Photosynthetic characteristics of Flindersia brayleyana and Castanospermum australe from tropical lowland and upland sites

Tree Physiology 18, 341--347 © 1998 Heron Publishing----Victoria, Canada

Photosynthetic characteristics of Flindersia brayleyana and Castanospermum australe from tropical lowland and upland sites P. W. SWANBOROUGH,1,2 D. DOLEY,1,2 R. J. KEENAN1,3 and D. J. YATES1,2 1

Cooperative Research Centre for Tropical Rainforest Ecology and Management, P.O. Box 6811, Cairns, Queensland 4870, Australia

2

Department of Botany, University of Queensland, Brisbane, Queensland 4072, Australia

3

Queensland Forest Research Institute, Department of Primary Industries, P.O. Box 210, Atherton, Queensland 4883, Australia

Received May 7, 1997

Keywords: black bean, carboxylation rate, electron transport rate, photosynthesis, Queensland maple, rainforest.

Introduction There has been a decline in supplies of valuable timbers from rainforests in many countries (Myers 1980, Rowe et al. 1991). Although Australian rainforests represent only a small fraction of the tropical timber resource, these forests provided several valuable timber species to the local cabinet timber market for many years (Baker 1913, Swain 1928). Two of the most prized Queensland species were Flindersia brayleyana F. Muell. (Queensland maple) and Castanospermum australe Cunn. & C. Fraser ex Hook. (black bean) (Francis 1974, Russell et al. 1993). Flindersia brayleyana occurs naturally at scattered locations in northeastern Queensland between 16°20′ S and 19° S, and

between sea level and 1200 m altitude (Boland et al. 1984). The climatic zones range from hot humid, with annual rainfall exceeding 3500 mm, to warm sub-humid, with annual rainfall in excess of 1100 mm, the greater part falling in summer. All climates are seasonally dry, but the driest month normally receives more than 25 mm rainfall. Soils associated with F. brayleyana vary in depth and in parent material from granite to basalt, but the species grows best on well-watered basalt soils (Cameron and Jermyn 1991). Flindersia brayleyana has characteristics typical of middle order forest succession species (Bazzaz and Pickett 1980, Thompson et al. 1988). Vigorous growth requires relatively open growing conditions (E. Volk, Queensland Dept. of Forestry, Atherton, Australia and R.J. Keenan, unpublished results), although F. brayleyana seedlings can tolerate a wide range of light conditions (Thompson et al. 1988). Castanospermum australe is widely distributed on the east coast of Australia, between 12°30′ S and 29° S, and is also found in New Caledonia and Vanuatu (Boland et al. 1984). Most occurrences are near the coast, at altitudes between 50 and 750 m, but this species also occurs in an isolated rainforest at about 1000 m altitude in the Bunya Mountains, southern Queensland. Climates are humid, with either a summer maximum or uniform rainfall pattern. At most natural locations, C. australe is a riparian species, although it also occurs in other locations where water is seldom limiting. Because F. brayleyana is a preferred species for inclusion in plantations in north Queensland, it has been the subject of several studies. For example, in a controlled environment study, Thompson et al. (1988) showed that apparent quantum efficiency of photosynthesis, α, was influenced by light regime and nitrogen availability. Thompson et al. (1992b) also showed that, for F. brayleyana and three other rainforest tree species, Argyrodendron trifoliolatum F.J. Muell., Argyrodendron sp. and Toona ciliata M. Roem., there was a relatively close linear relationship between maximum rate of electron transport (Jmax ), which is related to α, and maximum rate of ribulosebisphosphate (RuP2) carboxylation (Vcmax ), which is related to carboxylation efficiency. These relationships are influenced by structural characteristics of leaves that influence light penetration to the chloroplasts, and carbon dioxide diffusion from the

