why are sun leaves thicker than shade leaves ... · phase and the resistance to c02 diffusion per...

J. Plant Res. 114: 93-105, 2001 Journal of Plant Research 0 by The Botanical Society of Japan 2001

Invited Article

Why are Sun Leaves Thicker than Shade Leaves? - Consideration based on Analyses of CO, Diffusion in the Leaf

lchiro Terashima’*, Shin-lchi Miyazawa’ and Yuko T. Hanba‘

Department of Biology, Graduate School of Science, Osaka University, Machikaneyama-cho, Toyonaka, Osaka, 560-0043 Japan Research Institute for Bioresources, Okayama University, Chuo, Kurashiki, 710-0046 Japan

Light-saturated rates of photosynthesis on leaf area basis (A) depend not only on photosynthetic biochemistry but also on mesophyll structure. Because resistance to CO2 diffusion from the substomatal cavity to the stroma is substantial, it is likely that mesophyll structure affects A through affect- ing diffusion of COZ in the leaf. To evaluate effects of various aspects of mesophyll structure on photosynthesis, we constructed a one-dimensional model of COZ diffusion in the leaf. When mesophyll thickness of the leaf is changed with the Rubisco content per unit leaf area kept constant, the maximum A occurs at an almost identical mesophyll thickness irrespective of the Rubisco contents per leaf area. On the other hand, with an increase in Rubisco content per leaf area, the mesophyll thickness that realizes a given photosynthetic gain per mesophyll thickness (or per leaf cost) increases. This probably explains the strong relationship between A and mesephyll thickness. In these simulations, an increase in mesophyll thickness simultaneously means an increase in the diffusional resistance in the intercellular spaces ria^), an increase in the total surface area of chloroplasts facing the intercellular spaces per unit leaf area (S,), and an increase in construction and maintenance cost of the leaf. Leaves can increase S, and decrease R h also by decreasing cell size. Leaves with smaller cells are mechanically stronger. However, actual leaves do not have very small cells. This could be because actual leaves exhibiting considerable rates of leaf area expansion, ade- quate heat capacitance, high efficiency of N and/or P use, etc, are favoured. Relationships between leaf longevity and mesophyll structure are also discussed.

Key words: Cell size - C O 2 diffusion - &&/benefit analysis - Leaf photosynthesis - Mesophyll

When expressed on leaf area basis, the light-saturated rate of leaf photosynthesis (A) in C3 plants strongly depends on variables such as leaf nitrogen and Rubisco contents.

Because more than 70% of leaf nitrogen exists in chloroplasts and because Rubisco is the key enzyme limiting photosynthesis, we can understand these strong relationships (for a review see Evans 1996, for theoretical consideration see Hikosaka and Terashima 1995).

There are also strong correlations between A and structural parameters such as leaf thickness (Koike 1988), leaf mass per area (Enriquez et a/. 1996), mesophyll surface area (Nobel 1991, Hanba et a/. 1999) and chloroplast surface area (Laisk et a/. 1970, Kariya and Tsunoda 1972, Evans et a/. 1994). The importance of these structural parameters has been argued (for reviews, see Parkhurst 1994, Evans and Loreto 2000). However, we still need more quantitative evaluations.

The most important issue of the present study is C02 concentration in the leaf. In this paper, C02 concentration in the gas phase is expressed by C (in mol m-3). The C02 concentration in an adjacent liquid phase equilibrated with the gas phase is expressed as q5 C (in mol m-3), where q5 is the Bunsen coefficient corrected for temperature. q5 values calculated from Bunsen coefficients given in Edwards and Walker (1983) are 0.954,0.829,0.739,0.666, and 0.609 at 20,25,30,35 and 40C, respectively. When the C02 concentrations in various regions including gas and liquid phases are compared, it is useful to express all the concentrations as those in the gas phase and omit q5 (for the symbols and units see Table 1).

In photosynthesizing C3 leaves, C02 concentration in the substomatal cavity, C,, is lower than that in the ambient air Ca, and CO2 diffuses into the leaf along the gradient in C02 concentratrion. C, can be estimated by the gas exchange techniques (von Caemmerer and Farquhar 1981, Sharkey et a/. 1982). In this paper, we refer to the value that is estimated by the gas exchange techniques as C, rather than the C02 concentration in the intercellular spaces, C,. This is because the bulk C, in a vigorously photosynthesizing leaf should be lower than C, due to the resistance to C02 diffusion in the intercellular spaces (Fig. 1).

CO2 concentration in the chloroplaststroma, q5 C,, in & plants is lower than that at the mesophyll surface, # Ci. Recent technological innovations, including pulse-modulat- ed fluorometry (DiMarco et a/. 1990, Loreto et a/. 1992, Epron

* Corresponding author: fax, 81-(0)6-6850-5808; e-mail, itera

Abbreviation: Rubism, ribulae-l,5-bisphaphate c ~ o x y l ~ e / Ochaos.bio.sci.osaka-u.ac.jp

oxygenase. For the symbols, see Table 1.

94 1. Terashima et a/.

Table 1 Symbols and units used.

