Publications

Recent Publications

To view Dr. Long's curriculum vitae, click here.

Zhu, X-G, de Sturler, E., Long, S.P. (2007) Optimizing the distribution of resources between enzymes of carbon metabolism can dramatically increase photosynthetic rate: A numerical simulation using an evolutionary algorithm. Plant Physiology, 145:513-526.

The distribution of resources between enzymes of photosynthetic carbon metabolism might be assumed to have been optimized by natural selection. However, natural selection for survival and fecundity does not necessarily select for maximal photosynthetic productivity. Further, the concentration of a key substrate, atmospheric CO2, has changed more over the past 100 years than the past 25 million years, with the likelihood that natural selection has had inadequate time to reoptimize resource partitioning for this change. Could photosynthetic rate be increased by altered partitioning of resources among the enzymes of carbon metabolism? This question is addressed using an "evolutionary'' algorithm to progressively search for multiple alterations in partitioning that increase photosynthetic rate. To do this, we extended existing metabolic models of C-3 photosynthesis by including the photorespiratory pathway (PCOP) and metabolism to starch and sucrose to develop a complete dynamic model of photosynthetic carbon metabolism. The model consists of linked differential equations, each representing the change of concentration of one metabolite. Initial concentrations of metabolites and maximal activities of enzymes were extracted from the literature. The dynamics of CO2 fixation and metabolite concentrations were realistically simulated by numerical integration, such that the model could mimic well-established physiological phenomena. For example, a realistic steady-state rate of CO2 uptake was attained and then reattained after perturbing O-2 concentration. Using an evolutionary algorithm, partitioning of a fixed total amount of protein-nitrogen between enzymes was allowed to vary. The individual with the higher light-saturated photosynthetic rate was selected and used to seed the next generation. After 1,500 generations, photosynthesis was increased substantially. This suggests that the "typical'' partitioning in C-3 leaves might be suboptimal for maximizing the light-saturated rate of photosynthesis. An overinvestment in PCOP enzymes and under-investment in Rubisco, sedoheptulose-1,7-bisphosphatase, and fructose-1,6-bisphosphate aldolase were indicated. Increase in sink capacity, such as increase in ADP-glucose pyrophosphorylase, was also indicated to lead to increased CO2 uptake rate. These results suggest that manipulation of partitioning could greatly increase carbon gain without any increase in the total protein-nitrogen investment in the apparatus for photosynthetic carbon metabolism.

Wittig, V.E., Ainsworth, E.A., Long, S.P. (2007) To what extent do current and projected increases in surface ozone affect photosynthesis and stomatal conductance of trees? A meta-analytic review of the last three decades of experiments. Plant, Cell & Environment, 30:1150-1162.

The surface concentration of ozone ([O3]) has risen from less than 10 ppb prior to the industrial revolution to a day-time mean concentration of approximately 40 ppb over much of the northern temperate zone. If current global emission trends continue, surface [O3] is projected to rise a further 50% over this century, with larger increases in many locations including Northern Hemisphere forests. This review uses statistical meta-analysis to determine mean effects, and their confidence limits, of both the current and projected elevations of [O3] on light-saturated photosynthetic CO2 uptake (Asat) and stomatal conductance (gs) in trees. In total, 348 measurements of Asat from 61 studies and 266 measures of gs from 55 studies were reviewed. Results suggested that the elevation of [O3] that has occurred since the industrial revolution is depressing Asat and gs by 11% (CI 9–13%) and 13% (CI 11–15%), respectively, where CI is the 95% confidence interval. In contrast to angiosperms, gymnosperms were not significantly affected. Both drought and elevated [CO2] significantly decreased the effect of ambient [O3]. Younger trees (<4 years) were affected less than older trees. Elevation of [O3] above current levels caused progressively larger losses of Asat and gs, including gymnosperms. Results are consistent with the expectation that damage to photosynthesis depends on the cumulative uptake of ozone (O3) into the leaf. Thus, factors that lower gs lessen damage. Where both gs and [O3] were recorded, an overall decline in Asat of 0.21% per mmol m-2 of estimated cumulative O3 uptake was calculated. These findings suggest that rising [O3], an often overlooked aspect of global atmospheric change, is progressively depressing the ability of temperate and boreal forests to assimilate carbon and transfer water vapour to the atmosphere, with significant potential effects on terrestrial carbon sinks and regional hydrologies.

Bernacchi, CJ; Kimball, BA; Quarles, DR; Long, SP; Ort, DR (2007) Decreases in stomatal conductance of soybean under open-air elevation of [CO2] are closely coupled with decreases in ecosystem evapotranspiration, Plant Physiology. 143: 134-144.

