On the Reason for the Different Photosynthetic Rates of ... - NCBI

Plant Physiol. (1980) 66, 940-944 0032-0889/80/66/0940/05/$00.50/0

On the Reason for the Different Photosynthetic Rates of Seedlings of Pinus silvestris and Betula verrucosa1 Received for publication January 11, 1980 and in revised form June 19, 1980

LAILA BRUNES, GUNNAR OQUIST, AND LENNART ELIASSON Department of Plant Physiology, University of Umed, S-901 87 Umed, Sweden of different morphological and metabolic factors for determining a certain rate of net photosynthesis is today usually looked for by The growth and net photosynthetic properties of seedlings of Pinus correlative analyses in statistical probability terms (e.g. refs. 5, 17, silvestris L. and Betula verrucosa Ehrh., grown under identical conditions in and 19). a controUled environment chamber, were compared. The relative growth Here, an approach for measuring the extent to which differences rate of birch was about twice that of pine. The rates of in situ net in the rates of in situ net photosynthesis between species depend photosynthesis were 1.50 and 2.30 micromoles CO2 meter 2 second-' and on morphological or metabolic factors is described. First, the rate the photosynthetic quantum yields under light-limiting conditions were of in situ net photosynthesis, which is dependent on both morpho0.022 and 0.032 for pine and birch, respectively. The total leaf surface logical and metabolic factors, is measured. Second, the absorbed areas were used for calculating the CO2 flux densities. The difference in flux densities of both and quanta are CO2 determined in totally the rates of in situ net photosynthesis depended equally on morphological diffuse radiation of different intensities. Diffuse radiation factors. It was assumed and metabolic that a pronounced mutual shading mizes mutual shading between leaves and branches (20, 22),minithus and an unfavorable leaf inclination made the pine seedlings less efficient in minimizing the influence on the measurements by the morphologthe unidirectional absorbing light of the climate chamber than the broad- ical factors. This is achieved by placing the gas-exchange cuvette leaved seedlings of birch. Both pine and birch were adapted to the growth in the center of a light-integrating Ulbricht sphere (18). By comconditions so the flux densities of absorbed quanta were rate-limiting for paring the photosynthetic properties determined by the two in situ net photosynthesis. It was concluded that the difference in the cedures, it can be determined in what respect differences inprothe photosynthetic quantum yields (ie. the linear slope of the photosynthetic rate of net photosynthesis between plants depend on morphologlight curve) of the two species defined the metabolically controlled part of ical and/or metabolic factors. The method is exemplified in a the difference in the rate of in situ net photosynthesis. The quantum yield comparative study between seedlings of Pinus silvestris and Betula of pine was lower than that of birch and was partly explained by pine verrucosa. having a higher rate of photorespiration than birch. The remaining differAmong woody plants, evergreen conifers usually show lower ence was most likely controlled by the properties of the chloroplast rates of net photosynthesis and growth than do deciduous broadthylakoids, e.g. energy transfer efficiency between pigments, photosynthetic leaved trees, shrubs, and herbaceous plants (9, 12, 14). This electron transport, or coupling between electron transport and photophos- difference may be attributed to a more pronounced mutual shadphorylation. ing between leaves and branches in conifers than in broad-leaved species (13). Zelawski et al. (22) showed that the rates of lightsaturated net photosynthesis in seedlings of P. silvestris and Picea abies can be comparable with those of the most efficient broadleaved trees if mutual shading is minimized in diffuse light. The different rates of net photosynthesis between the two species may The question often arises whether differences in the rate of net also depend on metabolic factors as the photosynthetic quantum photosynthesis (or photosynthetic productivity) between clones, yield of P. silvestris seems to be lower than that of B. verrucosa ABSTRACT

varieties, or species depend on morphological (anatomical, structural) or metabolic (physiological, biochemical) factors. Exposure of plants to different environmental conditions also induces adaptation in photosynthesis, both on the morphological and on the metabolic levels (4, 21). The morphological factors, ie. shape and angle of leaves and branches, leaf internal structure, and Chl

content determine the amount of quanta absorbed by the plant per unit of time in a particular light environment, whereas the capacities of the metabolic factors, i.e. photosynthesis, photorespiration, and respiration, determine the efficiency with which absorbed light quanta are transformed into chemical equivalents. The reason for differences in the rate of net photosynthesis between plants in a particular environment is important in the construction and application of models for photosynthesis, and it is particularly important to know in selecting and crossing for varieties with high photosynthetic productivity. The importance ' This work was supported by the Swedish Natural Science Research Council.

(18).