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Summary Photosynthetic responses to temperature, light and carbon dioxide partial pressure were studied in two-yearold Flindersia brayleyana F. Muell. and Castanospermum australe Cunn. & C. Fraser ex Hook. growing on coastal lowland and upland rainforest sites in tropical Queensland, Australia. Climatic conditions ranged from moist and cool (17--19 °C) to dry and warm (22--24 °C). The optimum temperature for photosynthesis was 23.7--25.6 °C for C. australe and 21.2--24.6 °C for F. brayleyana. Mean maximum rate of electron transport for each species did not differ between sites but was higher (60--62 µmol m −2 s −1) in F. brayleyana than in C. australe (42--44 µmol m −2 s −1). Ribulose-bisphosphate carboxylation rate did not differ significantly between sites or species. Maximum rates of photosynthesis at 1000 µPa Pa −1 CO2 did not differ significantly between sites for each species, but did differ significantly between species. At 350 µPa Pa −1 CO2, photosynthetic light use efficiencies of F. brayleyana and C. australe were 0.05 and 0.015, respectively, at the upland site, and the corresponding values at the lowland site were 0.025 and 0.05. In C. australe, these differences were reflected in significantly greater maximum rates of photosynthesis at 350 µPa Pa −1 CO2 at the lowland site than at the upland site (5.2 versus 3.3 µmol m −2 s −1).

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Materials and methods Plant material Plant stock was raised in Queensland Department of Primary Industries (QDPI) nurseries, from seed collections made in northern Queensland. Both species were planted out at a spacing of 2 × 3 m when the seedlings were 30 to 40 cm in height.

Table 1. Description of the sites of physiological studies with Flindersia brayleyana and Castanospermum australe in north Queensland. Attribute

Upland

Lowland

Location 17°15′ S 145°31′ E 17°35′ S 145°59′ E Altitude (m a.s.l) 800 20 Topographic position Broad upper ridge Lower confined valley 20° to 100° Aspect 330° Slope 5% 5--10% Friable red earth Friable colluvial loam Soil type1 Meteorological station Kairi South Johnstone Mean annual rainfall (mm) 1190 3375 Temperature annual mean 21.12, 10.30 24.13, 8.25 and range (°C) Upland

Lowland

Species studied C. australe F. brayleyana C. australe F. brayleyana Planting 03/94 03/94 (month/year) Measurement 08/95 06/95 month/year Total rain in 34.5 40.9 study period (mm) Temperature 19.2, 10.9 17.0, 9.7 mean and range in month of study (°C) 1

02/93

01/93

09/95

07/95

137.9

160.3

24.0, 8.5

20.6, 8.5

Soil descriptions derived from Isbell et al. (1966).

Gas exchange system Site conditions Details of the upland and lowland sites are presented in Table 1. The upland site, part of an experimental plantation established on a former permanent pasture in March 1994, was unfertilized. At the lowland site, F. brayleyana and C. australe were planted in separate plots in January and February 1993, respectively. The F. brayleyana plots were unfertilized, the C. australe plots were fertilized with nitrogen and potassium. Five trees of each species at each site were selected for study at random. Measurements For each of the study trees, photosynthetic gas exchange data were collected from one leaf. Both species produce compound leaves, with leaflets reaching 40 cm2 in area in F. brayleyana and 20 cm2 in C. australe. For each tree, the third leaflet from the tip of the second or third youngest fully expanded leaf was selected on the northern side of the mainstem or a large erect upper branch. The instruments and measured branches were sheltered from the often inclement weather by an open tent. Observations at both sites were made between June and September 1995, with F. brayleyana being studied first at each site, and then C. australe. This procedure minimized the seasonal differences in functions within species, but resulted in different seasonal conditions being associated with measurements of F. brayleyana at the upland site and C. australe at the lowland site (Table 1).

Photosynthetic gas exchange rates were measured with a portable infrared gas analysis system (Model CI-301, CID Inc., Vancouver, WA) in open-system configuration operating in differential mode. The leaf chamber had a square aperture of 6.2 cm2. A quantum sensor (CI-324LN35DH, CID Inc.) was located outside the chamber, 5 mm from the edge of the aperture. Photosynthetic photon flux density (PPFD) was regulated by a light-emitting diode (LED) array located directly above the leaf chamber, which provided radiation mostly in the red portion of the spectrum. Leaf temperature in the chamber, which was controlled by a Peltier heat exchange unit (CI-301 CS, CID Inc.) attached to the lower surface of the leaf chamber, was calculated from the temperature of the air stream leaving the chamber, and allowed for the latent heat of vaporization associated with the measured transpiration rate, the quantum flux incident in the leaf chamber, and a standard boundary layer conductance of 3.3 mol m −2 s −1. Carbon dioxide partial pressure of air entering the chamber was regulated by adding pure CO2 through a mass flow meter to an air stream scrubbed by passage through soda lime. The leaf-air vapor pressure difference (LAVPD) within the chamber was regulated by a humidity controller (CI-301 AD, CID Inc.) inserted in the supply air pathway. Air flow rate through the chamber and analyzer was monitored by a mass flow meter incorporated in the analyzer system. The system recorded environmental variables and calculated rates of photosynthesis and transpiration, stomatal conductance and LAVPD at 5-s intervals. Data were saved for further processing when at least 10 consecutive

