Leaf-age effects on seasonal variability in photosynthetic parameters and its relationships with leaf mass per area and leaf nitrogen concentration within a Pinus densiflora crown

In the temperate zone of Japan, Pinus densiflora Sieb. et Zucc. bears needles of up to three age classes in the upper crown and up to five age classes in the lower crown. To elucidate the effects of leaf age on photosynthetic parameters and its relationships with leaf mass per unit area (LMA) and leaf nitrogen (N(l)) concentration on an area (N(a)) and mass (N(m)) basis, we measured seasonal variations in LMA, N(l), light-saturated photosynthetic rate (A(max)), stomatal conductance (g(s)), maximum rate of carboxylation (V(cmax)) and maximum rate of electron transport (J(max)) in leaves of all age classes in the upper and lower crown. Leaf mass per unit area increased by 27% with increasing leaf age in the lower crown, but LMA did not depend on age in the upper crown. Leaf age had a significant effect on N(m) but not on N(a) in both crown positions, indicating that decreases in N(m) resulted from dilution. Photosynthetic parameters decreased significantly with leaf age in the lower crown (39% for A(max) and 43% for V(cmax)), but the effect of leaf age was not as great in the upper crown, although these parameters exhibited seasonal variation in both crown positions. Regression analysis indicated a close relationship between LMA and N(a), regardless of age class or when each age class was pooled (r(2) = 0.57-0.86). Relationships between LMA and N(a) and among A(max), V(cmax) and J(max) were weak or not significant when all age classes were examined by regression analysis. However, compared with older leaves, relationships among LMA, N(a) and A(max) were stronger in younger leaves. These results indicate that changes in LMA and N(l) mainly reflect light acclimation during leaf development, but they are only slightly affected by irradiance in mature leaves. In conclusion, LMA and N(l) are useful parameters for estimating photosynthetic capacity, but age-related effects need to be taken into account, especially in evergreen conifers.     

Regulation of photosynthesis in interior spruce during water stress: changes in gas exchange and chlorophyll fluorescence

Photosynthetic response to water stress was analyzed in 1-year-old interior spruce (Picea glauca (Moench) Voss x P. engelmanni Parry hybrid complex) seedlings and emblings produced from somatic embryogenesis. Carbon dioxide uptake, oxygen evolution and chlorophyll fluorescence at 20 degrees C were monitored as predawn shoot water potential (Psi) decreased. Concurrently with stomatal closure, carbon assimilation declined rapidly as Psi decreased to -1.0 MPa. Oxygen evolution at 10,000 micro l CO(2) l(-1) declined continuously as Psi decreased to -1.6 MPa. At photon flux densities (PFD) above 50 micro mol m(-2) s(-1), photochemical efficiency of photosystem (PS) II observed during actinic light exposure (Phi(II), calculated as DeltaF/F(m)') decreased as Psi decreased. At the same PFDs, photochemical quenching (q(P)) declined with decreasing Psi and nonphotochemical quenching (q(N)) increased steadily. At PFDs below 50 micro mol m(-2) s(-1), major decreases in q(N) were not observed until Psi decreased below -1.6 MPa. We identified three phases of photosynthetic response to progressive water stress in interior spruce: a pronounced decline in gas exchange, subsequent photoprotective changes in chlorophyll fluorescence as primary photochemistry was down-regulated, and a decline in photochemical efficiency of dark-adapted needles.     

Effects of potassium supply on limitations of photosynthesis by mesophyll diffusion conductance in Carya cathayensis

Potassium (K) influences the photosynthesis process in a number of ways; however, the mechanisms underlying the photosynthetic response to differences in K supply are not well understood. Concurrent measurements of gas exchange and chlorophyll fluorescence were made to investigate the effect of K nutrition on photosynthetic efficiency and mesophyll conductance (g(m)) in hickory seedlings (Carya cathayensis Sarg.) in a greenhouse. The results show that leaf K concentrations < 0.7-0.8% appeared to limit the leaf net CO2 assimilation rate (A), and that the relative limitation of photosynthesis due to g(m) and stomatal conductance (g(s)) decreased with increasing supplies of K. However, a sensitivity analysis indicated that A was most sensitive to the maximum carboxylation rate of Rubisco (V(c,max)) and the maximum rate of electron transport (J(max)). These results indicate that the photosynthetic rate is primarily limited by the biochemical processes of photosynthesis (V(c,max) and J(max)), rather than by g(m) and g(s) in K-deficient plants. Additionally, g(m) was closely correlated with g(s) and the leaf dry mass per unit area (M(A)) in hickory seedlings, which indicates that decreased g(m) and g(s) may be a consequence of leaf anatomical adaptation.     

