The response of nodulated alfalfa to water supply, temperature and elevated CO2: photosynthetic downregulation

Plants grown in an environment of elevated CO₂ and temperature often show reduced CO₂ assimilation capacity, providing evidence of photosynthetic downregulation. The aim of this study was to analyse the downregulation of photosynthesis in elevated CO₂ (700 µmol mol⁻¹) in nodulated alfalfa plants grown at different temperatures (ambient and ambient + 4°C) and water availability regimes in temperature gradient tunnels. When the measurements were taken in growth conditions, a combination of elevated CO₂ and temperature enhanced the photosynthetic rate; however, when they were carried out at the same CO₂ concentration (350 and 700 µmol mol⁻¹), elevated CO₂ induced photosynthetic downregulation, regardless of temperature and drought. Intercellular CO₂ concentration measurements revealed that photosynthetic acclimation could not be accounted for by stomatal limitations. Downregulation of plants grown in elevated CO₂ was a consequence of decreased carboxylation efficiency as a result of reduced rubisco activity and protein content; in plants grown at ambient temperature, downregulation was also induced by decreased quantum efficiency. The decrease in rubisco activity was associated with carbohydrate accumulation and depleted nitrogen availability. The root nodules were not sufficiently effective to balance the source–sink relation in elevated CO₂ treatments and to provide the required nitrogen to counteract photosynthetic acclimation.     

Growth in elevated CO2 enhances temperature response of photosynthesis in wheat

The temperature dependence of C₃ photosynthesis may be altered by the growth environment. The effects of long-term growth in elevated CO₂ on photosynthesis temperature response have been investigated in wheat ( Triticum aestivum L.) grown in controlled chambers with 370 or 700 μmol mol⁻¹ CO₂ from sowing through to anthesis. Gas exchange was measured in flag leaves at ear emergence, and the parameters of a biochemical photosynthesis model were determined along with their temperature responses. Elevated CO₂ slightly decreased the CO₂ compensation point and increased the rate of respiration in the light and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) V_cmax, although the latter effect was reversed at 15°C. With elevated CO₂, J_max decreased in the 15–25°C temperature range and increased at 30 and 35°C. The temperature response (activation energy) of V_cmax and J_max increased with growth in elevated CO₂. CO₂ enrichment decreased the ribulose 1,5-bisphosphate (RuBP)-limited photosynthesis rates at lower temperatures and increased Rubisco- and RuBP-limited rates at higher temperatures. The results show that the photosynthesis temperature response is enhanced by growth in elevated CO₂. We conclude that if temperature acclimation and factors such as nutrients or water availability do not modify or negate this enhancement, the effects of future increases in air CO₂ on photosynthetic electron transport and Rubisco kinetics may improve the photosynthetic response of wheat to global warming.     

The interaction of high temperature and elevated CO2 on photosynthetic acclimation of single leaves of rice in situ

Rice ( Oryza sativa L. cv. IR72) was grown at three different CO₂ concentrations (ambient, ambient + 200 μmol mol⁻¹, ambient + 300 μmol mol⁻¹) at two different growth temperatures (ambient, ambient + 4°C) from sowing to maturity to determine longterm photosynthetic acclimation to elevated CO₂ with and without increasing temperature. Single leaves of rice showed a cooperative enhancement of photosynthetic rate with elevated CO₂ and temperature during tillering, relative to the elevated CO₂ condition alone. However, after flowering, the degree of photosynthetic stimulation by elevated CO₂ was reduced for the ambient + 4°C treatment. This increasing insensitivity to CO₂ appeared to be accompanied by a reduction in ribulose-1.5-bisphosphate carboxylase/oxygenase (Rubisco) activity and/or concentration as evidenced by the reduction in the assimilation (A) to internal CO₂ (C₁) response curve. The reproductive response (e.g. percent filled grains, panicle weight) was reduced at the higher growth temperature and presumably reflects a greater increase in floral sterility. Results indicate that while CO₂ and temperature could act synergistically at the biochemical level, the direct effect of temperature on floral development with a subsequent reduction in carbon utilization may change sink strength so as to limit photosynthetic stimulation by elevated CO₂ concentration.     

