of growth temperature.     

Short‐term acclimation to warmer temperatures accelerates leaf carbon exchange processes across plant types

While temperature responses of photosynthesis and plant respiration are known to acclimate over time in many species, few studies have been designed to directly compare process‐level differences in acclimation capacity among plant types. We assessed short‐term (7 day) temperature acclimation of the maximum rate of Rubisco carboxylation (V_cmax), the maximum rate of electron transport (J_max), the maximum rate of phosphoenolpyruvate carboxylase carboxylation (V_pmax), and foliar dark respiration (R_d) in 22 plant species that varied in lifespan (annual and perennial), photosynthetic pathway (C₃ and C₄), and climate of origin (tropical and nontropical) grown under fertilized, well‐watered conditions. In general, acclimation to warmer temperatures increased the rate of each process. The relative increase in different photosynthetic processes varied by plant type, with C₃ species tending to preferentially accelerate CO₂‐limited photosynthetic processes and respiration and C₄ species tending to preferentially accelerate light‐limited photosynthetic processes under warmer conditions. R_d acclimation to warmer temperatures caused a reduction in temperature sensitivity that resulted in slower rates at high leaf temperatures. R_d acclimation was similar across plant types. These results suggest that temperature acclimation of the biochemical processes that underlie plant carbon exchange is common across different plant types, but that acclimation to warmer temperatures tends to have a relatively greater positive effect on the processes most limiting to carbon assimilation, which differ by plant type. The acclimation responses observed here suggest that warmer conditions should lead to increased rates of carbon assimilation when water and nutrients are not limiting.     

High thermal acclimation potential of both photosynthesis and respiration in two lowland Plantago species in contrast to an alpine congeneric

Thermal acclimation of photosynthesis and respiration can enable plants to maintain near constant rates of net CO₂ exchange, despite experiencing sustained changes in daily average temperature. In this study, we investigated whether the degree of acclimation of photosynthesis and respiration of mature leaves differs among three congeneric Plantago species from contrasting habitats [two fast‐growing lowland species (Plantago major and P. lanceolata), and one slow‐growing alpine species (P. euryphylla)]. In addition to investigating some mechanisms underpinning variability in photosynthetic acclimation, we also determined whether leaf respiration in the light acclimates to the same extent as leaf respiration in darkness, and whether acclimation reestablishes the balance between leaf respiration and photosynthesis. Three growth temperatures were provided: constant 13, 20, or 27°C. Measurements were made at five temperatures (6–34°C). Little acclimation of photosynthesis and leaf respiration to growth temperature was exhibited by P. euryphylla. Moreover, leaf masses per area (LMA) were similar in 13°C‐grown and 20°C‐grown plants of the alpine species. In contrast, growth at 13°C increased LMA in the two lowland species; this was associated with increased photosynthetic capacity and rates of leaf respiration (both in darkness and in the light). Alleviation of triose phosphate limitation and increased capacity of electron transport capacity relative to carboxylation were also observed. Such changes demonstrate that the lowland species cold‐acclimated. Light reduced the short‐term temperature dependence (i.e. Q₁₀) of leaf respiration in all three species, irrespective of growth temperature. Collectively, our results highlight the tight coupling that exists between thermal acclimation of photosynthetic and leaf respiratory metabolism (both in darkness and in the light) in Plantago. If widespread among contrasting species, such coupling may enable modellers to assume levels of acclimation in one parameter (e.g. leaf respiration) where details are only known for the other (e.g. photosynthesis).     

