Moderate shade can increase net gas exchange and reduce photoinhibition in citrus leaves

Daily variations in net gas exchange, chlorophyll a fluorescence and water relations of mature, sun-acclimated grapefruit (Citrus paradisi Macfady.) and orange (Citrus sinensis L. Osbeck) leaves were determined in tree canopies either shaded with 50% shade screens or left unshaded (sunlit). Mean daily maximum photosynthetic photon flux density (PPFD) under shade varied from 500 to 700 micromol m-2 s-1 and was sufficient to achieve maximum net CO2 assimilation rates (A CO2). Responses of grapefruit and orange leaves to shading were remarkably similar. At midday, on bright clear days, the temperatures of sunlit leaves were 2-6 degrees C above air temperature and 1-4 degrees C above the temperatures of shaded leaves. Although midday depressions of stomatal conductance (gs) and A CO2 were observed in both sunlit and shaded leaves, shaded leaves had lower leaf-to-air vapor pressure differences (D) along with higher gs, A CO2 and leaf water-use efficiency than sunlit leaves. Estimated stomatal limitation to A CO2 was generally less than 25% and did not differ between shaded and sunlit leaves. Leaf intercellular CO2 partial pressure was not altered by shade treatment and did not change substantially with increasing D. Radiation and high temperature stress-induced non-stomatal limitation to A CO2 in sunlit leaves was greater than 40%. Reversible photoinhibition of photosystem II efficiency was more pronounced in sunlit than in shaded leaves. Thus, non-stomatal factors play a major role in regulating A CO2 of citrus leaves during radiation and high temperature stress.     

Factors influencing red expression in autumn foliage of sugar maple trees

We evaluated factors influencing the development of autumn red coloration in leaves of sugar maple (Acer saccharum Marsh.) by measuring mineral nutrient and carbohydrate concentrations, water content, and phenology of color development of leaves from 16 mature open-grown trees on 12 dates from June through October 1999. Mean foliar nutrient and carbohydrate concentrations and water content were generally within the range published for healthy sugar maple trees. However, foliar nitrogen (N) concentrations were near deficiency values for some trees. The timing and extent of red leaf coloration was consistently correlated with both foliar N concentrations and starch or sugar concentrations, which also varied with N status. Leaves of trees with low foliar N concentrations turned red earlier and more completely than those of trees with high foliar N concentrations. Low-N trees also had higher foliar starch concentrations than high-N trees. During the autumn development of red leaf coloration, foliar starch, glucose and fructose concentrations were positively correlated with red leaf color expression. At peak red expression, the concentrations of glucose, fructose, sucrose and stachyose were all positively correlated with red color expressed as a percent of total leaf area.     

Characterization of a type II chlorophyll a/b-binding protein gene (Lhcb2*Pp1) in peach. II. mRNA abundance in developing leaves exposed to sun or shade

Leaf development of shoots exposed to full sunlight and shoots shaded by the canopy was followed in field-grown, mature peach trees (Prunus persica (L.) Batsch, cv. Loring) during the first half of the 1995 growing season. The architecture and size of shaded shoots and sun-exposed shoots differed significantly. Total number of leaves produced on shaded shoots was significantly less than on sun-exposed shoots throughout the season, and differences in leaf number between light conditions increased as the season progressed. The overall patterns of leaf development along sun-exposed and shaded shoots were qualitatively similar. The expression pattern of the type II chlorophyll a/b-binding protein gene, Lhcb2*Pp1, determined by RNA abundance in leaves at different positions along the shoot, was also similar between the two light conditions. The major difference between sun-exposed and shaded leaves was a lower abundance of Lhcb2*Pp1 RNA in mature, shaded leaves compared with sun-exposed leaves. Although the number of fruit per shoot was significantly lower on shaded shoots than on sun-exposed shoots, the rate of fruit drop was not substantially different during the growing season, indicating that quantitative differences in leaf initiation and growth caused by differences in light exposure did not adversely affect fruit retention. However, based on comparison with a previous study of leaf development in non-fruiting trees, reproductive development slowed the rate of vegetative growth without affecting the overall pattern of leaf development along the shoots.     