Stomatal responses to atmospheric change have been well documented through a range of laboratory- and field-based experiments. Increases in atmospheric concentration of CO2 ([CO2]) have been shown to decrease stomatal conductance (g(s)) for a wide range of species under numerous conditions. Less well understood, however, is the extent to which leaf-level responses translate to changes in ecosystem evapotranspiration (ET). Since many changes at the soil, plant, and canopy microclimate levels may feed back on ET, it is not certain that a decrease ing(s) will decrease ET in rain-fed crops. To examine the scaling of the effect of elevated [CO2] on g(s) at the leaf to ecosystem ET, soybean (Glycine max) was grown in field conditions under control (approximately 375 mu mol CO2 mol(-1) air) and elevated [CO2] (approximately 550 mu mol mol(-1)) using free air CO2 enrichment. ET was determined from the time of canopy closure to crop senescence using a residual energy balance approach over four growing seasons. Elevated [CO2] caused ET to decrease between 9% and 16% depending on year and despite large increases in photosynthesis and seed yield. Ecosystem ET was linked with g(s) of the upper canopy leaves when averaged across the growing seasons, such that a 10% decrease in g(s) results in a 8.6% decrease in ET; this relationship was not altered by growth at elevated [CO2]. The findings are consistent with model and historical analyses that suggest that, despite system feedbacks, decreased g(s) of upper canopy leaves at elevated [CO2] results in decreased transfer of water vapor to the atmosphere.

Bernacchi, CJ; Leakey, ADB; Heady, LE; Morgan, PB; Dohleman, FG; McGrath, JM; Gillespie, KM; Wittig, VE; Rogers, A; Long, SP; Ort, DR (2006) Hourly and seasonal variation in photosynthesis and stomatal conductance of soybean grown at future CO2 and ozone concentrations for 3 years under fully open-air field conditions, Plant Cell and Environment 29: 2077-2090.

It is anticipated that enrichment of the atmosphere with CO2 will increase photosynthetic carbon assimilation in C3 plants. Analysis of controlled environment studies conducted to date indicates that plant growth at concentrations of carbon dioxide ([CO2]) anticipated for 2050 (similar to 550 mu mol mol(-1)) will stimulate leaf photosynthetic carbon assimilation (A) by 20 to 40%. Simultaneously, concentrations of tropospheric ozone ([O-3]) are expected to increase by 2050, and growth in controlled environments at elevated [O-3] significantly reduces A. However, the simultaneous effects of both increases on a major crop under open-air conditions have never been tested. Over three consecutive growing seasons > 4700 individual measurements of A, photosynthetic electron transport (J(PSII)) and stomatal conductance (g(s)) were measured on Glycine max (L.) Merr. (soybean). Experimental treatments used free-air gas concentration enrichment (FACE) technology in a fully replicated, factorial complete block design. The mean A in the control plots was 14.5 mu mol m(-2) s(-1). At elevated [CO2], mean A was 24% higher and the treatment effect was statistically significant on 80% of days. There was a strong positive correlation between daytime maximum temperatures and mean daily integrated A at elevated [CO2], which accounted for much of the variation in CO2 effect among days. The effect of elevated [CO2] on photosynthesis also tended to be greater under water stress conditions. The elevated [O-3] treatment had no statistically significant effect on mean A, g(s) or J(PSII) on newly expanded leaves. Combined elevation of [CO2] and [O-3] resulted in a slightly smaller increase in average A than when [CO2] alone was elevated, and was significantly greater than the control on 67% of days. Thus, the change in atmospheric composition predicted for the middle of this century will, based on the results of a 3 year open-air field experiment, have smaller effects on photosynthesis, g(s) and whole chain electron transport through photosystem II than predicted by the substantial literature on relevant controlled environment studies on soybean and likely most other C3 plants.

Leakey, ADB; Bernacchi, CJ; Ort, DR; Long, SP (2006) Long-term growth of soybean at elevated [CO2] does not cause acclimation of stomatal conductance under fully open-air conditions, Plant Cell and Environment 29: 1794-1800.

Accurately predicting plant function and global biogeochemical cycles later in this century will be complicated if stomatal conductance (g(s)) acclimates to growth at elevated [CO2], in the sense of a long-term alteration of the response of g(s) to [CO2], humidity (h) and/or photosynthetic rate (A). If so, photosynthetic and stomatal models will require parameterization at each growth [CO2] of interest. Photosynthetic acclimation to long-term growth at elevated [CO2] occurs frequently. Acclimation of g(s) has rarely been examined, even though stomatal density commonly changes with growth [CO2]. Soybean was grown under field conditions at ambient [CO2] (378 mu mol mol(-1)) and elevated [CO2] (552 mu mol mol(-1)) using free-air [CO2] enrichment (FACE). This study tested for stomatal acclimation by parameterizing and validating the widely used Ball et al. model (1987, Progress in Photosynthesis Research, vol IV, 221-224) with measurements of leaf gas exchange. The dependence of g(s) on A, h and [CO2] at the leaf surface was unaltered by long-term growth at elevated [CO2]. This suggests that the commonly observed decrease in g(s) under elevated [CO2] is due entirely to the direct instantaneous effect of [CO2] on g(s) and that there is no longer-term acclimation of g(s) independent of photosynthetic acclimation. The model accurately predicted g(s) for soybean growing under ambient and elevated [CO2] in the field. Model parameters under ambient and elevated [CO2] were indistinguishable, demonstrating that stomatal function under ambient and elevated [CO2] could be modelled without the need for parameterization at each growth

Long, SP; Ainsworth, EA; Leakey, ADB; Nosberger, J; Ort, DR (2006) Food for thought: Lower-than-expected crop yield stimulation with rising CO2 concentrations, Science 312: 1918- 1921.