Seasonal photosynthetic gas exchange and leaf reflectance characteristics of male and female cottonwoods in a riparian woodland

Cottonwoods (Populus spp.) are dioecious phreatophytes of hydrological and ecological importance in riparian woodlands throughout the Northern Hemisphere. In streamside zones of southern Alberta, groundwater and soil water typically decline between May and September. To understand how narrowleaf cottonwoods (Populus angustifolia James) are adapted to this seasonal decrease in water availability, we measured photosynthetic gas exchange, leaf reflectance, chlorophyll fluorescence and stable carbon isotope composition (delta(13)C) in trees growing in the Oldman River valley of southern Alberta during the 2006 growth season. Accompanying the seasonal recession in river flow, groundwater table depth (Z(gw)) declined by 1.6 m, but neither mean daily light-saturated net photosynthetic rate (A(max)) nor stomatal conductance (g(s)) was correlated with this change. Both A(max) and g(s) followed a parabolic seasonal pattern, with July 24 maxima of 15.8 micromol m(-2) s(-1) and 559 mmol m(-2) s(-1), respectively. The early summer rise in A(max) was related to an increase in the chlorophyll pool during leaf development. Peak A(max) coincided with the maximum quantum efficiency of Photosystem II (F(v)/F(m)), chlorophyll index (CI) and scaled photochemical reflectance index (sPRI), but occurred one month after maximum volumetric soil water (theta(v)) and minimum Z(gw). In late summer, A(max) decreased by 30-40% from maximum values, in weak correlation with theta(v) (r(2) = 0.50). Groundwater availability limited late-season water stress, so that there was little variation in mean daily transpiration (E). Decreasing leaf nitrogen (% dry mass), CI, F(v)/F(m) and normalized difference vegetation index (NDVI) were also consistent with leaf aging effects. There was a strong correlation between A(max) and g(s) (r(2) = 0.89), so that photosynthetic water-use efficiency (WUE; A(max)/E) decreased logarithmically with increasing vapor pressure deficit in both males (r(2) = 0.75) and females (r(2) = 0.95). The male:female ratio was unequal (2:1, chi(2) = 16.5, P < 0.001) at the study site, but we found no significant between-sex differences in photosynthetic gas exchange, leaf reflectance or chlorophyll fluorescence that might explain the unequal ratio. Females tended to display lower NDVI than males (P = 0.07), but mean WUE did not differ significantly between males and females (2.1 +/- 0.2 versus 2.5 +/- 0.2 mmol mol(-1)), and delta(13)C remained in the -28.8 to -29.3 per thousand range throughout the growth season, in both sexes. These results demonstrate changes in photosynthetic and water-use characteristics that collectively enable vigorous growth throughout the season, despite seasonal changes in water supply and demand.     

Leaf conductance and CO(2) assimilation of Larix gmelinii growing in an eastern Siberian boreal forest

In July 1993, we measured leaf conductance, carbon dioxide (CO(2)) assimilation, and transpiration in a Larix gmelinii (Rupr.) Rupr. ex Kuzen forest in eastern Siberia. At the CO(2) concentration of ambient air, maximum values (mean of 10 highest measured values) for CO(2) assimilation, transpiration and leaf conductance for water vapor were 10.1 micro mol m(-2) s(-1), 3.9 mmol m(-2) s(-1) and 365 mmol m(-2) s(-1), respectively. The corresponding mean values, which were much lower than the maximum values, were 2.7 micro mol m(-2) s(-1), 1.0 mmol m(-2) s(-1) and 56 mmol m(-2) s(-1). The mean values were similar to those of Vaccinium species in the herb layer. The large differences between maximum and actual performance were the result of structural and physiological variations within the tree crowns and between trees that reduced maximum assimilation and leaf conductance by about 40 and 60%, respectively. Thus, maximum assimilation and conductance values averaged over the canopy were 6.1 micro mol m(-2) s(-1) and 146 mmol m(-2) s(-1), respectively. Dry air caused stomatal closure, which reduced assimilation by an additional 26%. Low irradiances in the morning and evening had a minor effect (-6%). Daily canopy transpiration was estimated to be 1.45 mm day(-1), which is higher than the value of 0.94 mm day(-1) measured by eddy covariance, but similar to the value of 1.45 mm day(-1) calculated from the energy balance and soil evaporation, and less than the value of 2.1 mm day(-1) measured by xylem flux. Daytime canopy carbon assimilation, expressed on a ground area basis, was 0.217 mol m(-2) day(-1), which is higher than the value measured by eddy flux (0.162 mol m(-2) day(-1) including soil respiration). We discuss the regulation of leaf gas exchange in Larix under the extreme climatic conditions of eastern Siberia (temperature > 35 degrees C and vapor pressure deficit > 5.0 kPa).     