MATERIALS AND METHODS Plant Materials and Growing Conditions. Seeds of P. silvestris L. (pine) and B. verrucosa Ehrh. (birch) were germinated on moist filter paper in Petri dishes in a controlled environment chamber (conditions as stated below). After germination (10 days for pine and 14 days for birch), the seedlings were mounted in cushions of plastic foam and placed on nutrient solution (1, 11) in aerated black plastic pots (1000 ml). In the beginning, six seedlings were placed in each pot but, after 3 weeks, the number was reduced to four seedlings/pot. The nutrient solution was renewed once a week. The seedlings were grown in a controlled environment chamber with conditions as follows: photoperiod, 17 h; temperature, 25 C in light and 15 C in dark; RH, 70 + 5 %; quantum flux density (400-750 nm), 230 ,umol m-2 s-'. Light sources were metal halogen lamps, Osram HQI-T 400 W/DW. Quantum flux density was measured with Quantaspectrometer QSM 2500-S (Techtum Instrument AB, Umea, Sweden). 940

Plant Physiol. Vol. 66, 1980

PHOTOSYNTHESIS OF PINUS and BETULA

Growth Analyses. Fresh weight and dry weight (oven-dried at 105 C for 24 h) of shoot and root were determined at weekly intervals during 6 weeks. Five seedlings were randomly chosen at each time of harvesting, and the experiment was replicated three times. RGR2 (g g-' day-) was calculated from dry weight measurement using the equation (6): In RGR

Wt2-In Wt I (1)

=

2- tl

where W is total dry weight at times t1 and t2. RGR was also calculated from the photosynthetic rates and respiration data of the seedlings when measured under the same light and temperature conditions as used when growing the plants in the controlled environment chamber (in situ). The following formula was used: RGR

=c X[(Pn

x

17)-(Rdx

7 + Rroot

x 24)]

941

used in photosynthetic measurements, the RGR (g g-' day-1) of pine and birch were 0.07 (day 50) and 0.15 (day 40), respectively (Fig. 2). RGR was also calculated (equation 2) from in situ steadystate values of net photosynthesis (Pn), dark respiration of shoot (Rd), and root respiration (Rr..t) (Table I), values which were measured under the same light and temperature conditions as used when cultivating the seedlings in the controlled environment chamber. The two different methods used to calculate RGR gave very similar results (Table I). Table I also gives the Chl contents per unit total leaf area (both sides) of pine and birch. Photosynthetic Measurements. The rate of photosynthesis was expressed as a function of the absorbed quantum flux density. The quantum yield (pmol absorbed CO2 m-2 s-' per umol absorbed quanta m2 s-1), as calculated from the slope of the linear part of

3.0 (2)

where P,n is the photosynthetic rate of the shoot in g CO2 g-1 dry weight, Rd is the dark respiration rate of the shoot in g CO2 g-1 dry weight, Rrot is the root respiration rate in g CO2 g-' dry weight, 17, 7, and 24 are h/day, 12 and 44 are the mol wt of C and C02, respectively, and c is 0.455 and 0.451 g carbon g-1 dry weight in birch and pine, respectively. The conversion factors c were determined for whole plants by the National Swedish Laboratory for Agricultural Chemistry, Uppsala, Sweden (Leco WR- 12 Carbon Determinator). Photosynthetic Measurement. Photosynthesis of seedlings of pine and birch were measured about 50 and 40 days after germination, respectively. Net photosynthesis, dark respiration, and root respiration of intact plants were determined in an open system (16) using an IR gas analyzer (URAS I, Hartmann and Braun). Air with 2.0 and 21.2 kPa 02 containing 600 ± 10 mg m-3 CO2 was taken from compressed air. The assimilation chamber was made of Plexiglas and water-cooled with a jacket (10). A wind speed of 0.5 m s-1 was produced by a fan. The light and temperature conditions of the climate chamber were used for the measurements (in situ). Root respiration was measured at 100%1o RH in

darkness. Quantum yield measurements were performed in a light-integrating Ulbricht sphere constructed for measuring the quantum flux absorbed by the plant (18). Inasmuch as the gas-exchange cuvette was placed in the center of the sphere, this set-up provided a totally diffuse radiation for the photosynthetic measurements. To get the absorbed quantum or CO2 flux densities, the fluxes were divided by the total leaf area, i.e. the combined areas of both sides of the leaf. For leaf area determination, all leaves of birch were photocopied, and the total leaf area was determined by planimetry. In pine, the relation between length and area of the needles was determined by optical planimetry for 400 needles. A correction factor of 2 x 1.09 x length was derived and used for calculating total needle area. Transpiration was measured using differential thermocouple psychrometers (3) or solid-state humidity sensors (Vaisala OY, Finland). Leaf and air temperatures were measured with copperconstantan thermocouples. Stomatal resistances for CO2 were calculated as described by Gastra (7). Chl content was determined according to Arnon (2) after extraction of needles or leaves with 80o acetone in a glass homogenizer.