Model projections suggest that although increased temperature and decreased soil moisture will act to reduce global crop yields by 2050, the direct fertilization effect of rising carbon dioxide concentration ([CO2]) will offset these tosses. The CO2 fertilization factors used in models to project future yields were derived from, enclosure studies conducted approximately 20 years ago. Free-air concentration enrichment (FACE) technology has now facilitated large-scale trials of the major grain crops at elevated [CO2] under fully open-air field conditions. In those trials, elevated [CO2] enhanced yield by similar to 50% less than in enclosure studies. This casts serious doubt on projections that rising [CO2] will fully offset losses due to climate change.

Rogers, A; Gibon, Y; Stitt, M; Morgan, PB; Bernacchi, CJ; Ort, DR; Long, SP (2006) Increased C availability at elevated carbon dioxide concentration improves N assimilation in a legume, Plant Cell and Environment 29: 1651-1658.

Plant growth is typically stimulated at elevated carbon dioxide concentration ([CO2]), but a sustained and maximal stimulation of growth requires acquisition of additional N in proportion to the additional C fixed at elevated [CO2]. We hypothesized that legumes would be able to avoid N limitation at elevated [CO2]. Soybean was grown without N fertilizer from germination to final senescence at elevated [CO2] over two growing seasons under fully open-air conditions, providing a model legume system. Measurements of photosynthesis and foliar carbohydrate content showed that plants growing at elevated [CO2] had a c. 25% increase in the daily integral of photosynthesis and c. 58% increase in foliar carbohydrate content, suggesting that plants at elevated [CO2] had a surplus of photosynthate. Soybeans had a low leaf N content at the beginning of the season, which was a further c. 17% lower at elevated [CO2]. In the middle of the season, ureide, total amino acid and N content increased markedly, and the effect of elevated [CO2] on leaf N content disappeared. Analysis of individual amino acid levels supported the conclusion that plants at elevated [CO2] overcame an early-season N limitation. These soybean plants showed a c. 16% increase in dry mass at final harvest and showed no significant effect of elevated [CO2] on leaf N, protein or total amino acid content in the latter part of the season. One possible explanation for these findings is that N fixation had increased, and that these plants had acclimated to the increased N demand at elevated [CO2].

Schroeder, JB; Gray, ME; Ratcliffe, ST; Estes, RE; Long, SP (2006) Effects of elevated Co-2 and O-3 on a variant of the western corn rootworm (Coleoptera : Chrysomelidae), Environmental Entomology 35: 7-44.

Global atmospheric carbon dioxide concentration [CO2] increased 20% in the last century and is expected to increase another 48% by 2050. Surface concentrations of ozone [0,] also are increasing rapidly in agricultural areas of the northern hemisphere and are expected to increase another 20% by 2050. To better understand the combined impact of increased [CO2] and [O-3] on crop production in the Midwest, we used a 32-ha experimental field planted with soybean (Glycines max L. Merr.). The field included sixteen 21.3-m-diameter plots using free air gas concentration enrichment (FACE) to simulate the changes in atmospheric composition forecast for 2050. We evaluated the impact of elevated [CO2] and [O-3] singly and in combination on adult and egg densities of the variant western corn rootworm, Diabrotica virgifera virgifera LeConte. During 2003-2004, each of the four blocks included four plots, representing the four treatments (ambient atmospheric control, elevated [O-3] elevated [CO2] and an elevated [CO2] and [O-3] combination). Although elevated [CO2] and [O-3] did not have an effect on adult female densities of the variant western corn rootworm, they did significantly affect egg densities. Approximately twice as many eggs were found in the soil of soybean plots exposed to elevated [CO2] compared with ambient control plots or those with elevated [O-3]. Egg densities were even greater (approximately three times) in plots with the elevated [CO2] and [O-3] combination treatment than in ambient plots. This suggests that rising [CO2] and [O-3] are stimulating egg-laying by the variant western corn rootworm, potentially increasing population densities and risk of damage to the subsequent corn crop.

Davey, PA; Olcer, H; Zakhleniuk, O; Bernacchi, CJ; Calfapietra, C; Long, SP; Raines, CA (2006) Can fast-growing plantation trees escape biochemical down-regulation of photosynthesis when grown throughout their complete production cycle in the open air under elevated carbon dioxide? Plant Cell and Environment 29: 1235-1244.

Poplar trees sustain close to the predicted increase in leaf photosynthesis when grown under long-term elevated CO2 concentration ([CO2]). To investigate the mechanisms underlying this response, carbohydrate accumulation and protein expression were determined over four seasons of growth. No increase in the levels of soluble carbohydrates was observed in the young expanding or mature sun leaves of the three poplar genotypes during this period. However, substantial increases in starch levels were observed in the mature leaves of all three poplar genotypes grown in elevated [CO2]. Despite the very high starch levels, no changes in the expression of photosynthetic Calvin cycle proteins, or in the starch biosynthetic enzyme ADP-glucose pyrophosphorylase (AGPase), were observed. This suggested that no long-term photosynthetic acclimation to CO2 occurred in these plants. Our data indicate that poplar trees are able to 'escape' from long-term, acclimatory down-regulation of photosynthesis through a high capacity for starch synthesis and carbon export. These findings show that these poplar genotypes are well suited to the elevated [CO2] conditions forecast for the middle of this century and may be particularly suited for planting for the long-term carbon sequestration

Farage, PK; Blowers, D; Long, SP; Baker, NR (2006) Low growth temperatures modify the efficiency of light use by photosystem II for CO2 assimilation in leaves of two chilling-tolerant C-4 species, Cyperus longus L. and Miscanthus x giganteus, Plant Cell and Environment 29: 720-728.