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stomata to the chloroplasts, and consequently by many aspects of the environment in which the leaves develop (Turnbull 1991, Thompson et al. 1992a, Westrup 1995). An association between J and Vc, which has been demonstrated in controlled environment studies of many plant species (Wullschleger 1992), has been interpreted as evidence that plants are able to optimize allocation of resources, particularly nitrogen, to preserve a balance between enzymatic (i.e., Rubisco) and light-harvesting (i.e., chlorophyll) capabilities (Thompson et al. 1992b). The patterns of distribution of resources among plant organs have important consequences for the survival of species in different environments (Givnish 1988), and for the commercial management of plants. To aid the development of suitable silvicultural regimes, we assessed photosynthetic responses to changes in temperature, light and carbon dioxide of the two most extensively planted cabinet timber species in Australia, F. brayleyana and C. australe.

PHOTOSYNTHETIC CHARACTERISTICS OF TWO TROPICAL SPECIES

Vc = (Al + Rd)(p i + Kc(1 + O / Ko)) / (p i − Γ∗),

photorespiration (Falge et al. 1996). The in situ rate of electron transport (J, µmol m −2 s −1) at 1000 µPa Pa −1 CO2 is described by: J = (Amax + Rd)(4.5p im + 10.5 Γ∗) / (pim − Γ∗),

(2)

where Amax is the maximum (light-saturated) rate of apparent photosynthesis at an ambient CO2 partial pressure of 1000 µPa Pa −1, and p im is the intercellular CO2 partial pressure associated with Amax . Mean values for photosynthetic attributes were compared between species and sites by standard analysis of variance procedures. Results and discussion The temperature responses of apparent photosynthesis at 350 µPa Pa −1 CO2 and 2000 µmol m −2 s −1 PPFD were described by quadratic curves (Figure 1). Flindersia brayleyana had a significantly lower photosynthetic optimum temperature than C. australe at the upland site (P < 0.05), but not at the lowland site, and exhibited a significant difference in mean optimum temperature of photosynthesis between the two sites (P < 0.05) (Table 2). There was no significant effect of site on the temperature optimum of photosynthesis in C. australe. The shapes of the quadratic curves that best described the mean temperature responses of photosynthesis for F. brayleyana at the lowland site and for C. australe at the upland site were almost identical. The light response curves of photosynthesis at 1000 µPa

(1)

where pi is the partial pressure of CO2 at the chloroplast, Al is the rate of apparent photosynthesis when CO2 is limiting (obtained by setting pi = pil (100 µPa Pa −1) and was derived from the curve of A versus pi), Kc is the Michaelis-Menten constant for ribulose 1,5-bisphosphate carboxylation (set at 252 µPa Pa −1 at 25 °C), Ko is the Michaelis-Menten constant for ribulose 1,5-bisphosphate oxygenation (set at 192 mPa Pa −1 at 25 °C), O is the oxygen partial pressure in the leaf (set at 200 mPa Pa −1), Γ* is the theoretical CO2 compensation partial pressure within the chloroplast (set at 35.4 µPa Pa −1 at 25 °C), and Rd is dark respiration, which is assumed to equal the daytime respiration in the absence of photorespiration (Thompson et al. 1992b). Light responses were determined at the optimum temperature for each species at each site, and a temperature correction was made for dark respiration and

Figure 1. Temperature response of apparent photosynthesis at 2000 µmol m −2 s −1 PPFD and 350 µPa Pa −1 ambient CO2 for Castanospermum australe (a, b) and Flindersia brayleyana (c, d) at upland (a, c) and lowland (b, d) sites in north Queensland. Vertical bars indicate standard errors of means for five trees. Continuous lines are quadratic curves of best fit used to estimate optimum temperature.