Seasonal CO(2) assimilation and stomatal limitations in a Pinus taeda canopy

Net CO(2) assimilation (A(net)) of canopy leaves is the principal process governing carbon storage from the atmosphere in forests, but it has rarely been measured over multiple seasons and multiple years. I measured midday A(net) in the upper canopy of maturing loblolly pine (Pinus taeda L.) trees in the piedmont region of the southeastern USA on 146 sunny days over 36 months. Concurrent data for leaf conductance and photosynthetic CO(2) response curves (A(net)-C(i) curves) were used to estimate the relative importance of stomatal limitations to CO(2) assimilation in the field. In fully expanded current-year and 1-year-old needles, midday light-saturated A(net) was constant over much of the growing season (5-6 &mgr;mol CO(2) m(-2) s(-1)), except during drought periods. During the winter season (November-March), midday A(net) of overwintering needles varied in proportion to leaf temperature. Net CO(2) assimilation at light saturation occurred when daytime air temperatures exceeded 5-6 degrees C, as happened on more than 90% of the sunny winter days. In both age classes of foliage, winter carbon assimilation accounted for approximately 15% of the daily carbon assimilation on sunny days throughout the year, and was relatively insensitive to year-to-year differences in temperature during this season. However, strong stomatal limitations to A(net) occurred as a result of water stress associated with freezing cycles in winter. During the growing season, drought-induced water stress produced the largest year-to-year differences in seasonal CO(2) assimilation on sunny days. Seasonal A(net) was more drought sensitive in current-year needles than in 1-year-old needles. Relative stomatal limitations to daily integrated A(net) were approximately 40% over the growing season, and summer drought rather than high temperatures had the largest impact on summer A(net) and integrated annual CO(2) uptake in the upper crown. Despite significant stomatal limitations, a long duration of near-peak A(net) in the upper crown, particularly in 1-year-old needles, conferred high seasonal and annual carbon gain.     

Growth and nutrition of small Betula pendula plants at different relative addition rates of manganese

In a series of experiments, growth of small birch plants (Betula pendula Roth) was controlled by the relative addition rate of manganese, R(Mn) (day(-1)). The R(Mn) treatments were 0.05, 0.10, 0.15 and 0.20 day(-1) with free access to all other nutrients. In an additional treatment, FA, there was free access to all nutrients including Mn. The pH of the nutrient solution ranged between 3.9 and 4.1, and the conductivity was between 100 and 200 micro S cm(-1). After an adjustment phase to steady-state growth, there was a one-to-one relationship between the relative growth rate, R(G) (day(-1)), and the supply of manganese, R(Mn) (day(-1)). The Mn concentration of the plants ranged from 6 to 13 micro g g(DW) (-1) in all treatments with limiting R(Mn) and was approximately 200 micro g g(DW) (-1) in the FA treatment. At steady-state growth, the plants showed specific Mn deficiency symptoms, including leaf mortality, that were more pronounced at severe Mn limitation. Total nonstructural carbohydrate concentrations were low, less than 7.5% of dry weight at Mn limitation, and the fraction of plant dry matter partitioned to roots was much less at Mn limitation than has previously been reported for equivalent rates of N or P supply. Manganese uptake rate per unit root growth rate, dMn/dW(r) ( micro mol g(DW) (-1)) was unaffected by the supply of Mn. At Mn limitation, low rates of plant growth were associated with high values of specific leaf area (37 versus 36 m(2) kg(DW) (-1)), and lower values of leaf weight ratio (40 versus 61%) and net assimilation rate (3 versus 10 kg(DW) (-1) m(-2) day(-1)) than were found at higher R(Mn).     