A (mol m-z s-l) a (x) (mol COZ m-3 s-I) C, (mol C02 m-3 air) C, (mol COZ m-3 air) C, (mol CO;! m-3 air) C, (mol CO2 m-3 air) D (m2 s-I) GI (m s-I) GtaS (m s-9 Gw (m s-1)

gw (m s-I)

E, (mol m-3) kl (mol-1 m3 s-I) kz (s-9 I (m) 4

RI (S m-I)

R,= (S m-l) Rw (S m-1)

S (m2 m-3) So (m2 m-2) S, (m2 m-4

Rd (mol CO2 mo1-l Rubisco S-’)

Light-saturated rate of net photosynthesis of the leaf Rate of photosynthetic net CO;! uptake per unit mesophyll volume CO2 concentration in the ambient air C02 concentration in the hypothetical air equilibrated with the chloroplast stroma C02 concentration in the intercellular spaces CO2 concentration in the substomatal cavity (1 1 mmol C02 m-3) Binary diffusion coefficient of COZ in the air (1.55 x Internal conductance for CO2 diffusion of the leaf (expressed for the whole leaf) Conductance for COZ diffusion in the intercellular spaces of the leaf (expressed for the whole leaf) Conductance for COZ diffusion from the mesophyll cell surface to the chloroplast stroma (expressed for the

whole leaf) Conductance for C02 diffusion from cell wall surface to the chloroplast stroma across cell walls (expressed as per unit mesophyll cell surface area)

Rubisco content per unit mesophyll volume Constant (6.08 x 102 mol-1 m3 s-I) Constant (-3.09 x 10-1 s-l)

Mesophyll thickness Bunsen coefficient corrected for temperature (0.829 at 25°C) Dark respiration rate per Rubisco (1 mol C02 mol-l Rubisco s-l) Internal resistance to COZ diffusion in the leaf (expressed for the whole leaf) Resistance to COZ diffusion in the intercellular spaces of the leaf (expressed for the whole leaf) Resistance to COZ diffusion from cell wall surface to the chloroplast stroma across cell walls (expressed as

Mesophyll surface area per unit mesophyll volume (including intercellular air spaces, 105 m2 m-3) Area of chloroplast surfaces facing the intercellular air spaces per leaf area Area of mesophyll cell surfaces facing the intercellular air spaces per leaf area

m2 s-l at 25°C)

per unit mesophyll cell surface area)

‘stoma

Fig. 1. Diffusion of COP in the leaf and the framework of the model. The COZ flux by pathway b is negligible compared with that by pathway a.

Why are Sun Leaves Thicker than Shade Leaves ? 95

et a/. 1985) and on-line measurement of carbon isotope discrimination (Evans et a/. 1986, von Caemmerer and Evans 1991, Hanba et a/. 1999), enabled us to estimate C,. It is reported that, in some species, Cc can be as low as half the C, (Evans and Loreto 2000).

Rubisco is the enzyme that fixes C02. Affinity of this enzyme to COZ is low. The Km at 25°C of this enzyme from C3 plants ranges from 8 to 16,uM or mmol m-3 (von Caem- merer and Quick 2000). The range includes 11.9 mmol (mM), the COZ concentration in water that is equilibrated with the air containing 14.3 mmol COZ m-3 (corresponding to 35 Pa C02 when the atmospheric pressure was 101.3 kPa). Moreover, in the presence of 21% 0 2 , a competitive inhibitor, the apparent Km to COZ, increases further to 14-25 mmol m-3 (von Caemmerer and Quick 2000). The presence of oxygen thus lowers the carboxylation rate. Moreover, the photorespiration path, which detoxyfies phosphoglycolate (the prod- uct of RuBP oxygenation), consumes ATP, NADPH, and triose phosphate, and releases COZ (Heldt 1997). Thus, the resistance to COP diffusion in the leaf lowers efficiency of photosynthesis not only by lowering C, but by enhancing photorespiration. Clearly, it is advantageous for the leaf to minimize resistance to COz diffusion.

Because diffusion of COZ in the water is slower than that in the air by (Nobel 1991), the C02 flux from the mesophyll surface to the chloroplast stroma through the pathway like b in Fig. 1 is negligible compared with that of pathway a. Thus, it is useless to have mesophyll cell surfaces without chloroplasts. Actually, when grown with sufficient nutrients, most of the mesophyll surfaces facing the intercellular spaces are occupied by chloroplasts (Evans et a/. 1994). Thickness of chloroplasts is also important. The drawdown of C02 concentration from the intercellular spaces to the stroma, ,$ * C,-4 C,, is proportional to the flux of C02 across the liquid phase and the resistance to C02 diffusion per unit chloroplast surface area in the liquid phase. With the increase in the amount of Rubisco per unit mesophyll cell surface area, photosynthetic rate per unit area of mesophyll cell surface increases. However, the photosynthetic rate per Rubisco decreases because 4 C, decreases. In this sense, thick leaves with a greater area of chloroplast surfaces facing the intercellular spaces (Sc) would be advantageous because chloroplasts can be thin and the amount of Rubisco per unit area of chloroplast surface can be small. On the other hand, with an increase in leaf thickness, the bulk resistance to C02 diffusion in the gas phase (&) increases, which causes an decrease in bulk C,. In this respect, thick leaves are disadvantageous.

In the present study, using a simple one-dimensional model of COZ diffusion, we examined these two contrasting effects of leaf thickness on leaf photosynthesis. Based on the simulation results, we suggest the photosynthetic gain per construction and maintenance cost of the leaf as a factor responsible for the strong dependence of A on leaf structural parameters. We also discuss other constraints of leaf mesophyll architecture, in particular those determining mesophyll cell size and leaf thickness.