A Comparison of Dark Respiration between C(3) and C(4) Plants

Lower respiratory costs were hypothesized as providing an additional benefit in C₄ plants compared to C₃ plants due to less investment in proteins in C₄ leaves. Therefore, photosynthesis and dark respiration of mature leaves were compared between a number of C₄ and C₃ species. Although photosynthetic rates were generally greater in C₄ when compared to C₃ species, no differences were found in dark respiration rates of individual leaves at either the beginning or after 16 h of the dark period. The effects of nitrogen on photosynthesis and respiration of individual leaves and whole plants were also investigated in two species that occupy similar habitats, Amaranthus retroflexus (C₄) and Chenopodium album (C₃). For mature leaves of both species, there was no relationship between leaf nitrogen and leaf respiration, with leaves of both species exhibiting a similar rate of decline after 16 h of darkness. In contrast, leaf photosynthesis increased with increasing leaf nitrogen in both species, with the C₄ species displaying a greater photosynthetic response to leaf nitrogen. For whole plants of both species grown at different nitrogen levels, there was a clear linear relationship between net CO₂ uptake and CO₂ efflux in the dark. The dependence of nightly CO₂ efflux on CO₂ uptake was similar for both species, although the response of CO₂ uptake to leaf nitrogen was much steeper in the C₄ species, Amaranthus retroflexus. Rates of growth and maintenance respiration by whole plants of both species were similar, with both species displaying higher rates at higher leaf nitrogen. There were no significant differences in leaf or whole plant maintenance respiration between species at any temperature between 18 and 42°C. The data suggest no obvious differences in respiratory costs in C₄ and C₃ plants.     

Woody tissue maintenance respiration of four conifers in contrasting climates

We estimate maintenance respiration for boles of four temperate conifers (ponderosa pine, western hemlock, red pine, and slash pine) from CO₂ efflux measurements in autumn, when construction respiration is low or negligible. Maintenance respiration of stems was linearly related to sapwood volume for all species; at 10°C, respiration per unit sapwood volume ranged from 4.8 to 8.3 mol CO₂ m^–3 s^–1. For all sites combined, respiration increased exponentially with temperature (Q ₁₀ =1.7, r ²=0.78). We estimate that maintenance respiration of aboveground woody tissues of these conifers consumes 52–162 g C m^–2 y^–1, or 5–13% of net daytime carbon assimilation annually. The fraction of annual net daytime carbon fixation used for stem maintenance respiration increased linearly with the average annual temperature of the site.     

Effects of temperature on the gas exchange of leaves in the light and dark

Summary Evolution of CO₂ into CO₂-free air was measured in the light and in the dark over a range of temperatures from 15 to 50°. Photosynthetic rates were measured in air and O₂-free air over the same range of temperatures. Respiration in the light had a different sensitivity to temperature compared with respiration in the dark. At the lower temperatures the rate of respiration in the light was higher than respiration in the dark, whereas at temperatures above 40° the reverse was observed. For any one species the maximum rates of photosynthesis and photorespiration occur at about the same temperature. The maximum rate for dark respiration generally is found at a temperature about 10° higher. Zea mays and Atriplex nummularia showed no enhancement of photosynthesis in O₂-free air nor any evolution of CO₂ in CO₂-free air at any of the temperatures.     

Acclimation of snow gum (Eucalyptus pauciflora) leaf respiration to seasonal and diurnal variations in temperature: the importance of changes in the capacity and temperature sensitivity of respiration

We investigated the relationship between daily and seasonal temperature variation and dark respiratory CO₂ release by leaves of snow gum (Eucalyptus pauciflora Sieb. ex Spreng) that were grown in their natural habitat or under controlled‐environment conditions. The open grassland field site in SE Australia was characterized by large seasonal and diurnal changes in air temperature. On each measurement day, leaf respiration rates in darkness were measured in situ at 2–3 h intervals over a 24 h period, with measurements being conducted at the ambient leaf temperature. The rate of respiration at a set measuring temperature (i.e. apparent ‘respiratory capacity’) was greater in seedlings grown under low average daily temperatures (i.e. acclimation occurred), both in the field and under controlled‐environment conditions. The sensitivity of leaf respiration to diurnal changes in temperature (i.e. the Q₁₀ of leaf respiration) exhibited little seasonal variation over much of the year. However, Q₁₀ values were significantly greater on cold winter days (i.e. when daily average and minimum air temperatures were below 6° and –1 °C, respectively). These differences in Q₁₀ values were not due to bias arizing from the contrasting daily temperature amplitudes in winter and summer, as the Q₁₀ of leaf respiration was constant over a wide temperature range in short‐term experiments. Due to the higher Q₁₀ values in winter, there was less difference between winter and summer leaf respiration rates measured at 5 °C than at 25 °C. The net result of these changes was that there was relatively little difference in total daily leaf respiratory CO₂ release per unit leaf dry mass in winter and summer. Under controlled‐environment conditions, acclimation of respiration to growth temperature occurred in as little as 1–3 d. Acclimation was associated with a change in the concentration of soluble sugars under controlled conditions, but not in the field. Our data suggest that acclimation in the field may be associated with the onset of cold‐induced photo‐inhibition. We conclude that cold‐acclimation of dark respiration in snow gum leaves is characterized by changes in both the temperature sensitivity and apparent ‘capacity’ of the respiratory apparatus, and that such changes will have an important impact on the carbon economy of snow gum plants.     

