Abstract Growth and photosynthetic responses of dwarfapple saplings (Malus domestica Borkh. cv. Fuji) accli-mated to 3 years of exposure to contrasting atmosphericCO2 concentrations (360 and 650 µmol mol–1) in combi-nation with current ambient or elevated (ambient +5°C)temperature patterns were determined. Four 1-year-oldapple saplings grafted onto M.9 rootstocks were each en-closed in late fall 1997 in a controlled environment unitin nutrient-optimal soil. Soil moisture regimes were au-tomatically controlled by drip irrigation scheduled at50 kPa of soil moisture tension. For the elevated CO2concentration alone, overall tree growth was suppressed.However, tree growth was slightly enhanced whenwarmer temperatures were combined with the elevatedCO2 concentration. Neither temperature nor CO2 concen-tration affected leaf chlorophyll content and stomataldensity. The elevated CO2 concentration decreased meanleaf area, but increased starch accumulation, thus result-ing in a higher specific dry mass of leaves. An elevatedtemperature reduced starch accumulation. Light-saturat-ed rates of leaf photosynthesis were suppressed due tothe elevated CO2 concentration, but this effect was re-moved or enhanced with warmer temperatures. The ele-vated CO2 concentration increased the optimum temper-
ature for photosynthesis by ca. 4°C, while the warmertemperature did not. The results of this study suggestedthat the long-term adaptation of apple saplings to growthat an elevated CO2 concentration may be associated witha potential for increased growth and productivity, if adoubling of the CO2 concentration also leads to elevatedtemperatures.
Keywords Apple · Elevated carbon dioxide concentration · Temperature · Photosynthesis · Growth
Introduction
Evidence suggests that higher plants respond to a risingambient atmospheric CO2 concentration by increasingtheir CO2 uptake (Ciais et al. 1995). According to globalclimate change scenarios, atmospheric CO2 concentra-tions are predicted almost double within the twenty-firstcentury if current emissions are not reduced, and thisdoubling of the CO2 concentration will increase themean surface temperature of the earth by about 2–6°C(Burroughs 2001). The atmospheric CO2 concentrationand temperature are concomitant factors influencing theglobal environment (Morison and Lawlor 1999), so theresponse of higher plants to rising CO2 concentration,temperature, and their possible interactions is of signifi-cantinterest for future agricultural and natural productiv-ity (Fritschi et al. 1999). Recent studies confirm that theimpact of global warming beyond a certain limit mayhave serious consequences for agricultural productivity(Lal etal. 1998).
One of the more sensitive and intriguing responses ofplants to elevated CO2 and temperature is the acclima-tion of photosynthesis (Stitt 1991). There is abundant evidence that photosynthesis acclimates to elevated CO2(Curtis 1996; Weber et al. 1994). Short-term exposure ofplants to elevated CO2 stimulates the rate of photosyn-thesis and biomass production (DeLucia et al. 1999).However, the effects of long-term exposure among dif-
H.M. Ro (✉ ) · I.B. LeeDepartment of Horticultural Environment, National Horticultural Research Institute, RDA, 475 Imok-dong,Jangan-ku, Suwon 440-310, Koreae-mail: [emailprotected].: +82-31-2403716, Fax: +82-31-2403556
P.G. KimNational Instrumentation Center for Environmental Management,College of Agricultural and Life Sciences, Seoul National University, Suwon 441-744, Korea
M.S. YiemNational Alpine Agricultural Experiment Station, RDA, Phyongchang-kun, Gangwon-do 232-955, Korea
S.Y. WooDepartment of Forest Resources, Sangju National University,Sangju 742-711, Korea
Trees (2001) 15:195–203DOI 10.1007/s004680100099
O R I G I N A L A RT I C L E
Hee-Myong Ro · Pan-Gi Kim · In-Bog LeeMyoung-Soon Yiem · Su-Young Woo
Photosynthetic characteristics and growth responses of dwarf apple(Malus domestica Borkh. cv. Fuji) saplings after 3 years of exposure to elevated atmospheric carbon dioxide concentration and temperature
Received: 16 November 2000 / Accepted: 2 March 2001 / Published online: 6 April 2001© Springer-Verlag 2001
ferent plant species are conflicting. In general, prolongedexposure to elevated CO2 reduces the initial stimulationof photosynthesis in many species, and frequently sup-presses photosynthesis, due in part to excess accumula-tion of starch in leaves, which probably hinders CO2 dif-fusion within the chloroplast (Makino 1994; Nafzigerand Koller 1976). However, such photosynthetic suppression cannot be so great for species which have strong sink organs for carbohydrate accumulation(Makino and Mae 1999).
In general, an increase in temperature counters thesuppression of photosynthesis due to an elevated CO2concentration (Drake et al. 1997), but the effects on plantgrowth are either positive or negative depending on thespecies (Reddy et al. 1998). From this interaction, it isdeduced that the optimum temperature for the maximalrate of CO2 assimilation must increase by about 6°C withan increase in the CO2 concentration to 670 µmol mol–1
(Long 1991). This interaction between CO2 and tempera-ture could, therefore, be of profound importance for fu-ture agricultural productivity, but there is little informa-tion regarding the nature of this interaction (Morison andLawlor 1999).