The Model

One-dimensional expression of CO diffusion and photosynthetic processes (Fig. 1) can be written as:

where p is volumetric fraction of the intercellular spaces (mesophyll porosity), z is tortuosity of the C02 diffusion path in the mesophyll, D is the binary diffusion coefficient of COZ in the air, Cxx) is the C02 concentration in the intercellular spaces at the distance x from the substomatal cavity, and a(x) is the rate of photosynthetic COZ uptake per unit volume of mesophyll at x (for derivation of eqn. (I), see Appendix; for detailed mathematical treatments, see Parkhurst 1977 and Rand 1978). We assume that a(x) is linearly related to Rubisco content per unit mesophyll volume (Ev) and to the C02 concentration in chloroplast stroma, ,$ Cc(x). a(x) is also proportional to the conductance for COZ diffusion from the surface of mesophyll cell wall to the chloroplast stroma bW), the mesophyll surface area per unit mesophyll volume (S) and to the difference in COZ concentration between the mesophyll cell surface (,$ C,(x)) and the chloroplast stroma ($5 Cc(x)):

(2) a(x)=Ev ( k ~ ,$ cc(x)-kZ-&)

=gw s (,$ C,(X)- d a x ) ) where, k1 and k2 are constants, and &is the rate of dark respiration per unit amount of Rubisco. Although we do not know whether the liquid phase conductance bW) is mainly determined by the conductance in the cell walls (Nobel 1991), we simply refer to gw as wall conductance.

Eliminating C,(x) from eqn. (2), we obtain a(x) as a linear function of C,{x);

When solved analytically for C,{x), eqn (1) gives

C,{x)=k3+G exp(a x)+Cz exp(-a x),

where CI and CZ are constants and k3 and a are, (4)

and

From the boundary conditions for a hypostomatous leaf, C,O)=Cs and C,’(I)=O, and Ci and C2 can be expressed as

and

exp(2a I) (Cs-k3) ‘2= 1+exp(2a I)

where / is the thickness of the mesophyll. stomatous leaf, boundary conditions are Accordingly, we obtain

For an amphi- C,{O)= CXI)= cs .

96 I. Terashima et a/.

and

(9)

We first calculated the averaged Ci. For the hypostomatous leaves,

For the amphistomatous leaf, -

(1 2) ci=k3+ (Cs-k3) (exda 4+exp(-a 4-21 I a (exp(a I)+l)-exp(-a 4

The average COZ concentration in the water at mesophyll cell surfaces is expressed as q5 c,. The averaged q5 cc is then given by,

The rate of photosynthesis of the leaf is expressed as:

A= a(x)dx=f’, / (k1 q5 c - k ~ + R d ) ) (14) ”l A can be also expressed as:

- A = G ~ ~ c ~ - ~ , ) = G , ~ ( c ~ - ~ ~ ) = G ~ ( ~ G-q5 cc). (15)

where, GI, GI, and G, are internal conductance for COZ diffusion, conducatence in the intercellular spaces and conductance from the mesophyll surface to the chloroplast stroma, all for the whole leaf. We examined the effects of several parameters on averaged el, q5 cc and on A. We also compared the resistance to COZ diffusion in the intercel- Mar spaces, Rias(=I/Gias), and the resistance to COZ diffusion from the mesophyll cell surface to the chloroplast stroma, Rw

In this model, we assume that the leaf mesophyll consists of columnar cells of the uniform radius, r (see Fig. 2). The arrangement of the columns is orthogonal. Let us assume r is 14pm, and the distance between the column centers is 29.65 pm. Then, the volume occupied by the cells is 70% and the mesophyll surface area per unit volume of mesophyll is 105 m2 m-3. When the length of the columns (i.e. mesophyll thickness), I, is 200pm, the mesophyll surface area directly facing the intercellular spaces per leaf area, Smes or AmeS/A according to Nobel (1991), is 20m2 m-2. The volumetric fraction of the intercellular spaces (porosity, p) of this mesophyll is 0.3. Although the tortuosity for the COZ diffusion path (z) in the direction of leaf thickness in this model mesophyll would be small, we assume some tortuosity and adopt 1/z=0.7, unless otherwise stated. We assume that S,, is equal to area of chloroplast surfaces facing the intercellular spaces on leaf area basis, Sc. The values adopted here are realistic for leaves of higher plants (Esau 1965). Although we are fully aware of the heterogeneity of

(=I / G w ) .

TOD view ,, ,---. ,, ,,- --., ,.---,, ,*.---.,, ,,.--5,

\ I

I J (j \ / ij C/,) \\ ‘. -

Side view

Fig. 2. Structure of the model mesophyll. The model leaf is composed of columnar mesophyll cells arranged orth- ogonally. In most of the simulations, the radius of the column (r) is assumed to be 14pm and the distance between the column centers Is 29.65pm. Then, the volume occupied by the cells were 70°/o and the surface area of mesophyll exposed to the intercellular spaces per unit leaf volume is 1@m2 m-3. When the length of the columnes (mesophyll thickness) is 200 pm, mesophyll surface area per unit leaf area (SmeS) is 20 m2 m *. The area of chlroplasts facing the intercellular spaces (S,) is assumed to be equal to Sm,.

the mesophyll, we assume that the photosynthetic function is distributed evenly in the mesophyll. For the symbols and units, see Table 1.