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estimates of photosynthesis varied by less than 5% after the predetermined environmental conditions had been stable for at least 15 min. For each leaf, a curve of the temperature response of net photosynthesis was constructed from measurements at a PPFD of 2000 µmol m −2 s −1 and a CO2 partial pressure of 350 µPa Pa −1. The sequence of measurements began at 15 °C and proceeded in 1 °C increments until the rate had decreased to less than 50% of the maximum observed. The temperature was then reduced in 1 °C steps and a second data set obtained, terminating when the rate of apparent photosynthesis was less than 50% of the maximum. The two data sets were combined for the fitting of a temperature response curve and the determination of the temperature associated with zero gradient of the curve (Topt ). At the optimum temperature for each leaf, gas exchange rates were measured at 2000, 1000, 500, 200, 100, 50 and 0 µmol m −2 s −1 PPFD for each of the CO2 partial pressures 0, 50, 150, 250, 350, 450, 600, 750 and 1000 µPa Pa −1. All measurements were made at an LAVPD of 2 kPa. Quantum efficiency (α) was calculated as the initial gradient of the quadratic curve best describing the response of photosynthesis to incident PPFD between 0 and 200 µmol m −2 s −1 at an atmospheric CO2 partial pressure of 1000 µPa Pa −1. Carboxylation efficiency (τ, mol m −1 s −1) was calculated as the initial gradient of the curve best describing the response of photosynthesis to atmospheric CO2 partial pressures between 0 and 350 µPa Pa −1 at incident PPFD of 2000 µmol m −2 s −1. The light-saturated photosynthetic rate (Amax ) was determined at a PPFD of 2000 µmol m −2 s −1 and a CO2 partial pressure of 1000 µPa Pa −1. Dark respiration rate (Rd) was determined at 350 and 1000 µPa Pa −1 after enclosure of the leaflet in a black cloth bag for at least 20 min. For each leaf, photosynthetic parameters were calculated by the photosynthetic model developed by Farquhar et al. (1980) and von Caemmerer and Farquhar (1981), modified as described by Thompson et al. (1992b). The rate of ribulose 1,5-bisphosphate carboxylation (Vc, µmol m −2 s −1) at a PPFD of 2000 µmol m −2 s −1 is described by:

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Table 2. Photosynthetic characteristics of Flindersia brayleyana and Castanospermum australe leaves from upland and lowland field sites in north Queensland described in Table 1. Parameter

Topt 350

α350 α1000 τ350 A350 A1000 R d 350 R d 1000

Qo Γ gs,max

J1000 1 2

Castanospermum australe 1

Upland

Lowland

Thompson

Upland

Lowland

21.2 0.054 ± 0.013 a 0.036 ± 0.008 a 0.040 ± 0.010 a 5.5 ± 1.5 a 11.2 ± 1.5 0.3 ± 0.1 a 0.6 ± 0.4 a 13.2 ± 1.7 a 41.6 ± 8.8 a 0.196 ± 0.005 a 60.5 ± 6.1 a 16.48 ± 1.46 a

24.6 0.025 ± 0.008 b 0.029 ± 0.014 a 0.022 ± 0.005 b 4.1 ± 0.3 ab 11.6 ± 1.9 0.6 ± 0.1 b 0.5 ± 0.4 a 30.3 ± 5.3 b 39.1 ± 10.6 a 0.194 ± 0.011 a 62.1 ± 8.3 a 15.54 ± 1.02 a

-0.046

23.7 0.015 ± 0.002 c 0.019 ± 0.003 b 0.011 ± 0.001 c 3.3 ± 0.5 b 7.3 ± 0.6 0.9 ± 0.3 b 0.9 ± 0.2 a 28.2 ± 3.4 b 68.1 ± 1.8 b 0.168 ± 0.008 b 42.0 ± 3.8 b 13.18 ± 2.72 a

25.6 0.050 ± 0.002 a 0.038 ± 0.001 a 0.030 ± 0.003 a 5.2 ± 0.4 a 8.4 ± 0.4 0.4 ± 0.06 a 0.2 ± 0.07 b 30.6 ± 5.9 b 52.6 ± 7.1 c 0.174 ± 0.011 ab 43.9 ± 2.2 b 14.9 ± 1.79 a