The effect of an increased CO2 concentration on plantgrowth is primarily due to changes in the compositionand dry mass per unit area of leaves (Roderick et al.1999), and the factors influencing these changes are pri-marily temperature dependent. Stomatal density is indi-cative of the extent of the acclimation of photosynthesisto a changing CO2 concentration (Sage 1994), and gener-ally decreases with an increase in CO2. However, this de-crease is not universally observed, and varies amongspecies (Estiarte et al. 1994; Woodward et al. 1991). In contrast to the general response to an elevated CO2 concentration, Maherali and DeLucia (2000) ob-served an increased leaf-specific hydraulic conductivityof Ponderosa pine exposed to elevated temperature. They hypothesized that this response should increasestomatal conductance and, therefore, transpirationalcooling.
Apple, one of the commercially important temperatefruits, has been widely cultivated from prehistoric times.The global demand for apples and their products has notslowed down, and thus apple producers face the chal-lenge of producing more apples from less area in an energy-efficient way (Ro and Park 2000). However, it isnot clear whether such an increase in productivity can besustained or achieved if global warming occurs. As ele-vated atmospheric temperature and CO2 concentrationare expected to be part of our future climate, it is impor-tant to understand and quantify the responses of appletrees to these two interactive environmental factors.
We measured the growth responses of dwarf apple(Malus domestica Borkh. cv. Fuji) saplings after 3 yearsof exposure to elevated CO2,to elevated temperature, and to these factors in combination. To understand themechanisms of the responses, leaf photosynthesis wasmeasured. We hypothesized that: (1) high temperatureswould ameliorate the effects of elevated atmospheric
CO2; (2) fruit yield would interact with the effects of atmospheric CO2, temperature, and their interactions;and (3) long-term exposure to elevated CO2 would shiftthe optimum temperature for photosynthesis to a highertemperature.
Materials and methods
CO2- and temperature-controlled closed environment facility for plant growth
The closed-environment plant-growth facility, sunk into the soil,consists of two rows of four experimental units each. Each unitcomprises a soil compartment (3×3×3 m), a transparent canopyenclosure (4×3×6 m), and a utility space for the temperature-con-trol unit located on the north side. A weather station measures airtemperature and relative humidity, wind speed and direction, solarradiation, and rainfall using a datalogger (21X, Campbell Scientif-ic, USA). An open-architecture, distributed control system (DCS),POREX 6800 (POSCON Institute 1996), developed by POSCONof POHANG Steel, Korea, was used to control the facility and theautomated collection of the sensors’ data. POREX 6800 DCS consists of two UNIX-based workstations (SPARCstation 20, SunMicrosystems, USA) providing user-friendly man-machine inter-faces for process operation and engineering works, and a processcontrol station performing various real-time processing for fieldinput/output points, and scheduled transfer of real-time data to acentral database.
Each unit individually controls atmospheric CO2 and air tem-perature. Compressed CO2 gas is mixed with the flow of fresh airdepending on the preset CO2 concentration of the bulk air in theenclosure, and the mixture is brought into the enclosure by the airblower. The purity of the CO2 gas was regularly inspected. Chilledor heated water is supplied to the fan coil unit depending onwhether cooling or heating is required. Conditioned air passesthrough the plant canopy with sufficient flux to cause slight leafflutter, and returns to the outlet duct just above the soil level.
An atmospheric CO2 concentration at ±1 Pa of a predeterminedset value and temperature at ±0.5°C of an ambient regime were individually controlled in each unit. Reference values for the ambient regime were taken real-time from a weather station. Thesolar radiation, air temperatures and relative humidity inside theenclosure were measured and multiplexed to a 21X datalogger. Atmospheric CO2 concentrations in the enclosure are measuredusing infrared CO2 analysers (ZRH, Fuji Electric, Japan), and theaddition of CO2 prior to inlet points is thus controlled. Elevatedtemperatures included a 5°C step above the ambient regime. Fourprofiles of time-domain reflectometry probes were installed hori-zontally with respect to roots at 0.15-m intervals, 0.15–1.20 m below the soil surface, and three tensiometers at depths of 0.15,0.45, and 0.75 m were also installed. Soil temperatures were mea-sured from calibrated RTD temperature sensors.
Tree culture under controlled climate conditions
Apple (M. domestica) cv. Fuji was selected because itrepresents alarge portion of commercial apple production in Korea. Each soilcompartment was back-filled with a sandy loam soil (Ro and Park2000) in 1996, and was stabilized for 2 years. Four nursery appletrees each grafted onto a M.9 rootstock were transplanted in eachcompartment during late fall in 1997. Four units were maintainedat 360 µmol CO2 mol–1, while the other four were maintained at650 µmol CO2 mol–1. The air temperature inside two of the unitsmaintained at 360 µmol CO2 mol–1 mimicked the ambient temper-ature pattern, while that of the remaining two was kept at an ele-vated temperature of +5°C. The same temperature treatments wereused for the four elevated-CO2 units. Both CO2 and temperaturetreatments were initiated after transplanting and lasted three con-
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secutive growing seasons. The relative humidity inside the unitvaried mostly between 60% and 80%. Soil moisture regimes wereautomatically controlled by drip irrigation scheduled at 50 kPa ofsoil moisture tension, which is the current cultural practice fordrip-irrigated apples (Ro and Park 2000). During measurements,the leaf water potential of the trees ranged between –0.1 and–0.4 MPa. Currently recommended N-P-K fertilization rates forapple trees were chosen to maintain optimum foliar nutrient con-centrations.