Optimality and nitrogen allocation in a tree canopy

Physical and functional properties of foliage were measured at a variety of microsites in a broad-leaved Nothofagus fusca (Hook. f.) ?rst. canopy. The light climate of the foliage at these sites was monitored for 39 days in the late spring and early summer with in situ sensors. Foliage nitrogen content (N), mean leaf angle, and gas exchange characteristics were all correlated with the amount of light reaching the microsites during foliage development. Foliage N content on a leaf area basis ranged between ~1 and 2.5 g N m(-2) and was highest at the brightest sites. Light-saturated photosynthetic rates ranged between ~4 and 9 micro mol m(-2) s(-1), increasing from the darkest to brightest sites. A biochemical model of photosynthesis was fitted to foliage characteristics at the different microsites and used to integrate foliage assimilation among the sites over 39 days. The actual arrangement of foliage physiological characteristics in the observed microsites led to higher total canopy rates of net assimilation than > 99% of the combinations of observed foliage characteristics randomly assigned to the observed microsites. Additional simulations first related the maximum rates of electron transport (J(max)), ribulose bisphosphate turnover (V(c,max)), and dark respiration (R(d)) of Nothofagus fusca foliage to nitrogen content and then allowed foliage N (and consequently leaf gas exchange characteristics) to vary across the canopy. The observed N allocation pattern results in greater total canopy assimilation than uniform or > 99% of the simulations with random distributions of N among the microsites (constrained so that the total N allocated was equivalent to that observed in the microsites). However, the observed pattern of N allocation places less N in the brightest microsites and results in substantially less total assimilation than a simulated canopy in which N was allocated in an optimal manner where the N distribution is such that the partial derivative of leaf assimilation (A) with respect to leaf nitrogen content, partial differential A/ partial differential N, is constant among microsites. These results suggest that other factors such as wind or herbivory reduce the integrated assimilation of high-N foliage relatively more than lower-N foliage and that a partial differential A/ partial differential N optimality criteria based only on formulations of leaf gas exchange overestimate canopy assimilation.     

Photosynthetic traits around budbreak in pre-existing needles of Sakhalin spruce (Picea glehnii) seedlings grown under elevated CO2 concentration assessed by chlorophyll fluorescence measurements

To assess the effects of elevated CO(2) concentration ([CO(2)]) on the photosynthetic properties around spring budbreak, we monitored the total leaf sugar and starch content, and chlorophyll fluorescence in 1-year-old needles of Sakhalin spruce (Picea glehnii Masters) seedlings in relation to the timing of budbreak, grown in a phytotron under natural daylight at two [CO(2)] levels (ambient: 360?μmol mol(-1) and elevated: 720?μmol mol(-1)). Budbreak was accelerated by elevated [CO(2)] accompanied with earlier temporal declines in the quantum yield of PSII electron transport (Φ(PSII)) and photochemical quenching (q(L)). Plants grown under elevated [CO(2)] showed pre-budbreak leaf starch content twice as high with no significant difference in Φ(PSII) from ambient-CO(2)-grown plants when compared at the same measurement [CO(2)], i.e., 360 or 720?μmol mol(-1), suggesting that the enhanced pre-budbreak leaf starch accumulation might not cause down-regulation of photosynthesis in pre-existing needles under elevated [CO(2)]. Conversely, lower excitation pressure adjusted for the efficiency of PSII photochemistry ((1?-?q(P)) F(v)'/F(m)') was observed in plants grown under elevated [CO(2)] around budbreak when compared at their growth [CO(2)] (i.e., comparing (1?-?q(P)) F(v)'/F(m)' measured at 720?μmol mol(-1) in elevated-CO(2)-grown plants with that at 360?μmol mol(-1) in ambient-CO(2)-grown plants), which suggests lower rate of photoinactivation of PSII in the elevated-CO(2)-grown plants around spring budbreak. The degree of photoinhibition, as indicated by the overnight-dark-adapted F(v)/F(m), however, showed no difference between CO(2) treatments, thereby suggesting that photoprotection during the daytime or the repair of PSII at night was sufficient to alleviate differences in the rate of photoinactivation.     

Photosynthetic responses of Scots pine to elevated CO(2) and nitrogen supply: results of a branch-in-bag experiment

Naturally seeded Scots pine (Pinus sylvestris L.) trees, age 25-30 years, were subjected to two soil-nitrogen-supply regimes and to elevated atmospheric CO(2) concentrations by the branch-in-bag method from April 15 to September 15 for two or three years. Gas exchange in detached shoots was measured in a diffuse radiation field. Seven parameters associated with photosynthetic performance and two describing stomatal conductance were determined to assess the effects of treatments on photosynthetic components. An elevated concentration of CO(2) did not lead to a significant downward regulation in maximum carboxylation rate (V(cmax)) or maximum electron transport rate (J(max)), but it significantly decreased light-saturated stomatal conductance (g(sat)) and increased minimum stomatal conductance (g(min)). Light-saturated rates of CO(2) assimilation were higher (24-31%) in shoots grown and measured at elevated CO(2) concentration than in shoots grown and measured at ambient CO(2) concentration, regardless of treatment time or nitrogen-supply regime. High soil-nitrogen supply significantly increased photosynthetic capacity, corresponding to significant increases in V(cmax) and J(max). However, the combined elevated CO(2) + high nitrogen-supply treatment did not enhance the photosynthetic response above that observed in the elevated CO(2) treatment alone.     