Two C-4 plants, Miscanthus x giganteus and Cyperus longus L., were grown at suboptimal growth temperatures and the relationships between the quantum efficiencies of photosynthetic electron transport through photosystem II (PSII) (PSII operating efficiency; F-q'/F-m') and CO2 assimilation (phi(CO2)) in leaves were examined. When M. x giganteus was grown at 10 degrees C, the ratio of the PSII operating efficiency to phi(CO2) increased relative to that found in leaves grown at 14 and 25 degrees C. Similar increases in the F-q'/F-m' : phi(CO2) occurred in the leaves of two C. longus ecotypes when the plants were grown at 17 degrees C, compared to 25 degrees C. These elevations of F-q'/F-m' : phi(CO2) at low growth temperatures were not attributable to the development of anthocyanins, as has been suggested for maize, and were indicative of the operation of an alternative sink to CO2 assimilation for photosynthetic reducing equivalents, possibly oxygen reduction via a Mehler reaction, which would act as a mechanism for protection of PSII from photoinactivation and damage. Furthermore, in M. x giganteus grown at 10 degrees C, further protection of PSII was effected by a 20-fold increase in zeaxanthin content in dark-adapted leaves, which was associated with much higher levels of non-photochemical quenching of excitation energy, compared to that observed in leaves grown at 14 and 25 degrees C. These differences may explain the long growing season and remarkable productivity of this C-4 plant in cool climates, even in comparison to other C-4 species such as C. longus, which occur naturally in such climates.

Morgan, PB; Mies, TA; Bollero, GA; Nelson, RL; Long, SP (2006) Season-long elevation of ozone concentration to projected 2050 levels under fully open-air conditions substantially decreases the growth and production of soybean, New Phytologist 170: 333-343.

Mean surface ozone concentration is predicted to increase 23% by 2050. Previous chamber studies of crops report large yield losses caused by elevation of tropospheric ozone, and have been the basis for projecting economic loss. This is the first study with a food crop (soybean, Glycine max) using free-air gas concentration enrichment (FACE) technology for ozone fumigation. A 23% increase in ozone concentration from an average daytime ambient 56 p.p.b. to a treatment 69 p.p.b. over two growing seasons decreased seed yield by 20%. Total above-ground net primary production decreased by 17% without altering dry mass allocation among shoot organs, except seed. Fewer live leaves and decreased photosynthesis in late grain filling appear to drive the ozone-induced losses in production and yield. These results validate previous chamber studies suggesting that soybean yields will decrease under increasing ozone exposure. In fact, these results suggest that when treated under open-air conditions yield losses may be even greater than the large losses already reported in earlier chamber studies. Yield losses with elevated ozone were greater in the second year following a severe hailstorm, suggesting that losses caused by ozone might be exacerbated by extreme climatic events.

Long, SP; Zhu, XG; Naidu, SL; Ort, DR (2006) Can improvement in photosynthesis increase crop yields? Plant Cell and Environment 29: 315-330.

The yield potential (Y-p) of a grain crop is the seed mass per unit ground area obtained under optimum growing conditions without weeds, pests and diseases. It is determined by the product of the available light energy and by the genetically determined properties: efficiency of light capture (epsilon(i)), the efficiency of conversion of the intercepted light into biomass (epsilon(c)) and the proportion of biomass partitioned into grain (eta). Plant breeding brings eta and epsilon(i) close to their theoretical maxima, leaving epsilon(c), primarily determined by photosynthesis, as the only remaining major prospect for improving Y-p. Leaf photosynthetic rate, however, is poorly correlated with yield when different genotypes of a crop species are compared. This led to the viewpoint that improvement of leaf photosynthesis has little value for improving Y-p. By contrast, the many recent experiments that compare the growth of a genotype in current and future projected elevated [CO2] environments show that increase in leaf photosynthesis is closely associated with similar increases in yield. Are there opportunities to achieve similar increases by genetic manipulation? Six potential routes of increasing epsilon(c) by improving photosynthetic efficiency were explored, ranging from altered canopy architecture to improved regeneration of the acceptor molecule for CO2. Collectively, these changes could improve epsilon(c) and, therefore, Y-p by c. 50%. Because some changes could be achieved by transgenic technology, the time of the development of commercial cultivars could be considerably less than by conventional breeding and potentially, within 10-15 years.

Wall, GW; Garcia, RL; Kimball, BA; Hunsaker, DJ; Pinter, PJ; Long, SP; Osborne, CP; Hendrix, DL; Wechsung, F; Wechsung, G; Leavitt, SW; LaMorte, RL; Idso, SB (2006) Interactive effects of elevated carbon dioxide and drought on wheat, Agronomy Journal: 354-381.