Photosynthetic and growth responses, and leaf morphology
Three fully expanded, mature source leaves from three shoots (leafnumber 12 counted from each shoot apex) of each tree were taggedfor gas exchange measurements. Gas exchange measurements weredone simultaneously with four portable photosynthesis measure-ment systems (LI-6400, Li-Cor, USA) during a 2-h period, 0900–1100 hours, for several days during the fruit-maturation stage. Thelight responses of leaf photosynthesis were obtained under twoCO2 partial pressures of 36 Pa and 65 Pa by varying the irradianceinside the chamber of each portable photosynthesis measurementsystem. Responses were approximated by fitting Michaelis-Mentenkinetics to the measured CO2 fixation rate versus photosyntheticphoton flux density (PPFD). Irradiance was chosen in ten stepsfrom 2,000 to 0 µmol m–2 s–1 of PPFD on the adaxial surfaces ofleaves. The CO2 responses of leaf photosynthesis (A/Ci curves)were obtained at low (150 µmol m–2 s–1) and high PPFD(1,500 µmol m–2 s–1) with the CO2 injector system (LI-6400-01, Li-Cor) and liquid CO2 by varying the CO2 concentration in the chamber (Ca) from 1,000 to 0 µmol mol–1. The intercellularCO2 concentration (Ci) was calculated based on the equation of Farquhar and Sharkey (1982). The CO2 fixation response was alsomeasured by varying leaf temperature under a fixed CO2 concen-tration and a PPFD level greater than the light saturation point.
At the end of the experiment, tree growth was assessed usingfour trees (two treesper unit) per treatment. For each, tree height,crown diameter, shoot length, and numbers of shoots and leaveswere measured. Shoot length was determined as the cumulativelength of all shoots. A sub-sample of 30 leaves tree–1, taken from the middle, was measured with an automatic leaf area meter(LI-3100, Li-Cor), and dry mass of those leaves was determined tocalculate specific leaf mass (leaf dry mass per unit leaf area).
Three fully mature source leaves per tree were sampled neartagged leaves for gas-exchange measurements. Chlorophyll wasextracted with 10 ml dimethyl sulphoxide solution (Hiscox and Israelstam 1979). The total chlorophyll content of leaves was determined by adding chlorophyll a and b contents determinedspectroscopically (MacKinney 1941).Sixteen fully mature sourceleaves from four shoots (leaf number 13–16 counted from each shoot apex) per tree were sampled before dawn (0500–0530 hours) on 5 September during the fruit maturation stage. The
collected leaf samples were immediately oven-dried at 105°C for30 min, and subsequently at 70°C overnight. Dried leaf sampleswere ground to pass through a 40-mesh sieve. The starch concen-tration in leaves was determined by measuring the glucose concentration of amyloglucosidase digests following a modifiedprocedure of Reddy et al. (1998) with a HPLC (SP8800, Spectra-Physics, USA) equipped with a SugarPak-1 column and aRISE-61 refractive index detector. Stomatal density was countedon scanning electron photomicrographs (200×) taken with a S-2460 N SEM (Hitachi, Japan) on the same portion of abaxialsurfaces of three randomly selected leaves per tree. Ultra-thin sec-tions obtained from leaf transverse sections taken at the point ofmaximum leaf width across the main vein were stained and exam-ined with a transmission electron microscope (LEO-906E, Zeiss,Germany).
Statistical analysis
Data were evaluated using General Linear Models procedures(SAS Institute 1990). Data were analysed with Tukey’s studenti-zed range (honestly significant difference) test after a two-wayANOVA for the completely randomized design to compare thesignificance of two factors and the effects of temperature and CO2concentration treatments as well as the effects of their interactionat the significance level of 0.05. Additionally, least square differ-ence was applied to test the significance among treatments. Finally, data were tested by multivariate ANOVA (MANOVA) toexamine response variables.
Results and discussion
Tree growth
The elevated temperature significantly increased treeheight, crown diameter, shoot length, and numbers ofshoots and leaves, while elevated CO2 did not (Table 1).There was a significant temperature×CO2 interaction fortree height, shoot length, and numbers of shoots andleaves, but not for crown diameter. Despite the insignifi-cant effect with regard to crown diameter, tree growthwas enhanced under elevated CO2 when coupled with elevated temperature, but was suppressed under elevatedCO2 alone, compared to trees grown under ambient tem-perature and CO2 concentration (control). The decreasedtree growth after long-term exposure to elevated CO2was similar to results with other C3 plants, where bio-
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Table 1 Effects of atmospheric temperature and CO2 concentration on crown diameter, tree height, shoot length, and numbers of shootsand leaves. LSD0.05 least significant difference at 0.05 level
Temperature CO2 concentration Crown Tree Shoot Number Number(µmol mol–1) diameter height length of of
(cm) (cm) (cm) shoots leaves
Ambient 360 210.8 268.2 1,793.3 38.8 1,313.8650 187.4 273.4 1,299.8 34.0 864.5
Ambient+5°C 360 224.8 276.4 1,667.7 43.3 1,082.0650 236.0 332.0 1,961.7 50.3 1,383.5
LSD0.05 40.1 27.3 176.1 7.0 189.8ANOVATemperature * *** *** *** *CO2 concentration n.s. n.s. n.s. n.s. n.s.Temperature×CO2 concentration n.s. *** *** * ***
***P<0.001, **P<0.01, *P<0.05, n.s. not significant
mass production was reduced due to a decreased rate ofphotosynthesis (Drake et al. 1997).