Diurnal changes in leaf gas exchange characteristics in the uppermost canopy of a rain forest tree, Dryobalanops aromatica Gaertn. f

Dryobalanops aromatica Gaertn. f. is a major tropical canopy species in lowland tropical rain forests in Peninsular Malaysia. Diurnal changes in net photosynthetic rate (A) and stomatal conductance to water vapor (g(s)) were measured in fully expanded young and old leaves in the uppermost canopy (35 m above ground). Maximum A was 12 and 10 micro mol m(-2) s(-1) in young and old leaves, respectively; however, because of large variation in A among leaves, mean maximum A in young and old leaves was only 6.6 and 5.5 micro mol m(-2) s(-1), respectively. Both g(s) and A declined in young leaves when T(leaf) exceeded 34 degrees C and leaf-to-air vapor pressure deficit (DeltaW) exceeded 0.025, whereas in old leaves, g(s) and A did not start to decline until T(leaf) and DeltaW exceeded 36 degrees C and 0.035, respectively. Under saturating light conditions, A was linearly related to g(s). The coefficient of variation (CV) for the difference between the CO(2) concentrations of ambient air and the leaf intercellular air space (C(a) - C(i)) was smaller than the CV for A or g(s), suggesting that maximum g(s) was mainly controlled by mesophyll assimilation (A/C(i)). Minimum C(i)/C(a) ratios were relatively high (0.72-0.73), indicating a small drought-induced stomatal limitation to A and non-conservative water use in the uppermost canopy leaves.     

Effects of needle age, long-term temperature and CO(2) treatments on the photosynthesis of Scots pine

Naturally regenerated 20-25-year-old Scots pine (Pinus sylvestris L.) trees were grown in open-top chambers in the presence of an elevated temperature or CO(2) concentration, or both. The elevated temperature treatment was administered year-round for 3 years. The CO(2) treatment was applied between April 15 and September 15 for 2 years. The photosynthetic responses of 1- and 2-year-old needles to varying photon flux densities (0-1500 micro mol m(-2) s(-1)) and CO(2) concentrations (350, 700 and 1400 micro mol mol(-1)) during measurement were determined. The CO(2) treatment alone increased maximum photosynthetic rate and light-use efficiency, but decreased dark respiration rate, light compensation and light saturation regardless of needle age. In contrast, the temperature treatment decreased maximum photosynthetic rate and photosynthetic efficiency, but increased dark respiration rate, light compensation and light saturation. The aging of needles affected the photosynthetic performance of the shoots; values of all parameters except photosynthetic efficiency were less in 2- than in 1-year-old needles. The CO(2) treatment decreased and the temperature treatment enhanced the reduction in maximum photosynthesis due to needle aging.     

Water deficits at anthesis reduce CO(2) assimilation and yield of lychee (Litchi chinensis Sonn.) trees

Ten-year-old 'Tai So' lychee (Litchi chinensis Sonn.) trees growing on a sandy loam soil in subtropical South Africa (latitude 25 degrees S) were watered weekly (well-watered treatment) or droughted from late July until January (drought treatment). After 16 weeks, at which time the trees obtained most of their water from below 150 cm, average soil water content at 0 to 150 cm depth was 14.5 +/- 0.1% in the well-watered treatment and reached a minimum of 7.6% in the drought treatment. At Week 7, minimum leaf water potential (Psi(L)) in the morning and early afternoon declined to -2.6 and -2.8 MPa, respectively, in droughted trees compared with -1.5 and -2.2 MPa, respectively, in well-watered trees. From Week 9, stomatal conductance and net CO(2) assimilation rate ranged from 70 to 300 mmol m(-2) s(-1) and 3 to 13 micro mol CO(2) m(-2) s(-1), respectively, in well-watered trees. The corresponding values for droughted trees were 50 to 180 mmol m(-2) s(-1) and 2 to 6 micro mol CO(2) m(-2) s(-1). Five weeks after rewatering the droughted trees, gas exchange had not recovered to the rate in well-watered trees, although tree water status recovered within a week of rewatering. In the well-watered trees, water use (E(t)) was 26 +/- 1 mm week(-1) with evaporation (E(p)) of 20 to 70 mm week(-1) indicating a crop factor (k(c) = E(t)/E(p)) of 0.4 to 1.2. Before anthesis, tree water status did not affect extension growth of floral panicles or leafy shoots. In contrast, no vegetative shoots were initiated after fruit set in the droughted trees when Psi(L) in the morning declined to -2.5 MPa. Water deficits reduced initial fruit set by 30% and final fruit set by 70% as a result of fruit splitting (41.2 +/- 4.0% versus 10.0 +/- 1.3%). Water deficits did not alter the sigmoidal pattern of fruit growth, but reduced yield from 51.4 +/- 5.5 kg tree(-1) in well-watered trees to 7.4 +/- 3.3 kg tree(-1) in droughted trees.     