Acclimation of leaf characteristics of Fagus species to previous-year and current-year solar irradiances

To examine the effects of different solar irradiances on leaf characteristics at the leaf primordium and expansion stages, we shaded parts of branches in the upper canopies of two adult beech trees, Fagus crenata Blume and Fagus japonica Maxim., for 4 years. The treatments during the leaf primordium and leaf expansion stages, respectively, were: (1) high light and high light (H, control), (2) high light and low light (HL), (3) low light and low light (LL), and (4) low light and high light (LH). Both number of cell layers in palisade tissue and individual leaf area were affected by the previous-year irradiance, whereas cell length of palisade tissue was larger in LH leaves than in LL leaves, suggesting determination by current-year irradiance. Lamina chlorophyll/nitrogen ratio was higher in HL and LL leaves than in LH leaves, suggesting determination by current-year irradiance. Diurnal minimum values of leaf water potential measured under sunlit conditions were lower in H and LH leaves than in HL and LL leaves. Effective osmotic adjustment was found in H and LH leaves, suggesting that leaf water relations were affected by current-year irradiance. Net photosynthetic rate and stomatal conductance measured under sunlight conditions were higher in H and LH leaves than in HL and LL leaves. Thus, effects of current-year irradiance had a greater effect on leaf-area-based daily carbon gain than previous-year irradiance.     

Inhibitory effect of flowering and early fruit growth on leaf photosynthesis in mango

Carbohydrate and nitrogen contents, chlorophyll fluorescence and gas exchange were measured in leaves from both vegetative and reproductive terminal shoots of 12-year-old flowering mango trees. Reproductive shoot leaves were close to swelling floral buds, inflorescences or panicles bearing set fruits. Leaves close to inflorescences had lower rates of mitochondrial respiration (Rd) and net photosynthesis (Anet), and lower stomatal conductance (gs) and quantum efficiency of photosystem II under actinic light than vegetative shoot leaves. Leaf nitrogen concentration, which decreased from the beginning until the end of flowering, was lower in leaves close to inflorescences than in vegetative shoot leaves. However, these differences and changes were counterbalanced by an increase in leaf mass-to-area ratio so that leaf nitrogen per unit leaf area (Na) remained nearly constant during the whole flowering period, except in leaves close to panicles bearing set fruits. Net CO2 assimilation rate simulated by a biochemical model of leaf photosynthesis (Urban et al. 2003) was much higher than Anet measured at an ambient CO2 partial pressure (Ca) of either 36 or 70 Pa. The overestimation of Anet was more pronounced in leaves close to inflorescences, to panicles bearing set fruits and to reversing inflorescences (characterized by the appearance of leaves in terminal positions on inflorescences) than in vegetative shoot leaves. It is concluded that low Anet in leaves close to inflorescences was probably due neither to changes in Na nor to a decrease in Rubisco activity induced by low gs, but rather to a decrease in electron flow in photosystem II. This decrease was not directly associated with higher starch or soluble sugar contents.     

Sensitivity to chilling temperatures and distribution differ in the mangrove species Kandelia candel and Avicennia marina

We compared the effects of short-term (hours) and long-term (days) exposure to chilling temperatures on the photosynthetic gas exchange, leaf characteristics and chlorophyll a fluorescence of seedlings of the mangrove species Kandelia candel Druce and Avicennia marina (Forsk.) Vierh. Both species occur along the west coast of Taiwan, but K. candel occurs further north than A. marina. We hypothesized that temperature was one of the major environmental factors limiting the northern distribution of A. marina. Avicennia marina was more sensitive to chilling temperatures than K. candel. Leaves of both species showed reductions in light-saturated photosynthetic rates (Amax), stomatal conductance (gs) and quantum yield of photosystem II after a 1-h exposure to 15 degrees C, with A. marina showing significantly greater reductions in Amax and gs than K. candel. No significant differences in Amax, gs and electron transport rate (ETR) were found between leaves of K. candel grown at 15 and 30 degrees C for 10 days. However, leaves of A. marina grown for 10 days at 15 degrees C had significantly lower Amax, gs and ETR than plants grown at 30 degrees C. After 20 days at 15 degrees C, leaf mass per area of both species was increased significantly, whereas area-based chlorophyll concentrations were reduced, with significantly greater changes in A. marina than in K. candel. We concluded that sensitivity to low winter temperatures is a primary limiting factor in the distribution of A. marina along the western coast of Taiwan.     