-7.4 1.3 30

65 31

Data from Thompson et al. (1988). Values of each parameter followed by the same letter are not significantly different (P = 0.05); Topt 350 , optimum temperature for photosynthesis (°C) at ambient CO2 partial pressure ( pCO 2) of 350 µPa Pa −1; α 350 and α1000 , apparent quantum efficiency of photosynthesis (mol mol −1) at pCO 2 of 350 and 1000 µPa Pa −1, respectively; τ350 , apparent carboxylation efficiency of photosynthesis (mol m −2 s −1) at PPFD of 2000 µmol m −2 s −1; A350 and A1 0 0 0 , maximum rate of apparent photosynthesis (µmol m −2 s −1) at pCO2 of 350 and 1000 µPa Pa−1, respectively; Rd3 5 0 and R d 1000 , rate of dark respiration (µmol m −2 s −1) at Topt and pCO 2 of 350 and 1000 µPa Pa −1, respectively; Qo, photosynthetic light compensation point (µmol m −2 s−1) at pCO 2 of 1000 µPa Pa −1; Γ, photosynthetic carbon dioxide compensation concentration (µPa Pa −1) at PPFD of 2000 µmol m −2 s −1; gs,max maximum stomatal conductance to water vapor (mol m −2 s −1) at PPFD of 2000 µmol m −2 s −1 and pCO2 of 1000 µPa Pa −1; J1000 , maximum rate of electron transport (µmol m −2 s −1) at pCO 2 of 1000 µPa Pa −1; V c2000 maximum carboxylation rate (µmol m −2 s −1) at PPFD of 2000 µmol m −2 s −1.

Pa −1 CO2 differed between both species and sites, but there was no evidence of photoinhibition in either species (Figure 2). Light-saturated rates of photosynthesis (Amax ) did not differ significantly between sites, but were consistently higher and more variable for F. brayleyana than for C. australe. For F. brayleyana, mean dark respiration rates did not differ significantly between lowland and upland sites (Table 2), whereas for C. australe, dark respiration rates were higher at the upland site than at the lowland site, and the variability in respiration rates was less than for F. brayleyana. At high light, photosynthesis continued to increase with increasing ambient CO2 partial pressures up to 1000 µPa Pa −1, or intercellular concentrations of about 900 µPa Pa −1, the highest that could be achieved with the photosynthesis system used (Figure 3). For both species at the upland site, and for F. brayleyana at the lowland site, the relationships between apparent photosynthesis and intercellular CO2 partial pressure were almost linear (Figure 3), and the initial slopes were similar. In contrast, the initial slope of the response curve for C. australe at the lowland site was steeper than for the others, and there was evidence of the onset of CO2 saturation at intercellular concentrations approaching 1000 µPa Pa −1. For both F. brayleyana and C. australe, mean values of α and τ differed significantly (P < 0.01) between sites (Table 2); however, the directions of the differences differed for the two species, with higher values being associated with the upland site in F. brayleyana and with the lowland site in C. australe. At both sites, but particularly at the upland site, the mean values and variabilities of α and τ were greater for F. brayley-

ana than for C. australe. Within a site, the relationships between α and τ were positive and linear for individual trees of C. australe (Figure 4),

Figure 2. Light response of apparent photosynthesis at optimum temperature and 1000 (closed symbols) and 350 µPa Pa −1 (open symbols) ambient CO2 for Castanospermum australe (a, b) and Flindersia brayleyana (c, d) at upland (a, c) and lowland (b, d) sites in north Queensland. Vertical bars indicate standard errors of means for five trees. Continuous lines are rectangular hyperbolae of best fit.

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V c2000

2

Flindersia brayleyana

PHOTOSYNTHETIC CHARACTERISTICS OF TWO TROPICAL SPECIES

indicating high variation between sites, but limited variation within sites in both parameters for C. australe. In contrast to the data for C. australe, which were tightly clustered at each site with a marked separation between the two sites, there was a continuum of data when α was plotted against τ for F. brayleyana trees from both sites, but with the upland site having consistently higher values than the lowland site. The existence of both positive and negative relationships between Topt and α and τ in these two species (Table 2) suggests that other factors in addition to site effects contributed to the differences between species. Although the previously established relationship between J and Vc (Thompson et al. 1992b, Wullschleger 1992) was preserved when site variation was introduced (Figure 5), the ways in which these parameters varied between sites differed in the two species. Within a species, J did not differ between sites but was higher in F. brayleyana than in C. australe, whereas Vc did not differ significantly between sites or species. In F. brayleyana, higher values of J and Vc were associated with the upland site, whereas in C. australe, these values were higher at the lowland site. Thompson et al. (1992b) demonstrated a positive relationship between nitrogen supply and the values of these variables in F. brayleyana, suggesting that soil nutrient and water availability may be associated with these differences in J and Vc. However, if nitrogen availability was the sole cause of the variation in J and Vc, both F. brayleyana and C. australe would be expected to respond in the same manner. Thompson et al. (1992b) found generally consistent changes in Vc and J in response to nitrogen and irradiance treatments

Figure 4. Relationships between apparent quantum efficiency (α) and apparent carboxylation efficiency (τ) for individual trees of Flindersia brayleyana (circles) and Castanospermum australe (triangles) at upland (open symbols) and lowland (closed symbols) in north Queensland: τ = 0.5348α + 0.0069, R 2 = 0.762.