Photosynthetic responses to needle water potentials in Scots pine after a four-year exposure to elevated CO(2) and temperature

Effects of needle water potential (Psi(l)) on gas exchange of Scots pine (Pinus sylvestris L.) grown for 4 years in open-top chambers with elevated temperature (ET), elevated CO(2) (EC) or a combination of elevated temperature and CO(2) (EC + ET) were examined at a high photon flux density (PPFD), saturated leaf to air water vapor pressure deficit (VPD) and optimal temperature (T). We used the Farquhar model of photosynthesis to estimate the separate effects of Psi(l) and the treatments on maximum carboxylation efficiency (V(c,max)), ribulose-1,5-bisphosphate regeneration capacity (J), rate of respiration in the light (R(d)), intercellular partial pressure of CO(2) (C(i)) and stomatal conductance (G(s)). Depression of CO(2) assimilation rate at low Psi(l) was the result of both stomatal and non-stomatal limitations on photosynthetic processes; however, stomatal limitations dominated during short-term water stress (Psi(l) < -1.2 MPa), whereas non-stomatal limitations dominated during severe water stress. Among the nonstomatal components, the decrease in J contributed more to the decline in photosynthesis than the decrease in V(c,max). Long-term elevation of CO(2) and temperature led to differences in the maximum values of the parameters, the threshold values of Psi(l) and the sensitivity of the parameters to decreasing Psi(l). The CO(2) treatment decreased the maximum values of V(c,max), J and R(d) but significantly increased the sensitivity of V(c,max), J and R(d) to decreasing Psi(l) (P < 0.05). The effects of the ET and EC + ET treatments on V(c,max), J and R(d) were opposite to the effects of the EC treatment on these parameters. The values of G(s), which were measured simultaneously with maximum net rate of assimilation (A(max)), declined in a curvilinear fashion as Psi(l) decreased. Both the EC + ET and ET treatments significantly decreased the sensitivity of G(s) to decreasing Psi(l). We conclude that, in the future, acclimation to increased atmospheric CO(2) and temperature could increase the tolerance of Scots pine to water stress.     

Effects of season,needle age and elevated atmospheric CO(2) on photosynthesis in Scots pine (Pinus sylvestris)

Five-year-old Scots pine (Pinus sylvestris L.) seedlings were grown in open-top chambers at ambient and elevated (ambient + 400 &mgr;mol mol(-1)) CO(2) concentrations. Net photosynthesis (A), specific leaf area (SLA) and concentrations of nitrogen (N), carbon (C), soluble sugars, starch and chlorophyll were measured in current-year and 1-year-old needles during the second year of CO(2) enrichment. The elevated CO(2) treatment stimulated photosynthetic rates when measured at the growth CO(2) concentration, but decreased photosynthetic capacity compared with the ambient CO(2) treatment. Acclimation to elevated CO(2) involved decreases in carboxylation efficiency and RuBP regeneration capacity. Compared with the ambient CO(2) treatment, elevated CO(2) reduced light-saturated photosynthesis (when measured at 350 &mgr;mol mol(-1) in both treatments) by 18 and 23% (averaged over the growing season) in current-year and 1-year-old needles, respectively. We observed significant interactive effects of CO(2) treatment, needle age and time during the growing season on photosynthesis. Large seasonal variations in photosynthetic parameters were attributed to changes in needle chemistry, needle structure and feedbacks governed by whole-plant growth dynamics. Down-regulation of photosynthesis was probably a result of reduced N concentration on an area basis, although a downward shift in the relationship between photosynthetic parameters and N was also observed.     