Architectural and physiological mechanisms of reduced size inequality in CO2-enriched stands of common ragweed (Ambrosia artemisiifolia)

Testing whether and how subordinate individuals differ from dominants in the utilization of enriched CO₂ atmospheres is important for understanding future stand and community structure. We hypothesized that subordinate and dominant Ambrosia artemisiifolia L. (Asteraceae) (common ragweed) plants growing in dense stands would not equally acquire or utilize carbon gains from CO₂-enrichment, and that the resulting disproportionate growth gains to subordinates would reduce size inequalities in competing stands. We grew experimental stands of A. artemisiifolia in either ambient (360 μL L⁻¹) or twice ambient (720 μL L⁻¹) levels of atmospheric CO₂. We compared the relative growth, photosynthetic capacity, and architecture of subordinate and dominant plants in each treatment, and assessed size inequalities using the stand-level coefficient of variation (CV). In elevated CO₂, plants grew larger, but subordinate plants shifted more mass to upper stem allocation than dominants. Dominant plants demonstrated reduced leaf-level photosynthetic gains in elevated CO₂ compared with subordinate plants. Reduced CVs in plant size reflected smaller proportional growth gains by dominants over subordinates in elevated vs. ambient stands. We conclude that differences in the architectural and physiological responses of subordinate and dominant ragweed plants reduce competition and allow subordinate plants to catch up to dominants in elevated CO₂ conditions.     

Effect of elevated CO2, temperature and drought on dry matter partitioning and photosynthesis before and after cutting of nodulated alfalfa

The rising atmospheric CO₂ concentration resulting from industrial development may enhance photosynthesis and plant growth. However, there is a lack of research concerning the effect of combined factors such as CO₂, temperature and water availability on plant regrowth following cutting or grazing, which represent the usual methods of managing forage legumes like alfalfa. Elevated CO₂, temperature and drought can interact with cutting factors (e.g. cutting frequency or height), and source-sink balance differences before and after defoliation can modify photosynthetic behaviour and dry matter accumulation, as well as dry matter partitioning between above- and belowground organs. The aim of our study was to determine the interactive effect of CO₂ (ambient, around 350 μmol mol⁻¹ versus 700 μmol mol⁻¹), temperature (ambient versus ambient + 4 °C) and water availability (well-irrigated versus partially irrigated) on dry matter partitioning and photosynthesis in nodulated alfalfa after vegetative normal growth and during regrowth. At the end of vegetative normal growth, CO₂ enhanced dry matter accumulation despite photosynthesis being down-regulated at the end of this period. Photosynthesis was stimulated by elevated CO₂ and resulted in greater dry matter accumulation during the regrowth period. Aboveground organs were affected more by drought than belowground organs during the entire experiment, particularly during vegetative normal growth. The higher drought tolerance (greater growth) observed during the regrowth period may be related to higher mass and greater reserves accumulated in the roots of plants.     

Photosynthesis, growth and structural characteristics of holm oak resprouts originated from plants grown under elevated CO2

The physiological characteristics of holm oak ( Quercus ilex L.) resprouts originated from plants grown under current CO₂ concentration (350 μl l⁻¹) (A-resprouts) were compared with those of resprouts originated from plants grown under elevated CO₂ (750 μl l⁻¹) (E-resprouts). At their respective CO₂ growth concentration, no differences were observed in photosynthesis and chlorophyll fluorescence parameters between the two kinds of resprout. E-resprouts appeared earlier and showed lower stomatal conductance, higher water-use efficiency and increased growth (higher leaf, stem and root biomass and increased height). Analyses of leaf chemical composition showed the effect of elevated [CO₂] on structural polysaccharide (higher cellulose content), but no accumulation of total non-structural carbohydrate on area or dry weight basis was seen. Four months after appearance, downregulation of photosynthesis and electron transport components was observed in E-resprouts: lower photosynthetic capacity, photosystem II quantum efficiency, photochemical quenching of fluorescence and relative electron transport rate. Reduction in ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCo) activity, deduced from the maximum carboxylation velocity of RuBisCo, accounts for the observed acclimation. Increased susceptibility of photosynthetic apparatus to increasing irradiance was detected in E-resprouts.     