Influence of irradiance on photosynthesis, morphology and growth of mangosteen (Garcinia mangostana L.) seedlings

The influence of shading intensity on growth, morphology and leaf gas exchange of mangosteen (Garcinia mangostana L.) seedlings was investigated over a 2-year period. Diurnal gas exchange studies revealed significantly higher carbon gain for leaves grown in 20 or 50% shade compared to leaves grown in 80% shade. Seedlings grown in 20 or 50% shade accumulated significantly more dry weight than seedlings grown in 80% shade during the 2-year study period. Seedlings grown in decreased shade showed decreased leaf size, increased leaf thickness, lower specific leaf area (SLA) and higher stomatal frequency. Less shaded seedlings also allocated relatively more dry matter to roots than shaded seedlings and exhibited a significant reduction in leaf area relative to total plant dry weight (leaf area ratio). Increased leaf number, enhanced branching and shorter internodes resulted in a more compact appearance of less shaded seedlings. Irrespective of light conditions, mangosteen seedlings exhibited inherently slow growth because of low photosynthetic rates per unit leaf area, low SLA, low leaf area ratios and inefficient root systems.     

Light environment alters ozone uptake per net photosynthetic rate in black cherry trees

Foliar ozone uptake rates of different-sized black cherry (Prunus serotina Ehrh.) trees were compared within a deciduous forest and adjacent openings in north-central Pennsylvania during one growing season. Study trees included open-grown seedlings and saplings, forest understory seedlings and saplings, and sunlit and shaded portions of mature canopy tree crowns. Instantaneous ozone uptake rates were highest in high-light environments primarily because of higher stomatal conductances. Low ozone uptake rates of seedlings and saplings in the forest understory could be attributed partially to lower average ambient ozone concentrations compared to the canopy and open environments. Among the tree size and light combinations tested, ozone uptake rates were highest in open-grown seedlings and lowest in forest-grown seedlings. Despite lower ozone uptake rates of foliage in shaded environments, ozone uptake per net photosynthesis of foliage in shaded environments was significantly higher than that of foliage in sunlit environments because of weaker coupling between net photosynthesis and stomatal conductance in shaded environments. The potential for greater ozone injury in shaded environments as a result of greater ozone uptake per net photosynthesis is consistent with previous reports of greater ozone injury in shaded foliage than in sunlit foliage.     

Effects of leaf,shoot and fruit development on photosynthesis of lychee trees (Litchi chinensis)

Changes in gas exchange with leaf age and fruit growth were determined in lychee trees (Litchi chinensis Sonn.) growing in subtropical Queensland (27 degrees S). Leaves expanded in a sigmoid pattern over 50 days during spring, with net CO2 assimilation (A) increasing from -4.1 +/- 0.9 to 8.3 +/- 0.5 micromol m-2 s-1 as the leaves changed from soft and red, to soft and light green, to hard and dark green. Over the same period, dark respiration (Rd) decreased from 5.0 +/- 0.8 to 2.0 +/- 0.1 micromol CO2 m-2 s-1. Net CO2 assimilation was above zero about 30 days after leaf emergence or when the leaves were half fully expanded. Chlorophyll concentrations increased from 0.7 +/- 0.2 mg g-1 in young red leaves to 10.3 +/- 0.7 mg g-1 in dark green leaves, along with stomatal conductance (gs, from 0.16 +/- 0.09 to 0.47 +/- 0.17 mol H2O m-2 s-1). Fruit growth was sigmoidal, with maximum values of fresh mass (29 g), dry mass (6 g) and fruit surface area (39 cm2) occurring 97 to 115 days after fruit set. Fruit CO2 exchange in the light (Rl) and dark (Rd) decreased from fruit set to fruit maturity, whether expressed on a surface area (10 to 3 micromol CO2 m-2 s-1 and 20 to 3 micromol CO2 m-2 s-1, respectively) or on a dry mass basis (24 to 2 nmol CO2 g-1 s-1 and 33 to 2 nmol CO2 g-1 s-1, respectively). Photosynthesis never exceeded respiration, however, the difference between Rl and Rd was greatest in young green fruit (4 to 8 micromol CO2 m-2 s-1). About 90% of the carbon required for fruit growth was accounted for in the dry matter of the fruit, with the remainder required for respiration. Fruit photosynthesis contributed about 3% of the total carbon requirement of the fruit over the season. Fruit growth was mainly dependent on CO2 assimilation in recently expanded dark green leaves.     