Photorespiration studies in Cycas circinalis L. and Cycas beddomei Dyer

Photosynthetic carboxylating enzymes and the effects of light and temperature on ¹⁴CO₂ efflux in two species of gymnosperm leaves were studied. The activity of RuBP carboxylase was high and that of PEP carboxylase was very low when compared to C₄ plants. The CO₂ compensation point was high. ¹⁴CO₂ efflux was greater in light than in darkness and the ratio (L/D) increased with increase in temperature and light intensity. The inhibitors of glycolate metabolism showed decreased ¹⁴CO₂ evolution in light while dark respiration was unaffected. It is concluded that both Cycas circinalis, L. and Cycas beddomei Dyer are C₃ plants.     

Variation in leaf respiration in relation to growth and photosynthesis of Lolium

Six Lolium genotypes with contrasting apparent photorespiration and CO_a compensation concentration, [C0₂]_c, were compared for net photosynthesis, dark respiration, leaf starch accumulation, rate of leaf expansion and shoot regrowth. Plants were grown in day/night temperatures of 15/10 and 25/20 ^oC. There were significant (P < 0–05) differences between the genotypes in all these parameters. At 25/20 ^oC apparent photorespiration was correlated with [CO₂]_c. Correlation coefficients, pooled from both temperature regimes, revealed that genotypes with high rates of net photosynthesis accumulated more leaf starch during light periods than genotypes with slow photosynthesis, but rates of leaf expansion and dry matter increase were only correlated, negatively, with dark respiration. Apparent photorespiration was negatively correlated with dark respiration. These findings suggest that attributes related to photorespiration such as [CO₂]_c and O₂ uptake from CO₂-free air in the light are unlikely to be useful selection criteria for growth of C₃ grasses, that net photosynthesis was probably not limiting growth and that maintenance respiration may have been an important determinant of genotypic differences in growth rate. Selections for slow and fast rates of dark respiration of mature leaves were therefore made at 8 and at 25 ^oC from within two different populations of L. perenne, S.23. This characteristic showed repeatabilities (broad-sense heritability) of from 0–41 to o-66. Six independent comparisons of simulated swards of the slow- and fast-respiring selections were made under periodic cutting regimes, either in a growth room at 25 ^oC or in a glasshouse from August to May. Growth of all plots of slow-respiring genotypes was consistently more rapid than that of the fast-respiring, at 25 ^oC in the growth room, and during autumn and spring in the glasshouse. There was no difference in winter growth. The implications of these results for the use of gas exchange measurements as selection criteria in plant breeding programmes are discussed.     

The effect of carbon dioxide concentration on respiration of growing and mature soybean leaves

Soybean plants were grown continuously at 350 and 700cm³m^?3 CO₂ at constant temperature. Respiration rates of third trifoliolate leaves were measured at the growth CO₂ concentration for the whole dark period from 5d before through to 5d after full area expansion. The short-term response of respiration rate to the measurement CO₂ concentration was also determined at each age. Respiration rates per unit of dry mass declined with age and were significantly less at a given age or RGR in leaves grown and measured at the elevated CO₂. The difference in respiration rate was largest in mature leaves and resulted from the different measurement CO₂ concentrations. The respiratory costs of the tissue synthesis, estimated from the elemental composition of the tissue, did not differ substantially between CO₂ treatments. The response of respiration rate to carbon dioxide concentration was not strongly affected by the form of nitrogen supplied. Maintenance respiration calculated by subtracting growth respiration from total respiration was negative in rapidly growing leaves for both CO₂ treatments. This indicates that CO₂ efflux in the dark does not accurately reflect the average 24 h rate of energy expenditure on growth and maintenance for soybean leaves.     