Effects of source-sink relations on photosynthetic acclimation to elevated CO2

Abstract. While photosynthesis of C₃ plants is stimulated by an increase in the atmospheric CO₂ concentration, photosynthetic capacity is often reduced after long-term exposure to elevated CO₂. This reduction appears to be brought about by end product inhibition, resulting from an imbalance in the supply and demand of carbohydrates. A review of the literature revealed that the reduction of photosynthetic capacity in elevated CO₂ was most pronounced when the increased supply of carbohydrates was combined with small sink size. The volume of pots in which plants were grown affected the sink size by restricting root growth. While plants grown in small pots had a reduced photosynthetic capacity, plants grown in the field showed no reduction or an increase in this capacity. Pot volume also determined the effect of elevated CO₂ on the root/shoot ratio: the root/shoot ratio increased when root growth was not restricted and decreased in plants grown in small pots. The data presented in this paper suggest that plants growing in the field will maintain a high photosynthetic capacity as the atmospheric CO₂ level continues to rise.     

Photosynthetic acclimation to elevated CO2 is modified by source:sink balance in three component species of chalk grassland swards grown in a free air carbon dioxide enrichment (FACE) experiment

Artificial chalk grassland swards were exposed to either ambient air or air enriched to 600 μ mol mol^–1 CO₂, using free-air CO₂ enrichment technology, and subjected to an 8 week simulated grazing regime. After 14 months of treatment, ribulose-1,5-bisphosphate carboxylase (Rubisco) activity ( V _c,max) and electron transport mediated ribulose-1,5-bisphosphate (RuBP) regeneration capacity ( J _max), estimated from leaf gas exchange, were significantly lower in fully expanded leaves of Anthyllis vulneraria L. (a legume) and Sanguisorba minor Scop. grown in elevated CO₂. After a change in source:sink balance brought about by defoliation, photosynthetic capacity was fully restored in A. vulneraria and S. minor, but acclimation continued in the grass Bromopsis erecta (Hudson) Fourr. Changes in net photosynthesis ( P _n) with growth at elevated CO₂ ranged from a 1·6% reduction in precut leaves of A. vulneraria to a 47·1% stimulation in postcut leaves of S. minor . Stomatal acclimation was observed in leaves of A. vulneraria (reduced stomatal density) and B. erecta (reduced stomatal conductance). The results are discussed in terms of whole-plant resource-use optimization and chalk grassland community competitive interactions at elevated CO₂.     

Low vapour pressure deficit reduces the beneficial effect of elevated CO2 on growth of N2-fixing alfalfa plants

Plant responses to elevated CO₂ can be modified by many environmental factors, but very little attention has been paid to the interaction between CO₂ and changes in vapour pressure deficit (VPD). Thirty-day-old alfalfa plants ( Medicago sativa L. cv. Aragón), which were inoculated with Sinorhizobium meliloti 102F78 strain, were grown for 1 month in controlled environment chambers at 25/15°C, 14 h photoperiod, and 600 µmol m⁻² s⁻¹ photosynthetic photon flux (PPF), using a factorial combination of CO₂ concentration (400 µmol mol⁻¹ or 700 µmol mol⁻¹) and vapour pressure deficit (0.48 kPa or 1.74 kPa, which corresponded to relative humidities of 85% and 45% at 25°C, respectively). Elevated CO₂ strongly stimulated plant growth under high VPD conditions, but this beneficial effect was not observed under low VPD. Under low VPD, elevated CO₂ also did not enhance plant photosynthesis, and plant water stress was greatest for plants grown at elevated CO₂ and low VPD. Moreover, plants grown under elevated CO₂ and low VPD had a lower leaf soluble protein and photosynthetic activity (photosynthetic rate and carboxylation efficiency) than plants grown under elevated CO₂ and high VPD. Elevated CO₂ significantly increased leaf adaxial and abaxial temperatures. Because the effects of elevated CO₂ were dependent on vapour pressure deficit, VPD needs to be controlled in experiments studying the effect of elevated CO₂ as well as considered in the extrapolations of results to a warmer, high-CO₂ world.     