2.0c

0. 1.0 0

-J

_

I

.

.

A

60 50 30 40 20 Days from germination FIG. 1. The relationship between age of seedlings and mean total dry weight. (@), log mg dry weight of B verrucosa; (A), log mg dry weight of P. silvestris. Each point gives the average of 15 seedlings and vertical bars indicate SD of the mean.

10

_0.20 cn 0)

b-

_ 0.10

3:

0

cm

0)

x

RESULTS Growth Analyses. The growth of the seedlings was followed for 8 weeks (Figs. I and 2). During this time, the growth rate (equation 1) of birch was about twice that of pine. When the seedlings were 2

Abbreviation: RGR, relative growth rate.

I

.

I.

60 50 40 30 20 Days from germination FIG. 2. RGR (equation l; g dry weight g- dry weight day ') in seedlings of B. verrucosa (0) and P. silvestris (A). Each point is the average of five seedlings. SD of the mean is indicated. 10

BRUNES, OQUIST, AND ELIASSON

942


Table I. Photosynthetic Rate (Pn), Dark Respiration (Rd), Root Respiration (Rt), RGR, and Chl Content of Seedlings of P. silvestris and B. verrucosa Photosynthesis and respiration were measured under the same light and temperature conditions as used when cultivating the seedlings in the controlled environment chamber. The values are for three representative seedlings enclosed in the cuvette. RGR, and RGR2 were calculated from CO2 flux and dry weight measurements, respectively. SD is given for the Chl measurements. Species Pn (shoot) Rd (shoot) Rt RGR, RGR2 Chl g g' day-' g C02 g-' dry wt.h-' tsmol dm-2 P silvestris 9.66 x l0-3 2.22 x 10-3 1.27 x 10-3 0.07 0.07 1.52 ± 0.07

(n 9) B. verrucosa

17.42 x l0-3

5.44 X 10-3

the curve, was in normal air (21.2 kPa 02)0.022 for pine and 0.032 for birch (Figs. 3 and 4). When photorespiration was suppressed by a reduced O2 content (2.0 kPa 02), the quantum yield of pine and birch increased to 0.033 and 0.040, respectively (Figs. 5 and 6). The reduction of the partial pressure of 02 thus increased the photosynthetic quantum yield of pine by 33% and that of birch by

20%/o.

The in situ steady-state photosynthetic rates of pine and birch were 1.50 and 2.30,umol CO2 m-2 s-', respectively. Notice that total leaf area instead of projected leaf area was used for the calculations. The stomatal resistances for CO2 (during in situ photosynthesis) were 1422 and 1048 s m-' for pine and birch, respectively. These values are far too low to have any effect on the photosynthetic rates in situ. When the values of in situ photosynthesis were inserted in the photosynthetic curves of Figures 3 and 4, it could be determined (dotted lines) that the absorbed quantum flux densities for pine and birch were 70 and 88 Amol quanta m-2 s-', respectively, when grown in the controlled environment chamber. If pine and birch had absorbed equal flux densities of quanta, the difference in photosynthetic rate between the two species, according to Figures 3 and 4, would have been 0.40 instead of 0.80 ,umol CO2 m-2 s-'. This means that the difference in photosynthetic rates between pine and birch under the growing conditions depended approximately 50%o on factors

Un

E 0

E

m

I-,

U_

c x

U

Rd -1

150 250 Absorbed quantum flux density (Ijmol mn2 s-') FIG. 3. The relationship between CO2 flux density and absorbed quantum flux density in four seedlings of P. silvestris with different leaf areas: (A), 0.50 10-2 M2; (0), 0.67 10-2 M2; (@), 0.70 10-2 M2; and (A), 0.82 10-2 M2. Partial pressure of 02 was 21.2 kPa. The mean quantum yield (4 = 0.022) was calculated from the slope of the linear part of the curve (n = 22, r2 = 0.84). The absorbed quantum flux densities between 10 and 90 umnol m-2 s-' were used for the quantum yield calculation. Pn, photosynthetic rate of seedlings measured under the same light and temperature conditions as used in the growth chamber; Rd, dark respiration.