In contrast, simultaneous exposure to increased tem-perature and CO2 resulted in a significant increase inheight, diameter, and dry mass of seedlings in anotherstudy (Sheu and Lin 1999). Farrar and Williams (1991)suggested that higher temperatures would increase thedegree of inorganic phosphate recycling to the chloro-plast bystimulating sucrose synthesis and hence permitenhanced rates of assimilate production for CO2-en-riched leaves. In addition, temperature increased thetransport of assimilates and sink metabolism, and thismight explain increases in tree growth above that seenfor CO2-enriched trees alone.
Light-response curves of photosynthesis
Leaves grown in elevated CO2 demonstrated a lower rateof photosynthesis than leaves grown in ambient CO2.When measured at PPFD levels greater than the light sat-uration point under 36 Pa CO2 partial pressure (Fig. 1A),the photosynthetic mechanism of leaves of trees was acclimated to elevated CO2 and, as a result, leaves hadlower maximum net photosynthesis. Pan et al. (1998) observed a suppression of photosynthesis in 1-year-oldmaiden apple (cv. Gala) plants following 8 days of expo-sure to elevated CO2, but the values were still higher thancontrol values. In both their and our studies, when lightintensity was inadequate, the light-harvesting abilities ofleaves exposed to elevated CO2 were also lower thanthose exposed to ambient CO2. In general, suppression ofphotosynthesis of plants grown in elevated CO2 relativeto control plants is apparent when both are measured atambient CO2 levels (Drake et al. 1997). Elevated temper-ature, however, increased the rate of photosynthesis ofleaves exposed to ambient or elevated CO2 concentra-tions. This implies that the stimulatory effects of elevatedtemperature offsets the reduction in photosynthesis due toelevated CO2 (Farrar and Williams 1991).
On the other hand, the rate of photosynthesis mea-sured at 65 Pa CO2 was higher than that measured at am-bient CO2 due to the increase in the CO2 concentration(Fig. 1B). In particular, a 1-h exposure to elevated CO2during measurements stimulated the rate of photosynthe-sis of trees grown at ambient CO2 concentration, thussuggesting that the current ambient CO2 concentration isinsufficient to saturate Rubisco (Drake et al. 1997). Asmentioned, long-term acclimation to elevated CO2 sup-pressed photosynthesis unless accompanied by exposureto warmer temperatures. Nagy et al. (2000) observed thatthe maximum net photosynthesis of elevated-CO2 treesincreased more during warmer seasons than during cooler seasons under their experimental conditions. Itwas also noted that the light-saturated rate of photosyn-thesis seemed to increase even at 2,000 µmol m–2 s–1
under this elevated CO2 partial pressure (65 Pa).Compared to light-saturated responses under 36 Pa
CO2 (Fig. 1A), the increasing effect due to elevated
temperature was similar irrespective of treatment CO2levels. In particular, regardless of growth temperature, asudden exposure of elevated-CO2 grown leaves to 36 PaCO2 immediately increased the difference (2.0–5.4 µmolm–2 s–1 for ambient, and 0.9–3.2 µmol m–2 s–1 for elevat-ed temperature conditions) in the light-saturated rate ofphotosynthesis between the two CO2 treatments, sug-gesting a probable impedance to intracellular CO2 diffu-sion (Makino 1994).
Light saturation and compensation points, dark respiration, maximum net photosynthesis, and apparent quantum yield
The light saturation point was greater under exposure to65 Pa CO2 (816 µmol m–2 s–1 for control trees) than un-der exposure to 36 Pa CO2 (622 µmol m–2 s–1 for controltrees), while the reverse (14 µmol m–2 s–1 at 36 Pa and10 µmol m–2 s–1 at 65 Pa CO2 for control trees) was thecase for the light compensation point (Table 2).
An elevated temperature and CO2 concentration sig-nificantly increased dark respiration. Several reportshave found a decrease in dark respiration during suddenexposure to elevated CO2 (Amthor et al. 1992). How-ever, the dark respiration of Chamaecyparis obtusa in-creased with an increasing CO2 concentration and tem-perature (Nagy et al. 2000). Maximum net photosynthe-sis (Amax) ranged from 9.9 to 25.9 µmol m–2 s–1. For treesgrown at a given CO2 level, increasing the measurementCO2 level increased Amax (e.g. from 15.3 to 23.2 andfrom 9.9 to 21.2 µmol m–2 s–1). On the other hand, for agiven measurement CO2 level, increasing the treatmentCO2 level decreased Amax (from 15.3 to 9.9 and from23.2 to 21.2 µmol m–2 s–1). Long-term exposure to an elevated temperature did not greatly change the patterndescribed above, but significantly reduced the degree ofsuppression of Amax due to elevated CO2. Overall, tem-perature did have an appreciable effect on the light com-
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Fig. 1 Light-response curves for photosynthesis of leaves mea-sured at 36 Pa (A) and 65 Pa (B) partial CO2 pressure. Vertical barat each data point denotes ±SD of the mean when larger thansymbol size
Temperature CO2 Light Light Dark Maximum net Apparent concentration compensation point saturation point respiration photosynthesis quantum yield (µmol mol–1) (µmol m–2 s–1) (µmol m–2 s–1) (µmol CO2 m–2 s–1) (µmol CO2 m–2 s–1) (mmol CO2 mol–1)
36 Pa 65 Pa 36 Pa 65 Pa 36 Pa 65 Pa 36 Pa 65 Pa 36 Pa 65 Pa
Ambient 360 14 10 622 816 0.8 0.5 15.3 23.2 35.5 41.8650 18 9 371 652 0.8 0.6 9.9 21.2 26.3 44.3
Ambient 360 14 11 682 968 0.8 0.8 16.8 25.9 36.0 46.8+5°C 650 25 18 695 1,019 1.3 0.9 13.7 25.0 31.0 39.8LSD0.05 2 2 38 121 0.2 0.1 1.4 1.2 2.9 3.3
ANOVATemperature *** *** *** *** ** *** *** *** * n.s.CO2 concentration *** ** *** n.s. *** *** *** ** *** n.s.Temperature×CO2 *** *** *** * ** n.s. * n.s. * ***concentration
***P<0.001, **P<0.01, *P<0.05, n.s. not significant
pensation and saturation points, dark respiration, andAmax.