Results

The dependence of COz fixation on COz concentration in the stroma can be expressed by a combination of Michaelis- Menten type hyperbolic functions (von Caemmerer and Farquhar 1981, see the legend of Fig. 3). However, we use a linear function (see eqns. (2) and (3)). Fig. 3 shows the dependence of the COZ fixation rate on the CO2 concentration in the stroma. The COZ fixation rates were calculated using kinetic parameters obtained from wheat Rubisco at 25C (Makino et a/. 1988). Oxygen concentration is assumed to be 21%. We are going to mainly discuss photosynthesis at q5 C, ranging from about 4 to 9 mmol m-3 (or pM), the range which is realistic at the natural atmospheric COZ concentration. The dependence of A on Cc in this range is


0 2 4 6 8 1 0

CO, concentration in stroma ($.Cc) (mmol m''orpM)

Fig. 3. The rate of COZ fixation in the leaf by 1 mol of Rubisco as a function of COZ concentration in the stroma. COZ fixation rate (gross rate, A,) is expressed as,

where, Vcm, and Vom, are substrate-saturated rates of RuBP carboxylation and oxygenation, K c and KO are Michaelis-Menten constants for COZ and 0 2 , and 0 and C are 02 and COZ concentrations (expressed as the equiva- lent concentrations in air) in the chloroplast stroma (von Caemmerer and Farquhar 1981). The kinetic data of wheat rubisco are adopted from Makino et a/. (1988). The linear function, fitted to the data points of Ag against the COZ concentraion in the stroma ranging from 4 to 9 ,uM, is also shown (r2=0.998).

well expressed by a linear function (see the straight line in Fig. 3, r2=0.998). It is worth mentioning that, for eqn (1) to be solved analytically, the eqations (2) and (3) should be liniar functions of Cc and C,, respectively.

Let us first compare hypostomatous leaves with amphistomatous leaves. In this simulation we assume that leaves have 4mmol Rubisco m-' which is a typical value for sun leaves of well-fertilized plants (Terashima and Evans 1988). p / r is 0.2 and g, is 2.38X10-4m s-I. These are also typical values of the real leaf (see below). The effects of mesophyll thickness on el, 4 c, 4 cc and A are shown in Fig. 4. It is obvious that the c,, d c, and d cc are much higher in the amphistomatous leaf than those in the hypostomatous leaf of the same mesophyll thickness. In the hypostomatous leaves, decreases considerably with an increase in mesophyll thickness. 4 cc increases with thickness (up to the mesophyll thickness of 0.662 mm) and gradually decreases with a further increase in the thickness. - When compared with hypostomatous leaves, the decrease in C, in the amphistomatous leaf is smaller and the cc increases up to the mesophyll thickness of 1.31 mm (Fig. 4A).

The effects of mesophyll thickness on leaf photosynthesis (A) is similar to those on q5 cc because A is linearly related to cc. There are clear peaks at the mesophyll thickness of 0.662 and 1.31 mm for A of the hypostomatous leaf and for the amphistomatous leaf, respectively. With a further

P L A 1 2 , I I I I

8 4t1 ' 1

18 -4 0

B I I I I

0 0.5 1 .o 1.5 2.0

Mesophyll thickness (mm) Fig. 4. Effects of mesophyll thickness on Ei, q5 and q5

(A) and A (B) of amphistomatous (dotted lines) and hypostomatous (solid lines) leaves. It is assumed that all leaves have 4pmol Rubisco m-2 leaf area. p / z is assumed to be 0.21. g, is 2.38XlO-4m s-I. COZ concentration in the substomatal cavities is asuumed to be 11 mmol COZ m-3.

increase in leaf thickness, A gradually decreases. The ratio of C8-G in the amphistomatous leaf to Cs-c, in

the hypostomatous leaf changes from about 25 to 30% with an increase in leaf thickness up to 2 mm. The resistance to C02 diffusion in the intercellular spaces, therefore, is less severely limiting photosynthesis in the amphistomatous leaves than in hypostomatous leaves. Since the drawdown of C02 concentration is more marked in the hypostomatous leaf, we will deal with hypostomatous leaf.

The total internal conductance expressed on leaf area basis (Gi) has been reported for various species (Hanba et a/. 1999, Evans and Loreto 2000), and ranges from 0.04 to 0.5 mol CO:! m-2 s-l. When expressed in m s-I, these values at 25 C correspond to 9.78X10-4 and 1.25X10-2 m s-I, respectively. We used these Gi values to obtain reasonable g, values for our model simulation. If Gi were solely determined by the wall resistance, g, would be expressed as Gi/ Sm,,. S,, would range from 5 to 50m2 m-2 (Nobel 1991). We also know that there is a positive dependence of G, on Sm, in the real leaves (See Fig. 7 in Hanba et a/. 1999, see also Fig. 14 in Evans and Loreto 2000). The Gi/Smes in the

98 I. Terashima et a/.

actual leaves so far reported ranges from 4.9X10-5 to 5.3X m s-l. In the present model, therefore we used g,

values ranging from 5X10-5 to 1.6X10-3 m s-l. The g, value, ~ . Z B X I O - ~ m s-I, which is used in the simulation shown in Fig. 4,7,8,9, and 10, corresponds to G, of 4.76X

m s-I or 0.195 mol m-2 s-I when mesophyll thickness I is 0.2 mm. This G, value is large for the GI values of the leaves of evergreen trees but small for those of annual herbs including crop species (Hanba et a/. 1999, Evans and Loreto

1 2 r I I I I

6 t 1 1 1

4 1 I I I I I 101 I I I I I

1.6 x 1 0 ' ~ B _........" ...................... " ..................