Foliar respiration acclimation to temperature and temperature variable Q10 alter ecosystem carbon balance

The response of respiration to temperature in plants can be considered at both short‐ and long‐term temporal scales. Short‐term temperature responses are not well described by a constant Q₁₀ of respiration, and longer‐term responses often include acclimation. Despite this, many carbon balance models use a static Q₁₀ of respiration to describe the short‐term temperature response and ignore temperature acclimation. We replaced static respiration parameters in the ecosystem model photosynthesis and evapo‐transpiration (PnET) with a temperature‐driven basal respiration algorithm (Rd_acclim) that accounts for temperature acclimation, and a temperature‐variable Q₁₀ algorithm (Q_10var). We ran PnET with the new algorithms individually and in combination for 5 years across a range of sites and vegetation types in order to examine the new algorithms' effects on modeled rates of mass‐ and area‐based foliar dark respiration, above ground net primary production (ANPP), and foliar respiration–photosynthesis ratios. The Rd_acclim algorithm adjusted dark respiration downwards at temperatures above 18°C, and adjusted rates up at temperatures below 5°C. The Q_10var algorithm adjusted dark respiration down at temperatures below 15°C. Using both algorithms simultaneously resulted in decreases in predicted annual foliar respiration that ranged from 31% at a tall‐grass prairie site to 41% at a boreal coniferous site. The use of the Rd_acclim and Q_10var algorithms resulted in increases in predicted ANPP ranging from 18% at the tall‐grass prairie site to 38% at a warm temperate hardwood forest site. The new foliar respiration algorithms resulted in substantial and variable effects on PnETs predicted estimates of C exchange and production in plants and ecosystems. Current models that use static parameters may over‐predict respiration and subsequently under‐predict and/or inappropriately allocate productivity estimates. Incorporating acclimation of basal respiration and temperature‐sensitive Q₁₀ have the potential to enhance the application of ecosystem models across broad spatial scales, or in climate change scenarios, where large temperature ranges may cause static respiration parameters to yield misleading results.     

Photosynthesis of temperate Eucalyptus globulus trees outside their native range has limited adjustment to elevated CO2 and climate warming

Eucalyptus species are grown widely outside of their native ranges in plantations on all vegetated continents of the world. We predicted that such a plantation species would show high potential for acclimation of photosynthetic traits across a wide range of growth conditions, including elevated [CO₂] and climate warming. To test this prediction, we planted temperate Eucalyptus globulus Labill. seedlings in climate‐controlled chambers in the field located >700 km closer to the equator than the nearest natural occurrence of this species. Trees were grown in a complete factorial combination of elevated CO₂ concentration (eC; ambient [CO₂] +240 ppm) and air warming treatments (eT; ambient +3 °C) for 15 months until they reached ca. 10 m height. There was little acclimation of photosynthetic capacity to eC and hence the CO₂‐induced photosynthetic enhancement was large (ca. 50%) in this treatment during summer. The warming treatment significantly increased rates of both carboxylation capacity (V_cmax) and electron transport (J_max) (measured at a common temperature of 25 °C) during winter, but decreased them significantly by 20–30% in summer. The photosynthetic CO₂ compensation point in the absence of dark respiration (Γ*) was relatively less sensitive to temperature in this temperate eucalypt species than for warm‐season tobacco. The temperature optima for photosynthesis and J_max significantly changed by about 6 °C between winter and summer, but without further adjustment from early to late summer. These results suggest that there is an upper limit for the photosynthetic capacity of E. globulus ssp. globulus outside its native range to acclimate to growth temperatures above 25 °C. Limitations to temperature acclimation of photosynthesis in summer may be one factor that defines climate zones where E. globulus plantation productivity can be sustained under anticipated global environmental change.     