Apparent quantum yield (Table 2) was estimated byfitting a linear regression to the measured rate of photo-synthesis versus PPFD over the range 0–300 µ mol m–2
s–1 (Evans 1987). Elevated CO2 significantly suppressedthe apparent quantum yield at 36 Pa CO2, but the degreeof suppression was significantly reduced by elevatedtemperature. However, temperature and CO2 concentra-tion did not affect the apparent quantum yield at 65 PaCO2, indicating a significant interaction between temper-ature and CO2 concentration. Overall, the apparent quan-tum yield was greatest in trees grown at elevated CO2with a concurrent increase in temperature.
Leaf characteristics
Neither temperature nor CO2 concentration significantlyaffected chlorophyll content and stomatal density ofleaves (Table 3). However, there was a significant tem-perature×CO2 concentration interaction for stomatal den-sity. Woodward (1987) compared stomatal density in
herbarium material from the mid-nineteenth century tothe present day and observed declines, but several stud-ies found that atmospheric CO2 concentrations did notaffect stomatal density (Estiarte et al. 1994). ElevatedCO2 significantly decreased the mean area of individualleaves (Sheu and Lin 1999), but increased specific leafdry mass (Sage et al. 1989). Pan et al. (1998) observedan increase in specific dry mass of apple leaves, and sug-gested that it was due to the accumulation of starch.However, temperature did not significantly affect themean area and specific dry mass of leaves.
Starch accumulation
Elevated CO2 led to a significantly greater accumulationof starch in leaves. In contrast, elevated temperature ledto a decrease in starch (Table 3). Transmission electronphotomicrographs of mesophyll cells showed more starchgrains in the leaves grown in elevated CO2, and reducedstarch accumulation with concurrent exposure to elevatedtemperature (Fig. 2). Elevated CO2 usually led to an in-creased rate of photosynthesis in plants when measured at
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Table 2 Effects of atmospheric temperature and CO2 concentra-tion on light saturation and compensation points, dark respiration,maximum net photosynthesis, and apparent quantum yield of
leaves measured at 36 and 65 Pa CO2 partial pressure. LSD0.05least significant difference at 0.05 level
Table 3 Effects of atmospheric temperature and CO2 concentration on chlorophyll and starch contents, stomatal density, mean area, andspecific dry mass of leaves. LSD0.05 least significant difference at 0.05 level
Temperature CO2 Chlorophyll Starch Stomatal Mean leaf Specific leaf concentration content content density area dry mass (µmol mol–1) (g m–2) (g kg–1) (no mm–2) (cm2 leaf–1) (mg cm–2)
Ambient 360 2.07 7.73 340.3 42.0 8.2650 2.07 16.80 285.4 29.8 9.9
Ambient+5°C 360 2.18 8.60 301.1 42.0 9.1650 2.10 10.90 339.8 38.9 10.1
LSD0.05 0.34 1.28 42.0 6.9 0.9ANOVATemperature n.s. *** n.s. n.s. n.s.CO2 concentration n.s. *** n.s. ** ***Temperature×CO2 concentration n.s. *** ** n.s. n.s.
***P<0.001, **P<0.01, n.s. not significant
the same CO2 concentration at which they were grown(Fig. 1). In addition, Stitt (1991) noted increased starchaccumulation. In contrast, an elevated temperature de-creased starch accumulation in elevated-CO2 grownleaves (Farrar and Williams 1991). Starch accumulationmay increase the resistance to intracellular CO2 diffusionin elevated-CO2 grown leaves or lead to a down-regula-tion ofRubisco; however, the presence of a causal rela-tionship between starch accumulation and the inhibitionof photosynthesis remains controversial (Stitt 1991).
The accumulation of starch within leaves of CO2-enriched plants may reflect a temporary or permanentimbalance between sources and sinks. If sink capacity islow, an accumulation of photosynthate may occur withinsource leaves leading to feedback inhibition of photosyn-thesis; otherwise photosynthesis is stimulated (Herold1980). Reduced overall tree growth and significant accu-mulation of starch in leaves were observed in elevated-CO2 grown trees. In contrast, for trees grown under bothelevated CO2 and temperature, starch accumulation wasreduced, due probably to the increased utilization ofstored assimilates for the growth of sink organs within awhole tree (Tables 1, 3). For instance, fruit yield per treeaveraged for 2 years was 1.4 kg for trees grown in an elevated CO2 concentration alone, but increased to4.3 kg with a higher temperature (unpublished data).