01 I I I I I 20

n 7 u)

Y E - 10 0

Y 5 a

' ......... ....I ................ : .... y 1 . 6 ~ l o 3 ........... ...... -..9

""

.............." ...........................

.._C .... I.."...'. c -

I I I I I 0 0.2 0.4 0.6 0.8 1 .o

Mesophyll thickness (mm)

Fig. 5. Effects of gw and mesophyll thickness on e,(A), Ec (6) and A (C) in the hypostomatous leaves. All leaves have 4 Dmol Rubisco m-2 leaf area. p / z is assumed to be 0.21. C, is assumed to be 11 mmol COZ m-3. The values of gw examined are 5X10-5, l.0x10-4, 2.0X10-4, 4.0X10-4, 8.Ox10-4 and 1.6X10-3 m s I.

2000). In Fig. 5, the effects of different g, on leaf photosynthesis

are shown. Here, Rubisco content is fixed to be 4 pmol rn-' and p / r is 0.21. Ci decreases with an increase in mesophyll thickness. cf for the leaves of the same mesophyll thickness decreases with an increase in g, (Fig. 5A), because A at a given mesophyll thickness increases with an increase in g,. The mesophyll thickness gives maximum ~5 cc and thus A decreases with an increase in g, (Figs. 5B and C). A at the maximum decreases with an decrease in g,. It is also noted that, with the increase in mesophyll thickness, the values of A converge irrespective of the g, values because G, approaches infinity with an increase in mesophyll thickness.

A in these leaves relative to the photosynthetic rate under the hypothetical condition of Cc=Cs is shown (Fig. 6-A). In the leaf with the largest g,, the maximum A realizes about 85% of the ideal value. Rim increases with an increase in mesophyll thickness. On the other hand, R, is inversely related to the mesophyll thickness (Fig. 6B). Consequently, the relative contributuion of the wall resistance to total diffusional resistance expressed as (C Cc)/(Cs-cc) decreases with the mesophyll thickness (Fig. 6C). At the mesophyll thickness that gives the maximum A, the contribution of the wall resistance to the total resistance is about

The effects of the porosity of the mesophyll @) and tortuosity of the C02 diffusion path in the mesophyll (z) are shown in Figs. 7 and 8. p for one-dimensional diffusion can be easily assessed from paradermal sections. Examples of anatomical data are shown in Table2. The smallest p in Table 2 is 0.12. Estimation of z is difficult (for a theoretical treatments, see Ball 1981 and Parkhurst 1994 ), and so we did not deal with this parameter. However, it is obvious that, for the diffusion in the direction of leaf thickness, z of the palisade tissue is smaller than that of the spongy tissue. In ordinary leaves, the minimum p / z is probably around 0.1. In the present simulation, p / z ranges from 0.025 to 0.8. In this case, Rubisco content of the leaf is 4,umol m-2 and g, is assumed to be 2.83X10-4 m s-l. With a decrease in p /z , the decrease in ci with increasing mesophyll thickness becomes sharper (Fig. 7A). The mesophyll thickness realizing the maximum A and the value of maximum A both decrease with the decrease in p / z (Fig. 7B, C). For a given mesophyll thickness, the relative contribution of wall resistance to the total resistance decreases with a decrease in p / z (Fig.8C). At the peak mesophyll thickness, (Ci-cc)/

The effects of leaf Rubisco content are shown in Figs. 9 and 10. The content of Rubisco per unit leaf area is reported for many species (for a review see Evans 1989). In this simulation, Rubisco content per leaf area ranges from 0.125 to 8,umol m-*. At all Rubisco contents, the mesophyll thickness that gives the maximum A is around 0.66mm, although the peaks are not so evident in leaves with small amounts of Rubisco (Figs. 9B, C). In Fig. IOA, the ratios of the photosynthetic rates of the model leaves to the hypothetical, ideal rate with C,=C, are shown. With the increase in

-

_ _

50%.

(Cs-cc) % 50%.


100r I I 1 I 1

s Q) 80 Q

0 i-

60 8 Y

u) .- 4 0 E

c 5 20

,t.6 x 1 0 ' ~ A ....................................... ................

I I 1 I I

I I I I I 0 0.2 0.4 0.6 0.8 1 .o

Mesophyll thickness (mm) Fig. 6. Effects of gw and mesophyll thickness on the ratio of A

to the hypothetical leaf with no diffusional resistance (i.e., Cc=Cs=ll mmol COZ m 3), (A), R,, and Rw (S) and the contribution of Rw to the total internal resistance expressed as (&cc)/(Cs -cc) (C). For simulation conditions, see the legend of Fig. 5.

Rubisco content, the ratio decreases. The relative contribution of the wall resistance to the total resistance expressed as (Ci-Cc)/(Cs-Cc) is shown in Fig.1OC. At a mesophyll thickness of 0.66 mm, the contribution of the wall resistance to the total resistance is about 50% (Fig. IOC).