Girdling effects on fruit set and quantum yield efficiency of PSII in two Citrus cultivars

Girdling effects on fruitlet abscission, leaf chlorophyll, chlorophyll a fluorescence and carbohydrate concentration in various flowering and vegetative shoots were studied during natural fruit drop in two Citrus cultivars. Irrespective of shoot type, girdling delayed fruitlet abscission, but only fruitlets borne on leafy shoots had increased final fruit set. Chlorophyll a fluorescence analysis revealed differences in quantum yield efficiency of photosystem II of light adapted leaves (Phi(PSII)) among shoot types and in response to girdling. In young leaves of vegetative shoots, girdling decreased Phi(PSII), whereas Phi(PSII) increased from Day 30 after girdling in young leaves of leafy flowering shoots; however, Phi(PSII) did not change in mature leaves during fruit set in either control or girdled trees. Girdling altered leaf carbohydrate concentrations and the photosynthetic performance of the various shoot types. Our results indicate that, in Citrus, several carbohydrate-based regulatory mechanisms of photosynthesis coexist during carbohydrate accumulation brought about by girdling. It is concluded that the delay in fruitlet abscission and the increase in Phi(PSII )observed in girdled leafy flowering shoots are the mechanisms underlying the enhancement of fruit set after girdling.     

Photosynthetic acclimation of overstory Populus tremuloides and understory Acer saccharum to elevated atmospheric CO2 concentration: interactions with shade and soil nitrogen

We exposed Populus tremuloides Michx. and Acer saccharum Marsh. to a factorial combination of ambient and elevated atmospheric CO2 concentrations ([CO2]) and high-nitrogen (N) and low-N soil treatments in open-top chambers for 3 years. Our objective was to compare photosynthetic acclimation to elevated [CO2] between species of contrasting shade tolerance, and to determine if soil N or shading modify the acclimation response. Sun and shade leaf responses to elevated [CO2] and soil N were compared between upper and lower canopy leaves of P. tremuloides and between A. saccharum seedlings grown with and without shading by P. tremuloides. Both species had higher leaf N concentrations and photosynthetic rates in high-N soil than in low-N soil, and these characteristics were higher for P. tremuloides than for A. saccharum. Electron transport capacity (Jmax) and carboxylation capacity (Vcmax) generally decreased with atmospheric CO2 enrichment in all 3 years of the experiment, but there was no evidence that elevated [CO2] altered the relationship between them. On a leaf area basis, both Jmax and Vcmax acclimated to elevated [CO2] more strongly in shade leaves than in sun leaves of P. tremuloides. However, the apparent [CO2] x shade interaction was largely driven by differences in specific leaf area (m2 g-1) between sun and shade leaves. In A. saccharum, photosynthesis acclimated more strongly to elevated [CO2] in sun leaves than in shade leaves on both leaf area and mass bases. We conclude that trees rooted freely in the ground can exhibit photosynthetic acclimation to elevated [CO2], and the response may be modified by light environment. The hypothesis that photosynthesis acclimates more completely to elevated [CO2] in shade-tolerant species than in shade-intolerant species was not supported.     

Carbohydrate transfer through root grafts to support shaded trees

We investigated whether root grafts between lodgepole pine (Pinus contorta var. latifolia Dougl. ex. Loud.) trees can transfer sufficient carbohydrate reserves from a source tree to a grafted sink tree to affect the vigor of trees growing in a light-limited environment. Eleven plots were established in early spring and two grafted tree pairs and two independent non-grafted trees were selected at each plot. One tree in a grafted pair and one non-grafted tree were shaded at each plot, whereas the remaining trees were non-shaded during the experimental period. Shaded trees had significantly lower carbohydrate reserves and smaller crowns than non-shaded trees following one growing season. Grafted shaded trees had significantly higher root total nonstructural carbohydrate concentrations than non-grafted shaded trees, indicating that root grafts partially offset the effects of shading. Also, large root grafts transferred proportionately more carbohydrates to the shaded trees than small root grafts. Carbohydrates transferred through root grafts could allow grafted trees to persist under conditions where non-grafted trees would be removed by competition.     