Influence of elevated CO2 and nitrogen supply on the carbon assimilation performance and cell composition of the unicellular alga Dunaliella viridis

The carbon assimilation efficiency and the internal composition of the chlorophyte Dunaliella viridis have been studied under conditions of current (0.035%) and enriched (1%) levels of CO₂, with and without N limitation (supplied as nitrate). Results show that both photosynthesis and growth rates are enhanced by high CO₂, but the strategy of acclimation also involves the light harvesting machinery and the nutritional metabolism in an N supply dependent manner. D. viridis carried out a qualitative rather than a quantitative acclimation of the light harvesting system leading to increased PSII quantum yields. Total internal C decreased as a consequence of either active growth or organic carbon release to the external medium. The latter process allowed photosynthetic electron transport to proceed at higher rates than under normal CO₂ conditions, and maintained the internal C:N balance in a narrow range (under N sufficiency). N limitation generally prevented the effects of high CO₂, with some exceptions such as the photosynthetic O₂ evolution rate.     

The impact of nitrogen supply on the potential response of a noxious, invasive weed, Canada thistle (Cirsium arvense) to recent increases in atmospheric carbon dioxide

A recognized invasive weed, Canada thistle ( Cirsium arvense L. Scop.) was grown at ambient and pre-ambient concentrations of atmospheric carbon dioxide [CO₂] (373 and 287 μmol mol⁻¹, respectively) at three levels of supplemental nitrogen (N) (3, 6 and 14.5 m M ) from seeding until flowering [77 days after sowing (DAS)]. The primary objective of the study was to determine if N supply limited the potential photosynthetic and growth response of this species to the increase in atmospheric [CO₂] which occurred during the 20th century (i.e. approximately 290 to 370 μmol mol⁻¹ CO₂). Leaf photosynthesis increased both as a function of growth [CO₂] and N supply during the first 46 DAS. Although by 46 DAS photosynthetic acclimation was observed relative to a common measurement CO₂ concentration, there was no interaction with N supply. Both [CO₂] and N increased biomass, relative growth rates and leaf area whereas root : shoot ratio was increased by CO₂ and decreased by increasing N; however, N supply did not effect the relative response to [CO₂] for any measured vegetative parameter up to 77 DAS. Due to the relative stimulation of shoot biomass, total above-ground N increased at elevated [CO₂] for all levels of supplemental N, but nitrogen use efficiency (NUE) did not differ as a function of [CO₂]. Overall, these data suggest that any potential response to increased atmospheric [CO₂] in recent decades for this noxious weedy species was probably not limited by nitrogen supply.     

Effects of elevated CO2 concentration on photosynthesis, respiration and carbohydrate status of coppice Populus hybrids

To determine how increased atmospheric CO₂ will affect the physiology of coppiced plants, sprouts originating from two hybrid poplar clones ( Populus trichocarpa × P. deltoides - Beaupre and P. deltoides × P. nigra - Robusta) were grown in open-top chambers containing ambient or elevated (ambient + 360 μmol mol⁻¹) CO₂ concentration. The effects of elevated CO₂ concentration on leaf photosynthesis, stomatal conductance, dark respiration, carbohydrate concentration and nitrogen concentration were measured. Furthermore, dark respiration of leaves was partitioned into growth and maintenance components by regressing specific respiration rate vs specific growth rate. Sprouts of both clones exposed to CO₂ enrichment showed no indication of photosynthetic down-regulation. During reciprocal gas exchange measurements, CO₂ enrichment significantly increased photosynthesis of all sprouts by approximately 60% ( P < 0.01) on both an early and late season sampling date, decreased stomatal conductance of all sprouts by 10% ( P < 0.04) on the early sampling date and nonsignificantly decreased dark respiration by an average of 11%. Growth under elevated CO₂ had no consistent effect on foliar sugar concentration but significantly increased foliar starch by 80%. Respiration rate was highly correlated with both specific growth rate and percent nitrogen. Long-term CO₂ enrichment did not significantly affect the maintenance respiration coefficient or the growth respiration coefficient. Carbon dioxide enrichment affected the physiology of the sprouts the same way it affected these plants before they were coppiced.     