Atmospheric CO2 concentration (C-a) continues to rise. An imperative exists, therefore, to elucidate the interactive effects of elevated C-a and drought on plant water relations of wheat (Triticum aestivum L.). A spring wheat (cv. Yecora Rojo) crop was exposed to ambient (Control: 370 mu mol mol(-1)) and free-air CO2 enrichment (FACE: ambient + 180 mu mol mol(-1)) under ample (Wet), and reduced (Dry), water supplies (100 and 541% replacement of evapotranspiration, respectively) over a 2-yr study. Our objective was to characterize and quantify the responses of 26 edaphic, gas exchange, water relations, carbohydrate pool dynamics, growth, and development parameters to rising C-a and drought. Increasing C-a minimized the deleterious effects of soil-water depiction by increasing drought avoidance (i.e., lower stomatal conductance and transpiration rate, and growth and development of a more robust root system) and drought tolerance (i.e., enhanced osmoregulation and adaptation of tissue) mechanisms, resulting in a 30% reduction in water stress-induced midafternoon depressions in net assimilation rate. An elevated C-a-based increase in daily and seasonal carbon gain resulted in a positive feedback between source capacity (shoots) and sink demand (roots). Devoid of a concomitant rise in global temperature resulting from the rise in C-a, improved water relations for a herbaceous, cool-season, annual, C-3 cereal monocot grass (i.e., wheat) are anticipated in a future high-CO2 world. These findings are applicable to other graminaceous species of a similar function-type as wheat common to temperate zone grassland prairies and savannas, especially tinder dryland conditions.

Leakey, ADB; Uribelarrea, M; Ainsworth, EA; Naidu, SL; Rogers, A; Ort, DR; Long, SP (2006) Photosynthesis, productivity, and yield of maize are not affected by open-air elevation of CO2 concentration in the absence of drought, Plant Physiology 140: 779-790.

While increasing temperatures and altered soil moisture arising from climate change in the next 50 years are projected to decrease yield of food crops, elevated CO2 concentration ([CO2]) is predicted to enhance yield and offset these detrimental factors. However, C-4 photosynthesis is usually saturated at current [CO2] and theoretically should not be stimulated under elevated [CO2]. Nevertheless, some controlled environment studies have reported direct stimulation of C-4 photosynthesis and productivity, as well as physiological acclimation, under elevated [CO2]. To test if these effects occur in the open air and within the Corn Belt, maize (Zea mays) was grown in ambient [CO2] (376 mu mol mol(-1)) and elevated [CO2] (550 mmol mol(-1)) using Free-Air Concentration Enrichment technology. The 2004 season had ideal growing conditions in which the crop did not experience water stress. In the absence of water stress, growth at elevated [CO2] did not stimulate photosynthesis, biomass, or yield. Nor was there any CO2 effect on the activity of key photosynthetic enzymes, or metabolic markers of carbon and nitrogen status. Stomatal conductance was lower (-34%) and soil moisture was higher (up to 31%), consistent with reduced crop water use. The results provide unique field evidence that photosynthesis and production of maize may be unaffected by rising [CO2] in the absence of drought. This suggests that rising [CO2] may not provide the full dividend to North American maize production anticipated in projections of future global food supply.

Zhu, XG; Govindjee; Baker, NR; deSturler, E; Ort, DR; Long, SP (2005) Chlorophyll a fluorescence induction kinetics in leaves predicted from a model describing each discrete step of excitation energy and electron transfer associated with photosystem II, Planta 223: 114-133.

Chlorophyll a fluorescence induction (FI) is widely used as a probe for studying photosynthesis. On illumination, fluorescence emission rises from an initial level O to a maximum P through transient steps, termed J and I. FI kinetics reflect the overall performance of photosystem II (PSII). Although FI kinetics are commonly and easily measured, there is a lack of consensus as to what controls the characteristic series of transients, partially because most of the current models of FI focus on subsets of reactions of PSII, but not the whole. Here we present a model of fluorescence induction, which includes all discrete energy and electron transfer steps in and around PSII, avoiding any assumptions about what is critical to obtaining O J I P kinetics. This model successfully simulates the observed kinetics of fluorescence induction including O J I P transients. The fluorescence emission in this model was calculated directly from the amount of excited singlet-state chlorophyll in the core and peripheral antennae of PSII. Electron and energy transfer were simulated by a series of linked differential equations. A variable step numerical integration procedure (ode15s) from MATLAB provided a computationally efficient method of solving these linked equations. This in silico representation of the complete molecular system provides an experimental workbench for testing hypotheses as to the underlying mechanism controlling the O J I P kinetics and fluorescence emission at these points. Simulations based on this model showed that J corresponds to the peak concentrations of Q(A)(-)Q(B) (Q(A) and Q(B) are the first and second quinone electron acceptor of PSII respectively) and Q(A)(-)Q(B)(-) and I to the first shoulder in the increase in concentration of Q(A)(-)Q(B)(2-). The P peak coincides with maximum concentrations of both Q(A)(-)Q(B)(2-) and PQH(2). In addition, simulations using this model suggest that different ratios of the peripheral antenna and core antenna lead to differences in fluorescence emission at O without affecting fluorescence emission at J, I and P. An increase in the concentration of Q(B)-nonreducing PSII centers leads to higher fluorescence emission at O and correspondingly decreases the variable to maximum fluorescence ratio (F-v/F-m).

Long, SP; Ainsworth, EA; Leakey, ADB; Morgan, PB (2005) Global food insecurity. Treatment of major food crops with elevated carbon dioxide or ozone under large-scale fully open-air conditions suggests recent models may have overestimated future yields, Philosophical Transactions of the Royal Society B-Biological Sciences 360: 2011-2020.