Effects of CO2 enrichment on the photosynthetic light response of sun and shade leaves of canopy sweetgum (Liquidambar styraciflua) in a forest ecosystem

To investigate whether sun and shade leaves respond differently to CO2 enrichment, we examined photosynthetic light response of sun and shade leaves in canopy sweetgum (Liquidambar styraciflua L.) trees growing at ambient and elevated (ambient + 200 microliters per liter) atmospheric CO2 in the Brookhaven National Laboratory/Duke University Free Air CO2 Enrichment (FACE) experiment. The sweetgum trees were naturally established in a 15-year-old forest dominated by loblolly pine (Pinus taeda L.). Measurements were made in early June and late August 1997 during the first full year of CO2 fumigation in the Duke Forest FACE experiment. Sun leaves had a 68% greater leaf mass per unit area, 63% more leaf N per unit leaf area, 27% more chlorophyll per unit leaf area and 77% greater light-saturated photosynthetic rates than shade leaves. Elevated CO2 strongly stimulated light-saturated photosynthetic rates of sun and shade leaves in June and August; however, the relative photosynthetic enhancement by elevated CO2 for sun leaves was more than double the relative enhancement of shade leaves. Elevated CO2 stimulated apparent quantum yield by 30%, but there was no interaction between CO2 and leaf position. Daytime leaf-level carbon gain extrapolated from photosynthetic light response curves indicated that sun leaves were enhanced 98% by elevated CO2, whereas shade leaves were enhanced 41%. Elevated CO2 did not significantly affect leaf N per unit area in sun or shade leaves during either measurement period. Thus, the greater CO2 enhancement of light-saturated photosynthesis in sun leaves than in shade leaves was probably a result of a greater amount of nitrogen per unit leaf area in sun leaves. A full understanding of the effects of increasing atmospheric CO2 concentrations on forest ecosystems must take account of the complex nature of the light environment through the canopy and how light interacts with CO2 to affect photosynthesis.     

Seasonal dynamics,especially autumnal retranslocation,of nitrogen and phosphorus in foliage of dominant and suppressed trees of beech,Fagus sylvatica

Seasonal changes in the N and P content of foliage in a young forest of Fagus sylvatica were measured. Leaves from branches of the upper and lower crown of dominant trees and from suppressed trees were compared. Nutrient retranslocation rates during senescence differed considerably between trees. This variation appeared not to be related to any differences in environmental factors or tree vigour, and was probably genetically induced. In dominant trees the most efficient retranslocation of N was recorded in the upper crown and probably resulted from higher leaf temperatures and a longer senescent period in the sun leaves than in the shade leaves. Phosphorus retranslocation efficiency was higher in suppressed trees than in dominant ones, but no such tendency was observed with N. The most obvious difference between leaves at different crown levels concerned the time at which P translocation began; an outflow of P from leaves in the lower crown began in June, while in the upper crown this outflow did not begin until September/October.     

Effects of <Emphasis Type="Italic">Inga densiflora</Emphasis> on the microclimate of coffee (<Emphasis Type="Italic">Coffea arabica</Emphasis> L.) and overall biomass under optimal growing conditions in Costa Rica