CO2 response curves of photosynthesis
When the light intensity was limiting (i.e. 150 µmol m–2
s–1), there was no treatment-related difference in A/Cicurves (Fig. 3A). Analyses of these A/Ci curves showedthat the initial slope was as steep as that in Fig. 3B, andthe Ci value at which CO2 saturation occurred was notgreatly altered between treatments, thus indicating thatthe balance between Rubisco and RubP regeneration wasbeing maintained (Evans 1988). CO2 saturation occurredat a Ci value close to that found at ambient (36 Pa) CO2partial pressure (Fig. 1A).
When light intensity was not limiting (Fig. 3B),leaves grown at ambient CO2 and elevated temperaturehad slightly higher rates of photosynthesis compared towhen light was limiting. However, when photosynthesiswas CO2 saturated, the photosynthetic capacity increasedwith elevated temperature, regardless of the growth CO2conditions. Compared to control trees, despite the insig-nificance of the initial slope, elevated-CO2 grown treeshad a slightly lower initial slope and a less sensitive re-sponse when photosynthesis was measured at low Cithan at high Ci values. The decrease in the initial slope ofthe A/Ci response suggested that the amount of Rubiscowas lower in elevated-CO2 grown trees (Stitt 1991).
After the CO2 saturation point, the rate of photosyn-thesis of leaves exposed to elevated temperature and am-bient CO2 was significantly higher. In general, treesgrown at elevated temperature had a higher rate of pho-tosynthesis than trees grown at ambient temperature, re-gardless of the CO2 concentration in which they weregrown. Although at a given CO2 partial pressure (36 Pa),the elevated-CO2 grown trees had about two-thirds therate of photosynthesis of their counterparts grown at am-bient CO2 (Fig. 1A), they had similar intercellular CO2concentrations (about 260 µmol mol–1). This suggestedthat the lower supply of CO2 to the mesophyll cells, as aresult of increased stomatal resistance, was counter-balanced by a decreased utilization of CO2 molecules
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Fig. 2 Transmission electron photomicrographs of mesophyllcells showing starch grains (S), and arrangement of mitochondria(M) and plastoglobuli (P) within a cell. Ambient (A) or elevated(B) CO2 concentration with ambient temperature pattern, and am-bient (C) or elevated (D) CO2 concentration with a temperaturepattern 5°C above ambient. Scale bar=5 µm
Fig. 3 CO2 response curves for photosynthesis of leaves measuredat 150 µmol m–2 s–1 (A) and 1,500 µmol m–2 s–1 (B) photosyntheticphoton flux density. Vertical bar at each data point denotes ±SD ofthe mean when larger than symbol size
(Spencer and Bowes 1986). When Ci reached 600 µmolmol–1, the rate of photosynthesis in control trees de-clined, implying an inhibition to the RubP-regeneratingmechanism (Stitt 1991).
Neither temperature nor CO2 concentration affectedcarboxylation efficiency and the CO2 compensation pointmeasured at 150 µmol m–2 s–1, and photorespiration mea-sured at 1500 µmol m–2 s–1 (Table 4). Instead, they signif-icantly increased photorespiration at 150 µmol m–2 s–1.However, carboxylation efficiency and the CO2 compen-sation point at 1,500 µmol m–2 s–1 were significantly affected by CO2 concentration, but not by temperature.
Temperature-response curves of photosynthesis
The elevated CO2 concentration suppressed the rate ofCO2 fixation and increased the optimum temperature formaximal CO2 uptake of leaves of trees by 4°C, regard-less ofwhether they were grown at ambient or elevatedtemperature conditions (Fig. 4). These findings corrobo-rate the prediction of Long (1991) that the temperatureoptimum for photosynthesis must increase with CO2concentration. However, an elevated temperature did notchange the optimum temperature for maximal CO2 up-take. Leaves of trees grown at an elevated temperaturehad a significantly higher CO2 fixation rate than theleaves of trees grown at ambient temperature conditions,regardless of the CO2 concentration to which they wereexposed. This indicates that the CO2 concentration inter-acted with temperature, and that the optimum tempera-ture for photosynthesis increased with increasing CO2concentration. When elevated CO2 led to an accumula-tion of starch, an increased temperature caused starch todecrease (Fig. 2, Table 3).
Considerable research has been done on the effects ofelevated CO2 and elevated temperature on plant produc-tivity. However, little is known about the interaction be-tween elevated CO2 and elevated temperature on plantgrowth (Nagy et al. 2000). In addition, the responses of
the deciduous apple tree to elevated CO2 and tempera-ture may differ from other C3 plants, because apple, aRosaceae, synthesizes and translocates sorbitol, a sugaralcohol, in addition to sucrose (Webb and Burley 1962;Wallart 1980). Our study thus lacks supporting informa-tion about the partitioning of assimilates and their trans-location between source and sink organs within thewhole tree during growth. In particular, carbohydrate re-serves in apple trees play an essential role during the ear-ly part of the next spring’s growth (Hansen 1971). Somestudies suggested that higher carbohydrate levels associ-ated with low N availability might result in a large decrease in photosynthesis (Paul and Driscoll 1997).During our study, leaves were not deficient in N:20.5±1.4 g N kg–1 leaf for 3 years. Particularly, treesgrown under elevated CO2 alone had higher leaf N con-centrations than the rest: 19.4 g N kg–1 leaf for 1998,21.2 g N kg–1 leaf for 1999, and 21.5 g N kg–1 leaf for2000, respectively. However, the leaf N concentration intrees grown at elevated CO2 and temperature was lowerthan that of control trees as a result of the dilution of Nas a consequence of growth.