Relationships of leaf dark respiration with light environment and tissue nitrogen content in juveniles of 11 cold-temperate tree species

It has been argued that plants adapted to low light should have lower carbon losses via dark respiration (Rd) than those not so adapted, and similarly, all species would be expected to down-regulate Rd in deep shade, because the associated advantages of high metabolic potential cannot be realized in such habitats. In order to test these hypotheses, and to explore the determinants of intraspecific variation in respiration rates, we measured Rd, leaf mass per unit area (LMA), and nitrogen content of mature foliage in juveniles of 11 cold-temperate tree species (angiosperms and conifers), growing in diverse light environments in forest understories in northern Minnesota. Among the seven angiosperm species, respiration on mass, area, and nitrogen bases showed significant negative overall relationships with shade tolerance level. Mass-based respiration rates (Rd _mass) of angiosperms as a group showed a significant positive overall relationship with an index of light availability (percentage canopy openness, %CO). Rd _mass of most conifers also showed evidence of acclimation of Rd _mass to light availability. LMA of all species also increased with increasing %CO, but this response was generally much stronger in angiosperms than in conifers. As a result, the response of area-based respiration (Rd _area) to %CO was dominated by ΔRd _mass for conifers, and by ΔLMA for most angiosperms, i.e., functional types differed in the components of acclimation of Rd _area to light availability. Among the seven angiosperm species, the relationships of leaf N on a mass basis (N _mass) with %CO were modulated by shade tolerance: negative slopes in shade-tolerant species may be related to the steep increases in LMA of these taxa along gradients of increasing light intensity, and associated dilution of N-rich, metabolically active tissue by increasing investment in leaf structural components. Although N _mass was therefore an unreliable predictor of variation in Rd _mass along light gradients, respiration per unit leaf N (Rd/N) was significantly positively correlated with %CO for most species. This probably reflects variation in the proportion of leaf N allocated to protein and/or the influence of leaf carbohydrate status on Rd. Species shade tolerance differences were not significantly correlated with the magnitude of either ΔRd _mass or ΔRd _area, indicating that variation in acclimation potential of Rd is much less important than inherent differences in this trait. Acclimation of Rd _mass to light availability appears to be a generalized feature of juvenile trees, and the important ecological trade-off is likely between high metabolic capacity in high light and low respiratory losses in low light. Received: 15 April 1999 / Accepted: 24 October 1999     

Woody-tissue respiration for Simarouba amara and Minquartia guianensis,two tropical wet forest trees with different growth habits

We measured CO₂ efflux from stems of two tropical wet forest trees, both found in the canopy, but with very different growth habits. The species were Simarouba amara, a fast-growing species associated with gaps in old-growth forest and abundant in secondary forest, and Minquartia guianensis, a slow-growing species tolerant of low-light conditions in old-growth forest. Per unit of bole surface, CO₂ efflux averaged 1.24 mol m^–2 s^–1 for Simarouba and 0.83 mol m^–2s^–1 for Minquartia. CO₂ efflux was highly correlated with annual wood production (r ²=0.65), but only weakly correlated with stem diameter (r ²=0.22). We also partitioned the CO₂ efflux into the functional components of construction and maintenance respiration. Construction respiration was estimated from annual stem dry matter production and maintenance respiration by subtracting construction respiration from the instantaneous CO₂ flux. Estimated maintenance respiration was linearly related to sapwood volume (39.6 mol m^–3s^–1 at 24.6° C, r ²=0.58), with no difference in the rate for the two species. Maintenance respiration per unit of sapwood volume for these tropical wet forest trees was roughly twice that of temperate conifers. A model combining construction and maintenance respiration estimated CO₂ very well for these species (r ²=0.85). For our sample, maintenance respiration was 54% of the total CO₂ efflux for Simarouba and 82% for Minquartia. For our sample, sapwood volume averaged 23% of stem volume when weighted by tree size, or 40% with no size weighting. Using these fractions, and a published estimate of aboveground dry-matter production, we estimate the annual cost of woody tissue respiration for primary forest at La Selva to be 220 or 350 g C m^–2 year^–1, depending on the assumed sapwood volume. These costs are estimated to be less than 13% of the gross production for the forest.     