The advantages of associating shade trees in coffee agroforestry systems (AFS) are generally thought to be restricted mostly to poor soil and sub-optimal ecological conditions for coffee cultivation whereas their role in optimal conditions remains controversial. Thus, the objective of this study was to investigate, under the optimal coffee cultivation conditions of the Central Valley of Costa Rica, the impact of Inga densiflora, a very common shade tree in Central America, on the microclimate, yield and vegetative development of shaded coffee in comparison to coffee monoculture (MC). Maximum temperature of shaded coffee leaves was reduced by up to 5°C relative to coffee leaf temperature in MC. The minimum air temperature at night was 0.5°C higher in AFS than air temperature in MC demonstrating the buffering effects of shade trees. As judged by the lower relative extractable water (REW) in the deep soil layers during the dry season, water use in AFS was higher than in MC. Nevertheless, competition for water between coffee and associated trees was assumed to be limited as REW in the 0–150 cm soil layer was always higher than 0.3 in shaded coffee compared to 0.4 in monoculture. Coffee production was quite similar in both systems during the establishment of shade trees, however a yield decrease of 30% was observed in AFS compared to MC with a decrease in radiation transmittance to less than 40% during the latter years in the absence of an adequate shade tree pruning. As a result of the high contribution (60%) of shade trees to overall biomass, permanent aerial biomass accumulation in AFS amounted to two times the biomass accumulated in MC after 7 years. Thus provided an adequate pruning, Inga-shaded plantations appeared more advantageous than MC in optimal conditions, especially considering the fact that coffee AFS provides high quality coffee, farmers’ revenue diversification and environmental benefits.     

Black leaf streak disease and plantain fruit characteristics as affected by tree density and biomass management in a tropical agroforestry system

Black leaf streak disease (BLSD) affects both bananas and plantains. Anecdotal evidence suggests that when plantain is grown under shade in agroforestry systems, there is less damage from BLSD. In this study, BLSD severity and agronomic fruit characteristics of plantain were compared under high and low timber stand densities (TSD), and also under different crop management systems (mulching, burning, intercropping). By 11 months after planting, in high TSDs, plants had 1.2 more standing leaves, 1.5 more symptom-free leaves and a lower percentage of total leaf area attacked than in low TSDs. At harvest, plants in high TSDs had, on average, 1.4 more standing leaves than those in low TSDs. Percentage leaf area attacked by BLSD was 43.0 % in high TSD treatments, significantly lower than the 57.2 % in low TSD treatments. There was a significant and positive relationship between the number of leaves at flowering and dry fruit bunch mass for plantains grown under low TSD. However, there was no correlation between the number of leaves and fruit mass for the high TSD. The beneficial effects of shade in the high TSD, such as reduced severity of BLSD, were established. However, as there was bunch mass loss associated with reduced leaf production rates under the high TSD, a reduction in BLSD only partially compensated for this yield loss. Thus overall, at the high TSD, there was no yield gain from reduced BLSD.     

Influence of canopy light environment and nitrogen availability on leaf photosynthetic characteristics and photosynthetic nitrogen-use efficiency of field-grown nectarine trees

Relationships between CO(2) assimilation at light saturation (A(max)), nitrogen (N) content and weight per unit area (W(A)) were studied in leaves grown with contrasting irradiances (outer canopy versus inner canopy) and N supply rates in field-grown nectarine trees Prunus persica L. Batsch. cv. Fantasia. Both A(max) and N content per unit leaf area (N(A)) were linearly correlated to W(A), but leaves in the high-N treatment had higher N(A) and A(max) for the same value of W(A) than leaves in the low-N treatment. The curvilinear relationship between photosynthesis and total leaf N was independent of treatments, both when expressed per unit leaf area A(maxA) and N(A)) and per unit leaf weight (A(maxW) and N(W)), but the relationship was stronger when data were expressed on a leaf area basis. Both A(maxA) and N(A) were higher for outer canopy leaves than for inner canopy leaves and A(maxW) and N(W) were higher for leaves in the high-N treatment than for leaves in the low-N treatment. The relationship between A(max) and N resulted in a similar photosynthetic nitrogen-use efficiency at light saturation (A(max)NUE) for both N and light treatments. Photosynthetic nitrogen-use efficiency was similar among treatments throughout the whole light response curve of photosynthesis. Leaves developed in shade conditions did not show higher N-use efficiency at low irradiance. At any intercellular CO(2) partial pressure (C(i)), photosynthetic CO(2) response curves were higher for outer canopy leaves and, within each light treatment, were higher for the high-N treatments than for the low-N treatments. Consequently, most of the differences among treatments disappeared when photosynthesis was expressed per unit N. However, slightly higher assimilation rates per unit N were found for outer canopy leaves compared with inner canopy leaves, in both N treatments. Because higher daily irradiance within the canopies of the low-N trees more than compensated for the lower photosynthetic performances of these leaves compared to the leaves of high-N trees, daily carbon gain (and N-use efficiency on a daily assimilation basis) per leaf was higher for the low-N treatment than for the high-N treatment in both outer and inner canopy leaves.     