1.11

X

10-3

0.14

0.15

1.36 ± 0.05 (n = 9)

4 U) 0 3

E

=

2

0

x

1

103

Absorbed quantum flux density (amol m-2 s')

FIG. 4. The same relationship as for Figure 3 with four seedlings of B. verrucosa with different total leaf areas: (A), 0.60 10-2 M2; (0), 1.05 10-2 mi2; (0), 1.48 10-2 M2; (A), 1.72 10-2 M2. Partial presence of 02 was 21.2 kPa. The mean quantum yield is 0.032 (n = 20, r2 = 0.97) and absorbed quantum flux densities between 10 and 100lumol m-2 s-' were used for the quantum yield calculation. For Pn and Rd, see legend to Fig. 3.

affecting the number of quanta absorbed per unit leaf area and time and approximately 50%o on metabolic factors as defined by the photosynthetic quantum yields, i.e. the initial slopes of the curves (Figs. 3 and 4). By inhibiting photorespiration by reduced partial pressure of oxygen, the steady-state in situ photosynthetic rates of pine and birch would increase to 2.2 and 3.1 ,umol quanta m-2 s-', when 70 and 88 pmol quanta m-2 s-' are absorbed, respectively. The rates are marked in the photosynthetic curves of Figures 5 and 6 (dotted lines). Now, the difference in photosynthetic rates between the two species would be 0.9 ,umol CO2 m-2 s-' and would depend approximately 70%o on factors affecting the number of quanta absorbed per unit leaf area and time and approximately 30%o on metabolic factors (calculated as above). Thus, about 20%o of the metabolic factor, which constitutes 50%o of the difference of photosynthetic rates in situ in normal air, must depend on a more pronounced photorespiration in pine than in birch. It cannot depend on differences in intercellular CO2 concentration since, from the presented data, it can be estimated (4) that both pine and birch operate with an intercellular CO2 concentration of about 270 ,ubar. The remaining 30%o of the metabolic factor must depend on a less efficient photosynthetic process of pine than of birch. DISCUSSION To make reliable comparisons in photosynthesis between pine and birch, information on the physiological state of the seedlings


PHOTOSYNTHESIS OF PINUS and BETULA

5

u4

E

=23

Rd

-1 'sI 50

150

Absorbed

quantum

250 flux

density

(IJmol

m~2s'))

FIG. 5. The same relationship as for Figure 3 with four seedlings of P.

(i\), 0.60 10-2 in2, 10-2 in2. The mean

silvestris. Partial pressure of 02 was 2.0 kPa. Leaf areas:

(0),

0.65

10 2

in2; (0),

0.81

10-2

in2;

and (A), 0.82

quantum yield is 0.033 (n = 20, r2 = 0.89) and values between 10 and 90

,umol m-2 s-'

:1 X

of the absorbed quantum flux densities were used for the

quantum yield determination. For

FPn

and Rd, see legend to Fig. 3.

0)

Rd

/E3

50

150 250 Absorbed quantum flux density (pmol M2s') FIG. 6. The same relationship as for Figure 3 with four seedlings of B. verrucosa. Partial pressure of 02 was 2.0 kPa. Leaf areas: (A), 0.7910-2 2 (0), 0 10-2m2; (0), 124 10-2 M2; and (A), 1.67 -102 M2. The mean quantum yield is 0.040 (n = 22, r2 = 0.88) and values between 10 and 100 Imol m-2 s-' of the absorbed quantum flux densities were used for the quantum yield calculation. For P,,and Rd, see legend to Fig. 3.

is important. The nearly exponential growth of the two species

(Fig.

1) shows

that

the

seedlings

were

actively growing

at a

constant rate during the time when the photosynthetic measurements were performed. The observed RGR of the two species (Fig. 2) and the finding that RGR of birch was about 2 times that of pine at the time for photosynthetic measurements agree with earlier reports (12). Growth analyses carried out on dry weight