Predictions of yield for the globe's major grain and legume arable crops suggest that, with a moderate temperature increase, production may increase in the temperate zone, but decline in the tropics. In total, global food supply may show little change. This security comes from inclusion of the direct effect of rising carbon dioxide (CO2) concentration, [CO2], which significantly stimulates yield by decreasing photorespiration in C-3 crops and transpiration in all crops. Evidence for a large response to [CO2] is largely based on studies made within chambers at small scales, which would be considered unacceptable for standard agronomic trials of new cultivars or agrochemicals. Yet, predictions of the globe's future food security are based on such inadequate information. Free-Air Concentration Enrichment (FACE) technology now allows investigation of the effects of rising [CO2] and ozone on field crops under fully open-air conditions at an agronomic scale. Experiments with rice, wheat, maize and soybean show smaller increases in yield than anticipated from studies in chambers. Experiments with increased ozone show large yield losses (20%), which are not accounted for in projections of global food security. These findings suggest that current projections of global food security are overoptimistic. The fertilization effect of CO2 is less than that used in many models, while rising ozone will cause large yield losses in the Northern Hemisphere. Unfortunately, FACE studies have been limited in geographical extent and interactive effects of CO2, ozone and temperature have yet to be studied. Without more extensive study of the effects of these changes at an agronomic scale in the open air, our ever-more sophisticated models will continue to have feet of clay.

Dermody, O; Long, SP; DeLucia, EH (2005) How does elevated CO2 or ozone affect the leaf-area index of soybean when applied independently? New Phytologist 169: 145-155.

Changes in leaf-area index (LAI) may alter ecosystem productivity in elevated [CO2] or [O-3]. By increasing the apparent quantum yield of photosynthesis (phi(c,max)), elevated [CO2] may increase maximum LAI. However, [O-3] when elevated independently accelerates senescence and may reduce LAI.
Large plots (20 m diameter) of soybean (Glycine max) were exposed to ambient (approx. 370 mu mol mol(-1)) or elevated (approx. 550 mu mol mol(-1)) CO2 or 1.2 times ambient [O-3] using soybean free-air concentration enrichment (SoyFACE).
In 2001 elevated CO2 had no detectable effect on maximum LAI, but in 2002 maximum LAI increased by 10% relative to ambient air. Elevated [CO2] also increased the phi(c,max) of shade leaves in both years. Elevated [CO2] delayed LAI loss to senescence by approx. 54% and also increased leaf-area duration. Elevated [O-3] accelerated senescence, reducing LAI by 40% near the end of the growing season. No effect of elevated [O-3] on photosynthesis was detected.
Elevated [CO2] or [O-3] affected LAI primarily by altering the rate of senescence; knowledge of this may aid in optimizing future soybean productivity.

Gielen, B; Calfapietra, C; Lukac, M; Wittig, VE; De Angelis, P; Janssens, IA; Moscatelli, MC; Grego, S; Cotrufo, MF; Godbold, DL; Hoosbeek, MR; Long, SP; Miglietta, F; Polle, A; Bernacchi, CJ; Davey, PA; Ceulemans, R; Scarascia-Mugnozza, GE (2005) Net carbon storage in a poplar plantation (POPFACE) after three years of free-air CO2 enrichment, Tree Physiology 25: 1399-1408.

A high-density plantation of three genotypes of Populus was exposed to an elevated concentration of carbon dioxide ([CO2]; 550 mu mol mol(-1)) from planting through canopy closure using a free-air CO2 enrichment (FACE) technique. The FACE treatment stimulated gross primary productivity by 22 and 11% in the second and third years, respectively. Partitioning of extra carbon (C) among C pools of different turnover rates is of critical interest; thus, we calculated net ecosystem productivity (NEP) to determine whether elevated atmospheric [CO2] will enhance net plantation C storage capacity. Free-air CO2 enrichment increased net primary productivity (NPP) of all genotypes by 21% in the second year and by 26% in the third year, mainly because of an increase in the size of C pools with relatively slow turnover rates (i.e., wood). In all genotypes in the FACE treatment, more new soil C was added to the total soil C pool compared with the control treatment. However, more old soil C loss was observed in the FACE treatment compared with the control treatment, possibly due to a priming effect from newly incorporated root litter. FACE did not significantly increase NEP, probably as a result of this priming effect.

Morgan, PB; Bollero, GA; Nelson, RL; Dohleman, FG; Long, SP (2005) Smaller than predicted increase in aboveground net primary production and yield of field-grown soybean under fully open-air [CO2] elevation, Global Change Biology 11: 1856-1865.

The Intergovernmental Panel on Climate Change projects that atmospheric [CO2] will reach 550 ppm by 2050. Numerous assessments of plant response to elevated [CO2] have been conducted in chambers and enclosures, with only a few studies reporting responses in fully open-air, field conditions. Reported yields for the world's two major grain crops, wheat and rice, are substantially lower in free-air CO2 enrichment (FACE) than predicted from similar elevated [CO2] experiments within chambers. This discrepancy has major implications for forecasting future global food supply. Globally, the leguminous-crop soybean (Glycine max (L.) Merr.) is planted on more land than any other dicotyledonous crop. Previous studies have shown that total dry mass production increased on average 37% in response to increasing [CO2] to approximately 700 ppm, but harvestable yield will increase only 24%. Is this representative of soybean responses under open-air field conditions? The effects of elevation of [CO2] to 550 ppm on total production, partitioning and yield of soybean over 3 years are reported. This is the first FACE study of soybean (http://www.soyface.uiuc.edu) and the first on crops in the Midwest of North America, one of the major food production regions of the globe. Although increases in both aboveground net primary production (17-18%) and yield (15%) were consistent across three growing seasons and two cultivars, the relative stimulation was less than projected from previous chamber experiments. As in previous studies, partitioning to seed dry mass decreased; however, net production during vegetative growth did not increase and crop maturation was delayed, not accelerated as previously reported. These results suggest that chamber studies may have over-estimated the stimulatory effect of rising [CO2], with important implications on global food supply forecasts.