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Table 4 Effects of atmospheric temperature and CO2 concentra-tion on CO2 compensation point, carboxylation efficiency, andphotorespiration of leaves measured at photosynthetic photon flux
density levels of 150 and 1,500 µmol m–2 s–1. LSD0.05 least signifi-cant difference at 0.05 level
Fig. 4 Temperature-response curves for photosynthesis of leaves.Vertical bar at each data point denotes ±SD of the mean whenlarger than symbol size
Temperature CO2 concentration Photo-respiration Carboxylation efficiency CO2 compensation point (µmol mol–1) (µmol CO2 m–2 s–1) (µmol CO2 m–2 s–1) (µmol CO2 mol–1)
150 µmol 1,500 µmol 150 µmol 1,500 µmol 150 µmol 1,500 µmol m–2 s–1 m–2 s–1 m–2 s–1 m–2 s–1 m–2 s–1 m–2 s–1
Ambient 360 1.64 4.41 0.028 0.080 59 55650 1.88 4.44 0.029 0.081 65 57
Ambient+5°C 360 2.01 4.58 0.031 0.088 65 54650 2.07 4.73 0.029 0.077 65 61
LSD0.05 0.19 0.46 0.003 0.006 12 4ANOVATemperature *** n.s. n.s. n.s. n.s. n.s.CO2 concentration * n.s. n.s. * n.s. **Temperature×CO2 concentration n.s. n.s. n.s. ** n.s. *
***P<0.001, **P<0.01, *P<0.05, n.s. not significant
Similar to our study, Farrar and Williams (1991) foundthat the effects of increasing CO2 and temperature wereto some extent opposite. Plants grown in warmer environ-ments have less stored carbohydrates, particularly starch;plants in high-CO2 environments have more. Consequent-ly, the increased supply of assimilates provided under ahigh-CO2 environment and the increased sink strengthspermitted by warming should combine to produce largerplants (Table 1) with less inhibition of photosynthesis anda higher flux of carbohydrates whatever their pool size(Fig. 1 and Fig. 2). Our study showed that the rate of photosynthesis did not decrease in high-CO2 and -temper-ature grown leaves. However, without a study of the fluxof carbohydrate, this study could not clearly explain howa warmer temperature induces an increase in tree growthand productivity and a decrease in starch accumulation inleaves of CO2-enriched apple trees.
In the long-term, the ability of apple leaves to sustainhigher photosynthetic rates would depend on the sink-source status of the whole tree and how this is regulated.Therefore, physiological and biological mechanisms thatregulate sink-source interactions need further study.
We concluded that: (1) warmer temperature (5°C aboveambient) counteracted the suppression of growth and pho-tosynthesis of apple saplings due to an enhanced CO2 con-centration; (2) fruit yield was reduced by an elevated CO2concentration, but enhanced with concurrent warmer tem-perature, thus in turn affecting starch accumulation inleaves and the growth of whole trees; and (3) 3 years ofexposure to elevated CO2 concentration increased the op-timum temperature of photosynthesis by 4°C, whilewarmer temperature did not. Despite the insignificance ofseveral response variables, MANOVA of all responsevariables showed that overall responses were significantlyinfluenced by temperature, CO2 concentration, and theirinteractions. Part of this study demonstrated that the stim-ulation of photosynthesis of dwarf apple saplings by an elevated CO2 concentration did not disappear for 3 years,and suggested that the long-term adaptation of apple sap-lings to growth at an elevated CO2 concentration may beassociated with a potential for increased growth and pro-ductivity, if a doubling of the CO2 concentration causes asimultaneous increase in the atmospheric temperature.