Effects of light availability on leaf gas exchange and expansion in lychee (Litchi chinensis)

Effects of photosynthetic photon flux density (PPFD) on leaf gas exchange of lychee (Litchi chinensis Sonn.) were studied in field-grown "Kwai May Pink" and "Salathiel" orchard trees and young potted "Kwai May Pink" plants during summer in subtropical Queensland (27 degrees S). Variations in PPFD were achieved by shading the trees or plants 1 h before measurement at 0800 h. In a second experiment, potted seedlings of "Kwai May Pink" were grown in a heated greenhouse in 20% of full sun (equivalent to maximum noon PPFD of 200 micromol m(-2)xs(-1)) and their growth over three flush cycles was compared with seedlings grown in full sun (1080 micromol m(-2)xs(-1)). Young potted plants of "Kwai May Pink" were also grown outdoors in artificial shade that provided 20, 40, 70 or 100% of full sun (equivalent to maximum PPFDs of 500, 900, 1400 and 2000 micromol m(-2)xs(-1)) and measured for shoot extension and leaf area development over one flush cycle. Net CO2 assimilation increased asymptotically in response to increasing PPFD in both orchard trees and young potted plants. Maximum rates of CO2 assimilation (11.9 +/- 0.5 versus 6.3 +/- 0.2 micromol CO2 m(-2) s(-1)), dark respiration (1.7 +/- 0.3 versus 0.6 +/- 0.2 micromol CO2 m(-2) s(-1)), quantum yield (0.042 +/- 0.005 versus 0.027 +/- 0.003 mol CO2 mol(-1)) and light saturation point (1155 versus 959 micromol m(-2) s(-1)) were higher in orchard trees than in young potted plants. In potted seedlings grown in a heated greenhouse, shoots and leaves exposed to full sun expanded in a sigmoidal pattern to 69 +/- 12 mm and 497 +/- 105 cm(2) for each flush, compared with 27 +/- 7 mm and 189 +/- 88 cm(2) in shaded seedlings. Shaded seedlings were smaller and had higher shoot:root ratios (3.7 versus 3.1) than seedlings grown in full sun. In the potted plants grown outdoors in 20, 40, 70 or 100% of full sun, final leaf area per shoot was 44 +/- 1, 143 +/- 3, 251 +/- 7 and 362 +/- 8 cm(2), respectively. Shoots were also shorter in plants grown in shade than in plants grown in full sun (66 +/- 5 mm versus 101 +/- 2 mm). Photosynthesis in individual leaves of lychee appeared to be saturated at about half full sun, whereas maximum leaf expansion occurred at higher PPFDs. We conclude that lychee plants can persist as seedlings on the forest floor, but require high PPFDs for optimum growth.     

Growth and photosynthesis of tropical forest tree seedlings (Bischofia javanica Blume) as influenced by a change in light availability

Acclimation in seedlings of Bischofia javanica Blume, which are commonly found in canopy gaps in the moist forests of tropical Asia, to a change in light availability was examined in a controlled environment simulating forest shade and daylight. Seedlings were grown in a high (1000 micro mol m(-2) s(-1); red/far-red, 1.45) or low (40 micro mol m(-2) s(-1); red/far-red, 0.10) light regime and then transferred to the contrasting light environment for nine weeks. Control seedlings were maintained in the same light regime throughout the study. The availability of light influenced relative growth rate through morphological and physiological adjustments. Transferred seedlings retained the leaves that had been developed before transfer, and no leaf-shedding was observed till the end of the experiment. Leaves formed in the new light regime were physiologically and morphologically identical to those of the corresponding controls. High-light seedlings transferred to low light displayed significantly lower relative growth rate than the low-light controls because of a lower leaf area ratio carried over from the previous high-light environment. A reverse pattern of response with respect to relative growth rate was observed for the low-light seedlings transferred to high light compared to the high-light controls. The higher relative growth rate in the low-light seedlings transferred to high light was the result of higher net assimilation rate and higher leaf area ratio. The higher leaf area ratio in the low-light seedlings transferred to high light was the consequence of the effects of previous environment, and the relatively lower net assimilation rate in the high-light control seedlings was, at least partly, due to the effects of self-shading rather than to the photosynthetic capacity of the leaves. The results suggest that the species has a wide acclimation potential to a change in light availability that might occur in nature following gap creation or canopy closure.     