Wittig, V.E., Bernacchi, C.J., Zhu, X., Calfapietra, C., Ceulemans, R., DeAngelis, P., Gielen, B., Miglietta, F., Morgan, P.B., Long, S.P. (2005) Gross primary production is stimulated for three Populus species grown under free-air CO2 enrichment from planting through canopy closure. Global Change Biology, 11: 644-656.

How forests will respond to rising [CO2] in the long term is uncertain, most studies having involved juvenile trees in chambers prior to canopy closure. Poplar free-air CO2 enrichment (Viterbo, Italy) is one of the first experiments to grow a forest from planting through canopy closure to coppice, entirely under open-air conditions using free-air CO2 enrichment technology. Three Populus species: P. alba, P. nigra and P. x euramericana, were grown in three blocks, each containing one control and one treatment plot in which CO2 was elevated to the expected 2050 concentration of 550 ppm. The objective of this study was to estimate gross primary production (GPP) from recorded leaf photosynthetic properties, leaf area index (LAI) and meteorological conditions over the complete 3-year rotation cycle. From the meteorological conditions recorded at 30 min intervals and biweekly measurements of LAI, the microclimate of leaves within the plots was estimated with a radiation transfer and energy balance model. This information was in turn used as input into a canopy microclimate model to determine light and temperature of different leaf classes at 30 min intervals which in turn was used with the steady-state biochemical model of leaf photosynthesis to compute CO2 uptake by the different leaf classes. The parameters of these models were derived from measurements made at regular intervals throughout the coppice cycle. The photosynthetic rates for different leaf classes were summed to obtain canopy photosynthesis, i.e. GPP. The model was run for each species in each plot, so that differences in GPP between species and treatments could be tested statistically. Significant stimulation of GPP driven by elevated [CO2] occurred in all 3 years, and was greatest in the first year (223–251%), but markedly lower in the second (19–24%) and third years (5–19%). Increase in GPP in elevated relative to control plots was highest for P. nigra in 1999 and for P. x euramericana in 2000 and 2001, although in 1999 P. alba had a higher GPP than P. x euramericana. Our analysis attributed the decline in stimulation to canopy closure and not photosynthetic acclimation. Over the 3-year rotation cycle from planting to harvest, the cumulative GPP was 4500, 4960 and 4010 g C m2 for P. alba, P. nigra and P. x euramericana, respectively, in current [CO2] and 5260, 5800 and 5000 g C m2 in the elevated [CO2] treatments. The relative changes were consistent with independent measurements of net primary production, determined independently from biomass increments and turnover.

Pimentel, C; Davey, PA; Juvik, JA; Long, SP (2005) Gene loci in maize influencing susceptibility to chilling dependent photoinhibition of photosynthesis, Photosynthesis Research 85: 319-326.

Variation in tolerance in chilling-dependent photoinhibition has been associated with a wide range of traits in comparative physiological studies. A sweet corn (Zea mays L.) population of 214 F-2:3 families previously mapped to near-saturation with 93 RFLP DNA markers were subjected to low temperature and high-light events prior to measurement of the maximum dark-adapted quantum efficiency of PS II (F-v/F-m), to identify loci associated with variation in chilling-dependent photoinhibition. In the first assay with ten families varying in seedling growth and germination, significant differences were observed among families in their response to and recovery from exposure to high light at low temperature. All the 214 F-2:3 families from this population were then evaluated for tolerance of chilling-dependent photoinhibition in a controlled environment and then in three replicated trials in the field, each following naturally occurring chilling events during spring. The measured effects on F-v/F-m were analyzed with software that mapped segregating loci that regulate trait expression and linked to genetic markers (PLABQTL). QTL 3.096 (i.e. 96 cM on chromosome three) was consistently identified in both controlled environment and in the mean of the three field trails. Another QTL at 8.025, described the greatest percentage of total phenotypic variance (ca. 10%) for the mean reduction in F-v/F-m of all three periods of measurement in the field. A third QTL (4.136) showed a highly significant association in the third field trial. These three QTLs were closely associated with genes that have been mechanistically related to photoinhibition tolerance and repair. The results suggest that the ratio of F-v/F-m is an approach that may be used in establishing marker-assisted breeding for improved tolerance to chilling of maize in the light and in turn better early-season growth in cool temperate climates.

Ainsworth, EA; Long, SP (2005) What have we learned from 15 years of free-air CO2 enrichment (FACE)? A meta-analytic review of the responses of photosynthesis, canopy, New Phytologist 165: 351-371.