Figure 5. Relationships between Rubisco carboxylation rate (Vc) at 2000 µmol m −2 s −1 PPFD and limiting intercellular CO2 concentration, and electron transport rate (J) at 1000 µPa Pa −1 ambient CO2 in Castanospermum australe (triangles) and Flindersia brayleyana (circles) at upland (open symbols) and lowland (closed symbols) in north Queensland.

when plants of four rainforest species were grown under controlled conditions providing a day/night temperature of 25/15 °C. Flindersia brayleyana and Toona ciliata, identified as early to mid-successional species, both showed greater variations in Vc and J than the late-successional Argyrodendron

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Figure 3. Response of apparent photosynthesis to increasing ambient (open symbols) and intercellular (closed symbols) CO2 partial pressure at optimum temperature, and 2000 µmol m −2 s −1 for (a, c) Flindersia brayleyana and (b, d) Castanospermum australe at upland (a, b) and lowland (c, d) sites in north Queensland. Vertical bars indicate standard errors of means for five trees. Continuous lines are polynomial equations of best fit.

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Acknowledgments The South Johnstone Cooperative Mill Ltd. and Mr. Ken Favier, Atherton, made land available for the planting of trees on which this study was based.

References Baker, R.T. 1913. Cabinet timbers of Australia. Technological Museum, Sydney, Australia, 186 p. Bazzaz, F.A. and S.T.A. Pickett. 1980. Physiological ecology of tropical succession: a comparative review. Annu. Rev. Ecol. Syst. 11:287--310. Boland, D.J., M.I.H. Brooker, G.M. Chippendale, N. Hall, B.P.M. Hyland, R.D. Johnston, D.A. Kleinig and J.D. Turner. 1984. Forest trees of Australia, 4th Edn. CSIRO, Melbourne, 687 p. Cameron, D.M. and D. Jermyn. 1991. Review of plantation performance of high value rainforest species. CSIRO and Queensland For. Serv., 88 p. Cribb, A.B. and J.W. Cribb. 1974. Wild food in Australia. William Collins, Sydney, Australia, 240 p. Evans, J.R. and I. Terashima. 1988. Photosynthetic characteristics of spinach leaves grown with different nitrogen treatments. Plant Cell Physiol. 29:157--165. Falge, A., W. Graber, D. Siegwolf and J.D. Tenhunen. 1996. A model of the gas exchange response of Picea abies to habitat conditions. Trees 10:277--287. Farquhar, G.D. and S. von Caemmerer. 1982. Modelling of photosynthetic response to environmental conditions. In Physiological Plant Ecology II. Water Relations and Carbon Assimilation. Eds. O.L. Lange, P.S. Nobel, C.B. Osmond and H. Ziegler. Encyclopedia of Plant Physiology, New Series, Vol. 12B. Springer-Verlag, Berlin, pp 549--588. Farquhar, G.D., S. von Caemmerer and J.A. Berry. 1980. A biochemical model of photosynthetic CO2 assimilation in leaves of C3 species. Planta 149:78--90. Francis, W.D. 1974. Australian rainforest trees. Aust. Govt. Publ. Serv., Canberra, Australia, 468 p. Givnish, T. 1988. Adaptations to sun and shade: A whole-plant perspective. Aust. J. Plant Physiol. 15:63--92. Isbell, R.F., G.G. Murtha and A.A. Webb. 1966. Atlas of Australian soils. Division of National Mapping, Dept. of National Development, Canberra, 60 p. Myers, B.J., R.H. Robichaux, G.L. Unwin and I.E. Craig. 1987. Leaf water relations and anatomy of a tropical rainforest tree species vary with crown position. Oecologia 74:81--85. Myers, N. 1980. Conversion of tropical moist forests. National Academy of Sciences, Washington, DC, 205 p. Rowe, R., N. Sharma and J. Browder. 1991. Deforestation: problems, causes, and concerns. In Managing the World’s Forests. Ed. N. Sharma. Kendall/Hunt Publishing Co., Dubuque, pp 33--45. Russel, J.S., D.M. Cameron, I.F. Whan, D.F. Beech, D.B. Prestwidge and S.J. Rance. 1993. Rainforest trees as a new crop for Australia. For. Ecol. Manag. 60:41--58. Swain, E.F.H. 1928. Timbers and forest products of Queensland. Government Printer, Brisbane, Australia, 500 p. Thompson, W.A., G.C. Stocker and P.E. Kriedemann. 1988. Growth and photosynthetic response to light and nutrients of Flindersia brayleyana F. Muell., a rainforest tree with broad tolerance to sun and shade. Aust. J. Plant Physiol. 15:299--315. Thompson, W.A., P.E. Kriedemann and I.E. Craig. 1992a. Photosynthetic response to light and nutrients in sun-tolerant and shade-tolerant rainforest trees. I. Growth, leaf anatomy and nutrient content. Aust. J. Plant Physiol. 19:1--18. Thompson, W.A., L.-K. Huang and P.E. Kriedemann. 1992b. Photosynthetic response to light and nutrients in sun-tolerant and shadetolerant rainforest trees. II. Leaf gas exchange and component processes of photosynthesis. Aust. J. Plant Physiol. 19:19--42.