Discussion

The aim of the present study is to reveal why the rate of net photosynthesis in the leaf depends on structural charac-

Table2 Porosity of palisade and spongy tissues in several species

Species Palisade tissue Spongy tissue

Lithocatpus edulis Quercus glauca Quercus acutissima Quercus semta Maesa japonica Alocasia odora Rumex obtusifolius Chenopodium album

.25

.12

.19

.32

.12

.26

.24

.24

.64

.42

.45

.63

.43

.26

.30

.37

Leaf tisseus were fixed and embedded in Spurr's resin. Light micrographs of the paradermal sections were taken and analysed with a software (NIH Image). For explanation for species except for Quercus acutissima, see the legend of Fig.13. Q. acutissima Carruthers is a deciduous tree (Fagaceae). Data from Terashima et a/. (1995).

teristics such as leaf thickness, leaf mass per area, area of mesophyll cell surfaces facing the intercellular air spaces per leaf area (S,,,), and area of chloroplast surfaces facing the intercellular air spaces per leaf area (SJ

The simulations for the hypostomatous model leaves with different Rubisco contents (Figs. 9,lO) reveal that the mesophyll thickness that gives the maximum photosynthetic rate is around 0.66 mm, regardless of the Rubisco contents. However, in the leaves with small Rubisco contents, A increases sharply with increasing mesophyll thickness. For example, the leaves having Rubisco at 0.5 and 1 pmol m-2 attain A values that are 93.3 and 87.9% of the maximum values, respectively, at a mesophyll thickness of 0.1 mm (Fig. IOA). On the other hand, leaves having high Rubisco contents, the increase in A with respect to the mesophyll thickness is still evident at a mesophyll thickness of 0.4 mm. These simulations clearly indicate that the increment of A per unit input of energy or the resources needed to construct and maintain the leaf greatly differs depending both on Rubisco content and mesophyll thickness.

Assuming that the construction and maintenance carbon costs of the leaf are proportional to mesophyll thickness, the calculated increment of A per unit increment of leaf thickness (marginal gain, dA/dl) decreases with an increase in mesophyll thickness and the decreasing pattern depends on the Rubisco content (Fig. 11A). When the mesophyll thickness realizing the same marginal gain is plotted against the Rubisco content per leaf area, the mesophyll thickness increases with respect to the Rubisco content (Fig. 11B). That is, if a leaf is constructed to attain a given marginal gain, the mesophyll thickness must increase with Rubisco content.

When matter economy of the leaf is considered, the photosynthetic rate is best expressed on the basis of leaf dry mass rather than area. If the epidermis is sufficiently thin, the photosynthetic rate expressed on dry mass basis is roughly proportional to the rate of photosynthesis per mesophyll thickness. The rate of photosynthesis per mesophyll

1 2 , 1 I I

“I

-...-- ..... - ......_...._

6 -

4 1 I I I I I

10 I I I I

0.8

- I ......-..

I$ 2: 0

20 I I I I I I

h .;

(II u)

..-.... .........-.........

E ’-. .....-* .......... ............ -- - 0 10 5 0.025

C

01 I I I I I

0 0.2 0.4 0.6 0.8 1 .o

Mesophyll thickness (mm) Fig. 7. - Effects of p / z and mesophyll thickness on c/ (A),

C, (6) and A (C) in the hypostomatous leaves. All leaves have 4pmol Rubisco m-2 leaf area gw (wall conductance per unit mesophyll surface area) is 2.38X10-4 m s-I. Cs is assumed to be 11 mmol CO2 m-3. The values of p / z examined are 0.025,0.05, 0.1,0.2,0.4 and 0.8.

thickness decreases with an increase in mesophyll thickness (Fig. 12A). The mesophyll thickness realizing a given rate of photosynthesis per mesophyll thickness increases with an increase in the Rubisco content per leaf area (Fig.128). The trends shown in Figs.11 and 12 indicate that, with the increase in the Rubisco content, the mesophyll thickness realizing the given ratio of the photosynthetic benefit to the “cost” also increases. This probably underlies the well-


known, strong relationship between A and leaf thickness. _ _ _

l I I I

A 00

01 I I I 1 I

.... ....--- o*2 1 b t .:a” \/ ...... .......

I I I I I 0 0.2 0.4 0.6 0.8 1 .o

Mesophyll thickness (mm) Fig. 8. Effects of p / z and mesophyll thickness on the ratio of

A to the hypothetical leaf with no diffusional resistance (i.e., Cc=Cs=llmmol COz m-3, A), RI, and R, (6) and the contribution of Rw to the total internal resistance expressed as (&cc)/(Cs-cc) (C). Simulation conditions are the same as for Fig. 7.

The resistance to C02 diffusion from the substomatal cavity to the chloroplast stroma is divided into two components, the resistance from the substomatal cavity to the cell wall surfaces (Rim), and the resistance from the cell wall surfaces to the stroma (&). These resistances are nearly equal when A shows the maximum value (Figs. 6,8 and 10). The component conferring the greatest internal resistance is frequently debated (Parkhurst and Mott 1990, Syvertsen et a/. 1995). From the Present simulations, it is clear that variation


12 I I I

0.1 25

,- 10 E 0

g 8

- . s I I

-i

.*._.............................. .. ......-.. " .._.. "..