How forests will respond to rising [CO2] in the long term is uncertain, most studies having involved juvenile trees in chambers prior to canopy closure. Poplar free-air CO2 enrichment (Viterbo, Italy) is one of the first experiments to grow a forest from planting through canopy closure to coppice, entirely under open-air conditions using free-air CO2 enrichment technology. Three Populus species: P. alba, P. nigra and P. x euramericana, were grown in three blocks, each containing one control and one treatment plot in which CO2 was elevated to the expected 2050 concentration of 550 ppm. The objective of this study was to estimate gross primary production (GPP) from recorded leaf photosynthetic properties, leaf area index (LAI) and meteorological conditions over the complete 3-year rotation cycle. From the meteorological conditions recorded at 30 min intervals and biweekly measurements of LAI, the microclimate of leaves within the plots was estimated with a radiation transfer and energy balance model. This information was in turn used as input into a canopy microclimate model to determine light and temperature of different leaf classes at 30 min intervals which in turn was used with the steady-state biochemical model of leaf photosynthesis to compute CO2 uptake by the different leaf classes. The parameters of these models were derived from measurements made at regular intervals throughout the coppice cycle. The photosynthetic rates for different leaf classes were summed to obtain canopy photosynthesis, i.e. GPP. The model was run for each species in each plot, so that differences in GPP between species and treatments could be tested statistically. Significant stimulation of GPP driven by elevated [CO2] occurred in all 3 years, and was greatest in the first year (223-251%), but markedly lower in the second (19-24%) and third years (5-19%). Increase in GPP in elevated relative to control plots was highest for P. nigra in 1999 and for P. x euramericana in 2000 and 2001, although in 1999 P. alba had a higher GPP than P. x euramericana. Our analysis attributed the decline in stimulation to canopy closure and not photosynthetic acclimation. Over the 3-year rotation cycle from planting to harvest, the cumulative GPP was 4500, 4960 and 4010 g C m(-2) for P. alba, P. nigra and P. x euramericana, respectively, in current [CO2] and 5260, 5800 and 5000 g C m(-2) in the elevated [CO2] treatments. The relative changes were consistent with independent measurements of net primary production, determined independently from biomass increments and turnover.

Bernacchi CJ, Morgan PB, Ort DR, Long SP (2005) The growth of soybean under free air [CO2] enrichment (FACE) stimulates photosynthesis while decreasing in vivo Rubisco capacity Planta, 220: 434-446.

Down-regulation of light-saturated photosynthesis (A,a,) at elevated atmospheric CO2 concentration, [CO2], has been demonstrated for many C-3 species and is often associated with inability to utilize additional photosynthate and/or nitrogen limitation. In soybean, a nitrogen-fixing species, both limitations are less likely than in crops lacking an N-fixing symbiont. Prior studies have used controlled environment or field enclosures where the artificial environment can modify responses to [CO2]. A soybean free air [CO2] enrichment (FACE) facility has provided the first opportunity to analyze the effects of elevated [CO2] on photosynthesis under fully open-air conditions. Potential ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) carboxylation (V-c,(max)) and electron transport through photosystem II (J(max)) were determined from the responses of A(sat) to intercellular [CO2] (C-i) throughout two growing seasons. Mesophyll conductance to CO2 (g(m)) was determined from the responses of A(sat) and whole chain electron transport (J) to light. Elevated [CO2] increased Asat by 15-20% even though there was a small, statistically significant, decrease in Vc,max. This differs from previous studies in that Vc,(max)/J(max) decreased, inferring a shift in resource investment away from Rubisco. This raised the Ci at which the transition from Rubisco-limited to ribulose-1,5-bisphosphate regeneration-limited photosynthesis occurred. The decrease in V-c,(max) was not the result of a change in gm, which was unchanged by elevated [CO2]. This first analysis of limitations to soybean photosynthesis under fully open-air conditions reveals important differences to prior studies that have used enclosures to elevate [CO,,], most significantly a smaller response of A(sat) and an apparent shift in resources away from Rubisco relative to capacity for electron transport.

What have we learned from 15 years of free-air CO2 enrichment (FACE)? A meta-analytic review of the responses of photosynthesis, canopy (2005) Ainsworth EA, Long SP, New Phytologist: 351-371.

Free-air CO2 enrichment (FACE) experiments allow study of the effects of elevated [CO2] on plants and ecosystems grown under natural conditions without enclosure. Data from 120 primary, peer-reviewed articles describing physiology and production in the 12 large-scale FACE experiments (475-600 ppm) were collected and summarized using meta-analytic techniques. The results confirm some results from previous chamber experiments: light-saturated carbon uptake, diurnal C assimilation, growth and above-ground production increased, while specific leaf area and stomatal conductance decreased in elevated [CO2]. There were differences in FACE. Trees were more responsive than herbaceous species to elevated [CO2]. Grain crop yields increased far less than anticipated from prior enclosure studies. The broad direction of change in photosynthesis and production in elevated [CO2] may be similar in FACE and enclosure studies, but there are major quantitative differences: trees were more responsive than other functional types; C-4 species showed little response; and the reduction in plant nitrogen was small and largely accounted for by decreased Rubisco. The results from this review may provide the most plausible estimates of how plants in their native environments and field-grown crops will respond to rising atmospheric [CO2]; but even with FACE there are limitations, which are also discussed.

Stephen Long Lab