Importance of leaf versus whole plant CO2 environment for photosynthetic acclimation

The reduction of photosynthetic capacity in many plants grown at elevated CO₂ is thought to result from a feedback effect of leaf carbohydrates on gene expression. Carbohydrate feedback at elevated CO₂ could result from limitations on carbohydrate utilization at many different points, for example export of triose phosphates from the chloroplast, sucrose synthesis and phloem loading, transport in the phloem, unloading of the phloem at the sinks, or utilization for growth of sinks. To determine the relative importance of leaf versus whole plant level limitations on carbohydrate utilization at elevated CO₂, and the possible effects on the regulation of photosynthetic capacity, we constructed a treatment system in which we could expose single, attached, soybean leaflets to CO₂ concentrations different from those experienced by the rest of the plant. The single leaflet treatments had dramatic effects on the carbohydrate contents of the treated leaflets. However, photosynthetic capacity and rubisco content were unaffected by the individual leaflet treatment and instead were related to the whole plant CO₂ environment, despite the fact that the CO₂ environment around the rest of the plant had no significant affect on the total non-structural carbohydrate (TNC) contents of the treated leaflets. These results necessitate a re-evaluation of the response mechanisms to CO₂ as well as some of the methods used to test these responses. We propose mechanisms by which sink strength could influence leaf physiology independently of changes in carbohydrate accumulation.     

Increasing growth temperature reduces the stimulatory effect of elevated CO2 on photosynthesis or biomass in two perennial species

We examined how anticipated changes in CO₂ concentration and temperature interacted to alter plant growth, harvest characteristics and photosynthesis in two cold-adapted herbaceous perennials, alfalfa ( Medicago sativa L. cv. Arc) and orchard grass ( Dactylis glomerata L. cv. Potomac). Plants were grown at two CO₂ concentrations (362 [ambient] and 717 [elevated] μmol mol⁻¹ CO₂) and four constant day/night temperatures of 15, 20, 25 and 30°C in controlled environmental chambers. Elevated CO₂ significantly increased total plant biomass and protein over a wide range of temperatures in both species. Stimulation of photosynthetic rate, however, was eliminated at the highest growth temperature in M. sativa and relative stimulation of plant biomass and protein at high CO₂ declined as temperature increased in both species. Lack of a synergistic effect between temperature and CO₂ was unexpected since elevated CO₂ reduces the amount of carbon lost via photorespiration and photorespiration increases with temperature. Differences between anticipated stimulatory effects of CO₂ and temperature and whole plant single and leaf measurements are discussed. Data from this study suggest that stimulatory effects of atmospheric CO₂ on growth and photosynthesis may decline with anticipated increases in global temperature, limiting the degree of carbon storage in these two perennial species.     

Effects of long-term elevated [CO2] from natural CO2 springs on Nardus stricta: photosynthesis, biochemistry, growth and phenology

Plants of Nardus stricta growing near a cold, naturally emitting CO₂ spring in Iceland were used to investigate the long-term (> 100 years) effects of elevated [CO₂] on photosynthesis, biochemistry, growth and phenology in a northern grassland ecosystem. Comparisons were made between plants growing in an atmosphere naturally enriched with CO₂ (≈ 790 μ mol mol^–1) near the CO₂ spring and plants of the same species growing in adjacent areas exposed to ambient CO₂ concentrations (≈360 μ mol mol^–1). Nardus stricta growing near the spring exhibited earlier senescence and reductions in photosynthetic capacity (≈25%), Rubisco content (≈26%), Rubisco activity (≈40%), Rubisco activation state (≈23%), chlorophyll content (≈33%) and leaf area index (≈22%) compared with plants growing away from the spring. The potential positive effects of elevated [CO₂] on grassland ecosystems in Iceland are likely to be reduced by strong down-regulation in the photosynthetic apparatus of the abundant N. stricta species.     