30 I I I

n 7 u)

(y 2 0 E 0 - Y 5 a 10

0 0 0.2 0.4 0.6 0.8 0.1

Mesophyll thickness (mm) Fig. 9. Effects of Rubisco content per unit leaf area and

mesophyll thickness on c, (A), 4 cc (B) and A (C) in the hypostomatous leaves. gw is 2.38X10-4 m 5.'. p / z is assumed to be 0.21. C, is assumed to be 11 mmol C02 m-3. The value of Rubisco content on leaf area basis examined are 0.125, 0.25,0.5,1.0,2.0,4.0, and 8.0pmol Rubisco m-2.

in Ria8 is small. For ordinary hypostomatous leaves, Rim would be below 100 sm-l (Figs. 6B, 8B, IOB), while for amphistomatous leaves, Rim would be smaller than 30s m-I. On the other hand, variation in Rw is large. When g, values are large as in annual herbs including crop plants, R, can be smaller than RIas. However, in leaves with low gw values, such as those of evergreen trees and Mediterranean deciduous trees, R, should be considerably greater than Rim.

0

400

-n 300 E a d 200

8 d 100

0

100

n

8 80 Y

60 z 18 40

I

IT c3

IQ

- 2ot "

0 0.2 0.4 0.6 0.8 1 .o

Mesophyll thickness (mm) Fig. 10. Effects of Rubisco content per unit leaf area and

mesophyll thickness on the ratio of A to the hypathetical leaf with no diffusional resistance (i.e., Cc=C,=ll mmol COZ m-3, A), Ri, and Rw (B) and the contribution of &to the total internal resistance expressed as (C,-Cc)/(~s-~c) (c). For simulation conditions, see the legend for Fig. 7.

Increase in mesophyll surface area (or chloroplast area) The main result of the present study is that leaves with

high Rubisco contents need to have considerable S,, and S, to realize high A. However, the large mesophyll surface area on leaf area basis is attained not only by increasing mesophyll thickness but by decreasing mesophyll cell size.

But why are mesophyll cells not very small ? To answer this question, we first examine the effects of cell size on mechanical strength. Let radius and length (or height) of a

102 1. Terashii n a et a/.

0.2

0 0.1 0.2 0.3 0.4


0.6

n

E E v

$, 0.4 z 4 r, - s 0.2 .c Q

f I

0

I I I I I I I

B I

0 2 4 6 8

Rubisco content (pmol m") Fig. 11. Effects of Rubisco content per unit leaf area and

mesophyll thickness on marginal gain (dA/d/, A) and the relationship between values of marginal gains and Rubisco content (B). Simulation conditions are the same as for Figs 9 and 10.

model columnar cell be r and I (Fig. 2). We express the ratio of length to radius by A (A=l/r). we also denote the thickness of the cell wall as t and the number of the columnar cells per unit area as n (m-2). If both ends of the column are fixed and z (( r , Euler's critical buckling load (PJ is expressed as

where E is Young's elastic modulus and I, is the second moment of area (Niklas 1992).

Let us reduce the leaf size. The mesophyll surface area of this leaf is expressed as 2n r l n=2n A r2 n. If we reduce the size of the model leaf to l/p (/J 21) and if the wall thickness after the reduction is also t, wall conductance expressed on leaf area basis is identical because the mesophyll surface area does not change. The critical buckling load calculated with eqn (16) is

h - 0 0.2 0.4 0.6 0.8


1 .o n E E - 0.8 C 5 0.6 8 g

0 0 2 4 6 8

Rubisco content (pmol mQ) Fig. 12. Effects of Rubisco content per unit leaf area and

mesophyll thickness on the rate of photosynthesis per unit mesophyll volume (A) and the relationships between the rate of photosynthesis per unit mesophyll volume and Rubisco content (B). Simulation conditions are the same as for Figs 9 and 10.

Therefore, the reduced leaf is mechanically stronger than the original leaf by the factors of p. If the cell wall thickness is also reduced to l/p, mechanical strength does not change before and after the reduction. However, wall conductance expressed on leaf area basis increases by p times and the materials needed for cell wall construction are lesser.

The above cons'lderation concerning the mechanical strength of the cells indicates that there are no disadvan- tages for leaves with small cells. It is also obvious that leaves with thick-walled, small cells are stronger than those with thin-walled, large cells.

In Fig.13, silhouettes of the paradermal sections of the palisade tissues are shown. Leaves of tall ever-green trees, which have long longevity and considerable mechanical strength, have smaller cells than that of Chenopodium album, an annual herb, which has short-lived leaves. Many annual herbs maintain their leaf shapes by the turgor rather than the solid cell walls per se. Because cell walls are very resistant to tension (Niklas 1992), it is possible to construct the leaf

Why are Sun Leaves Thic :ker than Shade Leaves ? 103

Le

Qg

Qa

Qs

lvli

Am

Ro

Ca

50 ym ' Fig. 13. Silhuettes of paradermal sections of the palisade tis-

sues. Le, Lithocarpus edulis (Makino) Nakai (evergreen tree, Fagaceae); Qg, Quercus g/auca Thunb. ex Murray (evergreen tree, Fagaceae); Qs, Quercus serrata Thunb. ex Murray (deciduous tree, Fagaceae); Mj, Maesa japonica (Thunb.) Moritzi (evergreen shrub on the forest floor, Myrsinaceae); Am, Alocasia odom (Lodd.) Spach (perennial herb, Araceae); Ro, Rumex obtusifolius L. (perennial, Polygonaceae); Ca, Chenopodium album L. (annual, Chenopodiacea). Data reproduced from Terashima et a/. (1995).

with thin-walled mesophyll cells and maintain the leaf shape by the turgor. However, if these leaves are subject to the drought, the turgor loss of the cell causes withering. As has been established, the loss of cell volume is fatal to cell functions such as photosynthesis (Cornic and Massacci 1996). Thus, the maintenance of leaf shape by turgor, which is adopted in many annual herbs exhibiting rapid growth, is a dangerous strategy with respect to water relations. Also, thin-walled leaves are more susceptible to herbivory because such leaves typically are soft and have low C/N ratio (see below). This would also be an important reason why the long-lived leaves are not thin-walled.