943

increment were in good agreement with those calculated from C02-flux measurements (Table I). This shows that the measurements obtained here of in situ net photosynthetic and respiratory rates were correctly performed and can be used in determining the reasons for the differences in photosynthetic properties between the two species. It was found that approximately 50%1o of the reason for the lower rate of in situ net photosynthesis in pine than of birch depended on factors which determine that amount of quanta absorbed per unit leaf area and time (Figs. 3 and 4). The difference in Chl content per unit leaf area between the two species is too small (Table I) to have any major effect on the differences in flux density of quanta absorption (8). In the controlled environment chamber used for cultivating the seedlings and measuring in situ net photosynthesis, the lamps were mounted above the seedlings. Due to radiation reaching the seedlings from one direction mainly, mutual shading between leaves and branches is expected to have occurred. Mutual shading is known to be more pronounced in conifers than in broad-leaved plants (13, 22). The primary needles of the pine seedlings also have a more or less upright position. This gives more reflection on the needle surfaces than on the broadleaf surfaces of birch, which are mainly perpendicular to the light. It is concluded that the reason for the lower flux density of quanta absorption in pine is due mainly to the morphological differences, which makes pine a less efficient absorber than birch of the unidirectional light used in the controlled environment chamber. The remaining 50%o ofthe lower rate of in situ net photosynthesis in pine was metabolically controlled (Figs. 3 and 4). The rates of the dark respiration of the shoots of the two species were about equal, when expressed per unit leaf area and time (Figs. 3 and 4). The metabolic factor in the difference of the in situ net photosynthesis between pine and birch, therefore, is given by the difference in photosynthetic quantum yield between the two species, i.e. by the initial slope of the photosynthetic light curves. Part of the metabolic factor for determining the different photosynthetic properties of pine and birch was due to a more pronounced photorespiration in pine than in birch (Figs. 3-5). This can, however, only explain about 20%o of the different rates of the in situ net photosynthesis of the two species. The remaining 30%o must depend on a less efficient photosynthetic process in pine. As both pine and birch adapted the properties of their photosynthetic apparatuses, so that light was essentially rate-limiting for in situ photosynthesis (Figs. 3 and 4), it is assumed that the difference in photosynthetic efficiency is related to some property of the function of the chloroplast thylakoids, e.g. energy transfer efficiency between pigments, photosynthetic electron transport, or coupling between electron transport and photophosphorylation. None of these possibilities have been investigated so far. However, as the in situ rates of net photosynthesis of the two species are very close to the point where the photosynthetic light curves become nonlinear (Figs. 3-6), the possibility that the different convexities of the light curves to some extent may be included in the metabolic factor, explaining the different photosynthetic properties of pine and birch (15), cannot be excluded. The quantum yield values of pine and birch seedlings are lower than those reported earlier for single leaves of some herbaceous plants (4). This is most likely not primarily due to whole seedlings having leaves in different developmental stages (G. Oquist, L. Brunes, and J.-E. Hallgren, manuscript in preparation) but, rather, is related to woody plants generally showing lower rates of both net photosynthesis and growth than most herbaceous plants (12). It should be remembered that the results of these analyses are only valid for the investigated populations of pine and birch. Looking at these species in other developmental stages or after adaptation to other climatic conditions, one might observe other relations between the morphological and metabolic factors for

944

BRUNES,

OQUISTr, AND ELIASSON

determining differences in the rate of in situ net photosynthesis. However, the outlined approach should be a useful tool for investigating the nature of the factor or factors which determine differences in the rate of in situ net photosynthesis between species, varieties, developmental stages, etc. LITERATURE CITED

L-A, C BENGTSON, SO FALK, S LARSSON 1977 Cultivation of Pine and Spruce Seedlings in Climate Chambers, Technical Report 5. Swedish Coniferous Forest Project, ARNON DI 1949 Copper enzymes in isolated chloroplasts. Polyphenoloxidase in Beta vulgaris. Plant Physiol 24: 1-15 BIERHUIZEN JF, RO SLATYER 1964 An apparatus for continuous and simultaneous measurement of photosynthesis and transpiration under controlled environmental conditions. CSIRO Division Land Research, Regional Survey Technical Paper 24: 1-16 BJORKMAN 0 1973 Comparative studies on photosynthesis in higher plants. In AC Giese, ed, Photophysiology 8. Academic Press, New York, pp 1-63 CHARLES-EDwARDS DA, J CHARLES-EDWARDS, Fl SAN[r 1974 Leaf photosynthetic activity in six temperate grass varieties grown in contrasting light and temperature environments. J Exp Bot 25: 715-724 EVANS GC 1972 The Quantitative Analysis of Plant Growth. Blackwell Scientific Publications, Oxford GASrRA P 1959 Photosynthesis of crop plants as influenced by light, carbon dioxide, temperature, and stomatal diffusion resistances. Mededel. Landbouwhogesch (Wageningen) 59: 1-68 GABRIELSEN EK 1948 Effects of different chlorophyll concentrations on photosynthesis in foliage leaves. Physiol Plant 1: 5-37 GABRIELSEN EK 1960 Beleuchtungstarke und Photosynthese. In A Pirson, ed, Handbuch der Pflanzenphysiologie, Vol 2. Springer-Verlag, Heidelberg, pp 26-48

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2. 3.

4.

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7. 8. 9.


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