References
Amthor JS, Koch GW, Boom AJ (1992) CO2 inhibits respirationin leaves of Rumex crispus L. Plant Physiol 98:1–4
Burroughs WJ (2001) Climate change – a multidisciplinary approach. Cambridge University Press, New York
Ciais P, Trans PP, Trolier M, White JWC, Francey RJ (1995) A large northern hemisphere terrestrial CO2 sink indicated bythe 13C/12C ratio of atmospheric CO2. Science 269:1098–1102
Curtis PS (1996) A meta-analysis of leaf gas exchange and nitro-gen in trees grown under elevated CO2 in situ. Plant Cell Envi-ron 19:127–137
DeLucia EH, Hamilton JG, Naidu SL, Thomas RB, Andrews JA,Finzi A, Lavine m, Matamala R, Mohan JE, Hendrey GR,Schlesinger WH (1999) Net primary production of a forest ecosystem with experimental CO2 enrichment. Science 284:1177–1179
Drake BG, Gonzàlez-Meler MA, Long SP (1997) More efficientplants: a consequence of rising atmospheric CO2? Annu RevPlant Physiol Plant Mol Biol 48:609–639
Estiarte M, Penuelas J, Kimball BA, Idso SB, Lamorte RL, PinterPJ Jr, Wall GW (1994) Elevated CO2 effects on stomatal den-sity of wheat and sour orange trees. J Exp Bot 45:1665–1668
Evans JR (1987) The dependence of quantum yield on wavelengthand growth irradiance. Aust J Plant Physiol 14:68–79
Evans JR (1988) Acclimation by the thylakoid membranes togrowth irradiance and the partitioning of nitrogen between soluble and insoluble proteins. Aust J Plant Physiol 15:93–106
Farquhar GD, Sharkey TS (1982) Stomatal conductance and pho-tosynthesis. Annu Rev Plant Physiol 11:539–552
Farrar JF, Williams ML (1991) The effects of increased atmo-spheric carbon dioxide and temperatures on carbon partition-ing, source-sink relations and respiration. Plant Cell Environ14:819–830
Fritschi FB, Boote KJ, Sollenberger LE, Allen LH, Sinclair TR(1999) Carbon dioxide and temperature effects on forage es-tablishment: photosynthesis and biomass production. GlobalChange Biol 5:441–453
Hansen P (1971) 14C-studies on apple trees. VII. The early season-al growth in leaves, flowers, and shoots as dependent uponcurrent photosynthates and existing reserves. Physiol Plant25:469–473
Herold A (1980) Regulation of photosynthesis by sink activity: themissing link. New Phytol 86:131–144
Hiscox JD, Israelstam GF (1979) A method for the extraction ofchlorophyll from leaf tissue without maceration. Can J Bot57:1332–1334
Lal M, Singh KK, Rathore LS, Srinivasan G, Saseendram SA(1998) Vulnerability of rice and wheat yields in NW India to future changes in climate. Agric For Meteorol 89:101–114
Long SP (1991) Modification of the response of photosyntheticproductivity to rising temperature by atmospheric CO2 con-centration: has its importance been under-estimated? PlantCell Environ 14:729–739
MacKinney G (1941) Absorption of light by chlorophyll solutions.J Biol Chem 140:315–332
Maherali H, DeLucia EH (2000) Interactive effects of elevatedCO2 and temperature on water transport in ponderosa pine.Am J Bot 87:243–249
Makino A (1994) Biochemistry of C3-photosynthesis in high CO2.J Plant Res 107:79–84
Makino A, Mae T (1999) Photosynthesis and plant growth at ele-vated levels of CO2. Plant Cell Physiol 40:999–1006
Morison JIL, Lawlor DW (1999) Interactions between increasingCO2 concentration and temperature on plant growth. Plant CellEnviron 22:659–682
Nafziger ED, Koller HR (1976) Influence of leaf starch concentra-tion on CO2 assimilation in soybean. Plant Physiol 57:560–563
Nagy M, Ogawa K, Hagihara A (2000) Interactive effect of CO2enrichment and temperature on the photosynthesis of field-grown hinoki cypress (Chamaecyparis obtusa) branches. Trees14:282–289
Pan Q, Wang Z, Quebedeaux B (1998) Responses of the appleplant to CO2 enrichment: changes in photosynthesis, sorbitol,other sugars, and starch. Aust J Plant Physiol 25:293–297
Paul MJ, Driscoll SP (1997) Sugar repression of photosynthesis:the role of carbohydrates in signalling nitrogen deficiencythrough source:sink imbalance. Plant Cell Environ 20:110–116
POSCON Institute (1996) System overview of open architecturedistributed control system Porex 6000 series. POSCON, Korea
Reddy KR, Robana RR, Hodges HF, Liu XJ, McKinion JM (1998)Interactions of CO2 enrichment and temperature on cottongrowth and leaf characteristics. Environ Exp Bot 39:117–129
Ro HM, Park JM (2000) Nitrogen requirements and vegetativegrowth of pot-lysimeter-grown “Fuji” apple trees fertilized bydrip irrigation with three nitrogen rates. J Hortic Sci Biotech75:237–242
202
Roderick ML, Berry SL, Noble IR (1999) The relationship be-tween leaf composition and morphology at elevated CO2 con-centrations. New Phytol 143:63–72
Sage RF (1994) Acclimation of photosynthesis to increasing atmo-spheric CO2: the gas exchange prespective. Photosynth Res39:351–368
Sage RF, Sharkey TD, Seemann JR (1989) Acclimation of photo-synthesis to elevated CO2 in five C3 species. Plant Physiol89:590–596
SAS Institute (1990) SAS/STAT guide for personal computers.Version 6.03. SAS Institute, Cary, N.C.
Sheu BH, Lin CK (1999) Photosynthetic response of seedlings ofthe sub-tropical tree Schima superba with exposure to elevatedcarbon dioxide and temperature. Environ Exp Bot 41:57–65
Spencer W, Bowes G (1986) Photosynthesis and growth of waterhyacinth under CO2 enrichment. Plant Physiol 82:528–533
Stitt M (1991) Rising CO2 levels and their potential significancefor carbon flow in photosynthetic cells. Plant Cell Environ14:741–762
Wallart RAM (1980) Distribution of sorbitol in Rosaceae. Phyto-chemistry 19:2603–2610
Webb KL, Burley JWA (1962) Sorbitol translocation in apple. Science 137:766
Weber AN, Nie GY, Long SP (1994) Acclimation of photosynthet-ic proteins to rising atmospheric CO2. Photosynth Res 39:413–425
Woodward FI (1987) Stomatal numbers are sensitive to increasesin CO2 from preindustrial levels. Nature 327:617–618
Woodward FI, Thomas GB, McKee IF (1991) The effects of elevated concentrations of carbon dioxide on individual plants, populations, communities, and ecosystems. Ann Bot67:23–38
203