Mechanical strength is very important for the long-lived leaves. As is clear from eqn (16) for buckling, both thick cell walls and small cell size enhance-leaf strength. Thick cell walls are also important to maintain high C/N ratio of leaves because C/N ratio of cell walls is much higher than that of cytosol. Many insects and other animals have low C/N ratios such as 7-10. Thus, to construct their bodies, they need to diet materials with low C/N ratios (Hartley and Jones 1997). Thus, leaves with high C/N ratios are not suitable diets for such animals. Thick cell walls in the leaves of evergreen trees contain much lignin which is also effective in avoiding herbivory (Hartley and Jones 1997).

There are other reasons why leaves tend not to have very small cells. First, consider the rate of leaf expansion. If the doubling time of cells is uniform, the leaf area expansion rate

will increase with an increase in cell size. Annuals with short-lived leaves need to expand their leaves quickly and successively. Thus, it would be advantageous for these leaves to have big cells because they can attain large leaf area quickly for efficient light interception. In addition, to attain considerable mesophyll surface area with large cells, leaves would need to be thick, suggesting why the leaves of many annual herbs are amphistomatous (Meidner and Mansfield 1968, see also Figs. 4-6). On the other hand, stomata on the adaxial epidermis are more liable to be occluded by obstacles. The rate of transpiration is smaller in hypostomatous leaves than in amphistomatous leaves, in particular, in still air (Foster and Smith 1986).

In typical sun leaves, nitrogen in nuclei accounts for about 1O0/o of total leaf nitrogen (Evans and Seemann 1989). If the cells are very small and have their respective nuclei, the resources invested into chloroplasts relative to those invested into nuclei will decrease. This is obviously disadvantageous, since the ratio of N for photosynthetic biochemistry to that for genetic information would be very low. This could be another reason why leaves do not have very small cells.

There are other constraints determining the lower limit of leaf thickness. Let us consider the chloroplast volume as the minimum mesophyll volume. In most of the simulations presented here, we assume that leaves have 4 pmol Rubisco m-2. If the concentration of Rubisco in the stroma is 0.2 mM (Evans et a/. 1994), then the total stroma volume per unit leaf area is 2x10-5 m3 m-2. If the volume ratio of stroma to thylakoids is 1 : 1, the total volume of chloroplasts is 4X10-5 m3 m-2. In the present simulation, the volume ratio of mesophyll columns to the total leaf volume is 0.7. Taking this ratio into account, we see that the total chloroplast volume corresponds to a mesophyll thickness of 57 mm. Since other cell components are necessary, the leaves cannot be very thin. Sun leaves containing 4 pmol Rubisco m-2, would not be thinner than, say, 100pm.

It is also probable that heat capacitance per unit leaf area cannot be very small. Heat capacitance per leaf area increases with leaf' thickness. Very thin leaves heat up more easily (Jones 1992). When a strong light fleck hits the leaf, the leaf temperature increases considerably especially when stomata are closed. It is thus necessary for the leaf to have considerable heat capacitance. This could explain why shade leaves, which need not have thick mesophyll, have thick epidermes. Some shade tolerant species also have thick non-chlrophyllous tissues. Besides the merit in terms of light absorption by the whole leaf (Vogelmann 1993), the thick tissues can serve as a heat reservor.

These hypotheses concerning cell size and leaf thickness are testable. Studies along these lines are necessary to understand the functional basis of leaf anatomical properties.

We are grateful to Prof. H. Tobe (Kyoto University) for his invitation to contribute a paper to Journal of Plant Research. We thank Prof. K.J. Niklas (Cornell University) and Dr. A. Takenaka (National Institute for Environmental Studies) for acting as referees of this paper and for constructive comments. We also thank Drs. K. Noguchi and P. Vyas and Mr.


T. Saito of our laboratory for helpful comments. When I.T. tried to construct a model of CO:! diffusion in a leaf in 1982, Profs. Emeritus T. Saeki and M. Suzuki (University of Tokyo), Professor T. Oikawa (Tsukuba University), Dr. E. Maruta (Toho University), and Dr. T.Q.P. Uyeda (National Institute for Interdisciplinary Studies) gave invaluable suggestions, which are now incorporated in this paper.

References

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Appendix

dCi(x) dCi(x + Ax) J(x)-J(x+Ax)=-@/z) D (7- =a(x) Ax.

dx The gradient in C02 concentration at the distance x from

the substomatal cavitry is expressed as dCi(X)/dX. Then, the C02 flux through the plane at x, J(x), can be expressed

as Then,

043)

dCi(x) dCi(x + Ax) dx

(A1) dCi(x) dx '

J(x)=-@/z) D - lim

Note the sign of dCi(x)/dx in photosynthesizing leaf is OX-0 AX d2CI(X) =a(x) negative. The difference between J(x) and J(x+Ax) is the =@/z) D -

photosynthetic C02 uptake between x and x+Ax: dx2

why are sun leaves thicker than shade leaves ... · phase and the resistance to c02 diffusion per...

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