Increased photosynthetic capacity of Scirpus olneyi after 4 years of exposure to elevated CO2

Abstract. While a short-term exposure to elevated atmospheric CO₂ induces a large increase in photosynthesis in many plants, long-term growth in elevated CO₂ often results in a smaller increase due to reduced photosynthetic capacity. In this study, it was shown that, for a wild C₃ species growing in its natural environment and exposed to elevated CO₂ for four growing seasons, the photosynthetic capacity has actually increased by 31%. An increase in photosynthetic capacity has been observed in other species growing in the field, which suggests that photosynthesis of certain field grown plants will continue to respond to elevated levels of atmospheric CO₂     

Ecophysiological responses to simulated canopy gaps of two tree species of contrasting shade tolerance in elevated CO2

1. One-year-old seedlings of shade tolerant Acer rubrum and intolerant Betula papyrifera were grown in ambient and twice ambient (elevated) CO₂, and in full sun and 80% shade for 90 days. The shaded seedlings received 30-min sun patches twice during the course of the day. Gas exchange and tissue–water relations were measured at midday in the sun plants and following 20 min of exposure to full sun in the shade plants to determine the effect of elevated CO₂ on constraints to sun-patch utilization in these species.
2. Elevated CO₂ had the largest stimulation of photosynthesis in B. papyrifera sun plants and A. rubrum shade plants.
3. Higher photosynthesis per unit leaf area in sun plants than in shade plants of B. papyrifera was largely owing to differences in leaf morphology. Acer rubrum exhibited sun/shade differences in photosynthesis per unit leaf mass consistent with biochemical acclimation to shade.
4. Betula papyrifera exhibited CO₂ responses that would facilitate tolerance to leaf water deficits in large sun patches, including osmotic adjustment and higher transpiration and stomatal conductance at a given leaf-water potential, whereas A. rubrum exhibited large increases in photosynthetic nitrogen-use efficiency.
5. Results suggest that species of contrasting successional ranks respond differently to elevated CO₂, in ways that are consistent with the habitats in which they typically occur.     

Adjustments of net photosynthesis in Solanum tuberosum in response to reciprocal changes in ambient and elevated growth CO2 partial pressures

Single leaf photosynthetic rates and various leaf components of potato ( Solanum tuberosum L.) were studied 1–3 days after reciprocally transferring plants between the ambient and elevated growth CO₂ treatments. Plants were raised from individual tuber sections in controlled environment chambers at either ambient (36 Pa) or elevated (72 Pa) CO₂. One half of the plants in each growth CO₂ treatment were transferred to the opposite CO₂ treatment 34 days after sowing (DAS). Net photosynthesis (P_n) rates and various leaf components were then measured 34, 35 and 37 DAS at both 36 and 72 Pa CO₂. Three-day means of single leaf P_n rates, leaf starch, glucose, initial and total Rubisco activity, Rubisco protein, chlorophyll ( a + b ), chlorophyll ( a/b ), α -amino N, and nitrate levels differed significantly in the continuous ambient and elevated CO₂ treatments. Acclimation of single leaf P_n rates was partially to completely reversed 3 days after elevated CO₂-grown plants were shifted to ambient CO₂, whereas there was little evidence of photosynthetic acclimation 3 days after ambient CO₂-grown plants were shifted to elevated CO₂. In a four-way comparison of the 36, 72, 36 to 72 (shifted up) and 72 to 36 (shifted down) Pa CO₂ treatments 37 DAS, leaf starch, soluble carbohydrates, Rubisco protein and nitrate were the only photosynthetic factors that differed significantly. Simple and multiple regression analyses suggested that negative changes of P_n in response to growth CO₂ treatment were most closely correlated with increased leaf starch levels.