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     

Growth temperature affects O2 consumption rates and plasticity of respiratory flux to support shoot growth at various growth temperatures

The temperature dependence of respiration rates and their acclimation to growth temperature vary among species/ecotypes, but the details remain unclear. Here, we compared the temperature dependence of shoot O₂ consumption rates among Arabidopsis thaliana ecotypes to clarify how the temperature dependence and their acclimation to temperature differ among ecotypes, and how these differences relate to shoot growth. We examined growth analysis, temperature dependence of O₂ consumption rates, and protein amounts of the respiratory chain components in shoots of twelve ecotypes of A. thaliana grown at three different temperatures. The temperature dependence of the O₂ consumption rates were fitted to the modified Arrhenius model. The dynamic response of activation energy to measurement temperature was different among growth temperatures, suggesting that the plasticity of respiratory flux to temperatures differs among growth temperatures. The similar values of activation energy at growth temperature among ecotypes suggest that a similar process may determine the O₂ consumption rates at the growth temperature in any ecotype. These results suggest that the growth temperature affects not only the absolute rate of O₂ consumption but also the plasticity of respiratory flux in response to temperature, supporting the acclimation of shoot growth to various temperatures.     

Temperature responses of growth and photosynthetic CO2 exchange rates in coastal and desert races of Atriplex lentiformis

Summary Comparative measurements of CO₂ exchange and growth rates were made on Atriplex lentiformis (Torr.) Wats. plants from populations native to coastal as well as desert habitats in southern California. While both had similar CO₂ exchange rates at moderate growth temperatures, the desert plants had a substantially greater capacity to acclimate to high growth temperatures indicating that clear ecotypic differences in acclimation capacity are present in this species. This large capacity for photosynthetic acclimation resulted in nearly equal CO₂ exchange rates of the desert plants under the different day temperatures characteristic of the desert habitat during the summer and winter months. In contrast, the photosynthetic CO₂ exchange rates of the coastal plants was markedly reduced by high growth temperatures. The large acclimation capacity of the desert plants may function to maintain high productivities during both the winter and summer months but would not be required in the coastal plants because of the moderate temperatures throughout the year in their native habitat.Relative growth rates (RGR) of the coastal and desert plants were similar at 23°C day/18°C night and 33°C day/25°C night growth temperatures. At 43°C day/30°C night temperatures, however, the RGR of the desert plants was higher than that of the coastal plants. Thus, the larger acclimation capacity of the desert plants is related to a greater ability to maintain high growth rates over a wide range of temperatures as compared to the coastal plants. Small differences in allocation patterns could account for differences in the comparative photosynthetic responses and growth rates in each temperature regime.Supported by National Science Foundation grant # GB 36311     

Contrasting crop species responses to CO2 and temperature: rice,soybean and citrus

The continuing increase in atmospheric carbon dioxide concentration ([CO₂]) and projections of possible future increases in global air temperatures have stimulated interest in the effects of these climate variables on plants and, in particular, on agriculturally important food crops. Mounting evidence from many different experiments suggests that the magnitude and even direction of crop responses to [CO₂] and temperature is almost certain to be species dependent and very likely, within a species, to be cultivar dependent. Over the last decade, [CO₂] and temperature experiments have been conducted on several crop species in the outdoor, naturally-sunlit, environmentally controlled, plant growth chambers by USDA-ARS and the University of Florida, at Gainesville, Florida, USA. The objectives for this paper are to summarize some of the major findings of these experiments and further to compare and contrast species responses to [CO₂] and temperature for three diverse crop species: rice (Oryza sativa, L.), soybean (Glycine max, L.) and citrus (various species). Citrus had the lowest growth and photosynthetic rates but under [CO₂] enrichment displayed the greatest percentage increases over ambient [CO₂] control treatments. In all three species the direct effect of [CO₂] enrichment was always an increase in photosynthetic rate. In soybean, photosynthetic rate depended on current [CO₂] regardless of the long-term [CO₂] history of the crop. In rice, photosynthetic rate measured at a common [CO₂], decreased with increasing long-term [CO₂] growth treatment due to a corresponding decline in RuBP carboxylase content and activity. Rice specific respiration decreased from subambient to ambient and superambient [CO₂] due to a decrease in plant tissue nitrogen content and a decline in specific maintenance respiration rate. In all three species, crop water use decreased with [CO₂] enrichment but increased with increases in temperature. For both rice and soybean, [CO₂] enrichment increased growth and grain yield. Rice grain yields declined by roughly 10 % per each 1 °C rise in day/night temperature above 28/21 °C.