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trifoliolatum and Argyrodendron sp. The overall relationships between J and Vc in our study are consistent with the ecological placements of F. brayleyana as a mid-successional species and C. australe as a late-successional species (Bazzaz and Pickett 1980). The relationships between J, Vc and A350 differed between the two species. In C. australe, both photosynthetic efficiency indices were positively related to A350 (Table 2) (cf. Farquhar and von Caemmerer 1982), whereas in F. brayleyana there appeared to be no relationship, with considerable variations in J and Vc being associated with small changes in A350 . Because structural variation between leaves influences the conductance of CO2 from the stomata to the chloroplasts (Evans and Terashima 1988, Thompson et al. 1992a), we postulate that variation in leaf structure was greater among individuals of F. brayleyana (Thompson et al. 1992a) than among individuals of C. australe (Myers et al. 1987). A prominent feature of our field study was the relative uniformity in physiological parameters within a site for C. australe, and the high variation in physiological parameters within a site for F. brayleyana. In contrast, in a controlled environment study, Thompson et al. (1992b) demonstrated substantial variation in photosynthetic parameters for F. brayleyana and concluded that the variation was associated with light regime and nitrogen availability. The reason for the discrepancy between the field study and the controlled environment study is not known. For the field study, the leaves were selected carefully to ensure uniformity of age and exposure to solar radiation during development. The sites were former agricultural fields, with no detectable differences in soil characteristics between the plots. Differences between sites in nutrition and light environment could have occurred in addition to those of temperature and rainfall (Table 1). No records were available concerning the provenances of the planting material of the two species. However, because there are few mature stands of C. australe in north Queensland from which seed is likely to have been collected, the genetic variability of the C. australe trees is likely to be less than that of the F. brayleyana trees for which a large number of trees would have contributed to the seed supply. On the basis of the physiological criteria established here, C. australe might be judged to have a more tropical and restricted natural distribution than F. brayleyana, but the converse is the case (Boland et al. 1984). We speculate that the distribution of C. australe over a wide latitudinal range may be more closely associated with the food value of its large seed to the aboriginal people (Cribb and Cribb 1974) than with natural seed dispersion processes. Our data indicate that the preferred environment for F. brayleyana is typified by the uplands, which is consistent with the patterns of natural distribution of the species (Boland et al. 1984).

PHOTOSYNTHETIC CHARACTERISTICS OF TWO TROPICAL SPECIES Turnbull, M.H. 1991. The effect of light quantity and quality during development on the photosynthetic characteristics of six Australian rainforest tree species. Oecologia 87:110--117. von Caemmerer, S. and G.D. Farquhar. 1981. Some relationships between the biochemistry of photosynthesis and the gas exchange of leaves. Planta 153:376--387.

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Westrup, J. 1995. Leaf spectral properties and light acclimation of four Australian rainforest trees. Ph.D. Thesis, Univ. of Queensland, Brisbane, Australia, 196 p. Wullschleger, S.D. 1992. Biochemical limitations to carbon assimilation in C3 plants: a retrospective analysis of the A/Ci curves from 109 species. J. Exp. Bot. 44:907--920.

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