Estimates of net photosynthetic parameters for twelve tree species in mature forests of the southern Appalachians

Leaf gas exchange, temperature, and incident radiation were measured in situ for 20 mature trees of 12 deciduous species spanning a range of heights from 7.9 to 30.1 m and growing in the southern Appalachian Mountains. Air temperature, water vapor pressure, total radiation, photosynthetically active radiation, and carbon dioxide concentration were also measured. Estimated mean, light-saturated net assimilation rates ( micro mol m(-2) s(-1)) were: Quercus coccinea Muenchh. (10.3), Q. prinus L. (9.9), Q. rubra L. (8.9), Betula lenta L. (8.1), Liriodendron tulipifera L. (7.9), Q. alba L. (7.6), Carya glabra Mill. (7.2), Acer rubrum L. (5.6), Nyssa sylvatica Marsh. (3.9), Cornus florida L. (3.5), and Acer pensylvanicum L. (1.7). There were significant differences in both net assimilation rates and quantum yield efficiencies between species, with the understory species C. florida and A. pensylvanicum exhibiting lower net assimilation rates at saturation and higher estimated quantum yield efficiencies than the other species. Average temperature and light decreased from the canopy top to bottom, whereas ambient CO(2) concentration increased, and vapor pressure and vapor pressure deficits were inconsistent. We observed curvilinear effects of temperature and vapor pressure deficit on net assimilation response to light, and these effects varied by species. Errors in predicted net assimilation ranged from 1 to 3 micro mol m(-2) s(-1) under the environmental conditions prevailing during the study.     

Effect of photon flux density on carbon assimilation and chlorophyll a fluorescence of cold-stored white spruce and lodgepole pine seedlings

White spruce (Picea glauca (Moench.) Voss) and lodgepole pine (Pinus contorta Dougl. var. latifolia Engelm.) seedlings previously held in dark, frozen storage (-2 degrees C) for 2.5 or 6 months, and nursery-grown white spruce seedlings lifted in summer were exposed to photon flux densities (PFDs) similar to those that might be encountered at planting. Photosynthetic gas exchange and chlorophyll a (chl a) fluorescence were examined in cold-stored and summer-lifted seedlings before and after a 9 h-exposure to artificial illumination of high PFD (2000 micro mol m(-2) s(-1)) or low PFD (ca. 500 micro mol m(-2) s(-1)), and during exposure to 400 micro mol m(-2) s(-1) for 4-9 days. In the 2.5-month-stored and summer-lifted seedlings, the high-PFD treatment caused a small decrease in carbon fixation and a large decrease in the ratio of variable to maximum fluorescence (F(v)/F(m)) relative to the effect of the low-PFD treatment. In contrast, in the 6-month-stored seedlings the high-PFD treatment caused a significant decrease in rate of light-saturated carbon fixation but little decrease in F(v)/F(m) relative to the effect of the low-PFD treatment, indicating that the mechanisms for maintaining integrity of the photochemical apparatus had changed during the storage interval.     

Photosynthetic responses of cottonwood seedlings grown in glacial through future atmospheric [CO2] vary with phosphorus supply

Plants often exhibit proportionately larger photosynthetic responses to the transition from glacial to modern [CO(2)] than from modern to future [CO(2)]. Although this pattern may reflect increased nutrient demand with increasing [CO(2)], few studies have examined the role of nutrient supply in regulating responses to the range of [CO(2)] from glacial to future [CO(2)]. In this study, we examined the effects of P supply (0.004-0.5 mM) on photosynthetic responses of Populus deltoides (cottonwood) seedlings to glacial (200 micromol mol(-1)), modern (350 μmol mol(-1)) and future (700 micromol mol(-1)) [CO(2)]. The A(sat) (light-saturated net photosynthetic rates at the growth [CO(2)]) response to future [CO(2)] decreased with decreasing P supply such that there was no response at the lowest P supply. However, P supply did not affect A(sat) responses to an increase from glacial to modern [CO(2)]. Photosynthetic capacity [e.g., final rubisco activity, apparent, maximal Rubisco-limited rate of photosynthesis (V(cmax)), apparent, maximal electron transport-limited rate of photosynthesis (J(max))], stomatal conductance (g(s)) and leaf P generally increased with increasing P supply but decreased with increasing [CO(2)]. Measures of carbohydrate sink capacity (e.g., leaf mass per unit leaf area, leaf starch) increased with both increasing P supply and increasing [CO(2)]. Changes in V(cmax) and g(s) together accounted for 78% of the variation in A(sat) among [CO(2)] and P treatments, suggesting significant biochemical and stomatal controls on photosynthesis. However, A(sat) responses to increasing [CO(2)] did not reflect the changes in the carbohydrate sink capacity. These results have important implications because low P already constrains responses to increasing [CO(2)] in many ecosystems, and our results suggest that the P demand will increasingly affect A(sat) in cottonwood as [CO(2)] continues to increase.