Leaf senescence and late-season net photosynthesis of sun and shade leaves of overstory sweetgum (Liquidambar styraciflua) grown in elevated and ambient carbon dioxide concentrations

We examined the effects of elevated CO2 concentration ([CO2]) on leaf demography, late-season photosynthesis and leaf N resorption of overstory sweetgum (Liquidambar styraciflua L.) trees in the Duke Forest Free Air CO2 Enrichment (FACE) experiment. Sun and shade leaves were subdivided into early leaves (formed in the overwintering bud) and late leaves (formed during the growing season). Overall, we found that leaf-level net photosynthetic rates were enhanced by atmospheric CO2 enrichment throughout the season until early November; however, sun leaves showed a greater response to atmospheric CO2 enrichment than shade leaves. Elevated [CO2] did not affect leaf longevity, emergence date or abscission date of sun leaves or shade leaves. Leaf number and leaf area per shoot were unaffected by CO2 treatment. A simple shoot photosynthesis model indicated that elevated [CO2] stimulated photosynthesis by 60% in sun shoots, but by only 3% in shade shoots. Whole-shoot photosynthetic rate was more than 12 times greater in sun shoots than in shade shoots. In senescent leaves, elevated [CO2] did not affect residual leaf nitrogen, and nitrogen resorption was largely unaffected by atmospheric CO2 enrichment, except for a small decrease in shade leaves. Overall, elevated [CO2] had little effect on the number of leaves per shoot at any time during the season and, therefore, did not change seasonal carbon gain by extending or shortening the growing season. Stimulation of carbon gain by atmospheric CO2 enrichment in sweetgum trees growing in the Duke Forest FACE experiment was the result of a strong stimulation of photosynthesis throughout the growing season.     

Effects of elevated CO(2) and light availability on the photosynthetic light response of trees of contrasting shade tolerance

Photosynthetic light response curves (A/PPFD), leaf N concentration and content, and relative leaf absorbance (alpha(r)) were measured in 1-year-old seedlings of shade-intolerant Betula papyrifera Marsh., moderately shade-tolerant Quercus rubra L. and shade-tolerant Acer rubrum L. Seedlings were grown in full sun or 26% of full sun (shade) and in ambient (350 ppm) or elevated (714 ppm) CO(2) for 80 days. In the shade treatments, 80% of the daily PPFD on cloud-free days was provided by two 30-min sun patches at midday. In Q. rubra and A. rubrum, leaf N concentration and alpha(r) were significantly higher in seedlings in the shade treatments than in the sun treatments, and leaf N concentration was lower in seedlings in the ambient CO(2) treatments than in the elevated CO(2) treatments. Changes in alpha(r) and leaf N content suggest that reapportionment of leaf N into light harvesting machinery in response to shade and elevated CO(2) tended to increase with increasing shade tolerance of the plant. Shifts induced by elevated CO(2) in the A/PPFD relationship in sun plants were largest in B. papyrifera and least in A. rubrum: the reverse was true for shade plants. Elevated CO(2) resulted in increased light-saturated A in every species x light treatment combination, except in shaded B. papyrifera. The light compensation point (Gamma) decreased in response to shade in all species, and in response to elevated CO(2) in A. rubrum and Q. rubra. Acer rubrum had the greatest increases in apparent quantum yield (phi) in response to shade and elevated CO(2). To illustrate the effects of shifts in A, Gamma and phi on daily C gain, daily integrated C balance was calculated for individual sun and shade leaves. Ignoring possible stomatal effects, estimated daily (24 h) leaf C balance was 218 to 442% higher in the elevated CO(2) treatments than in the ambient CO(2) treatments in both sun and shade seedlings of Q. rubra and A. rubrum. These results suggest that the ability of species to acclimate photosynthetically to elevated CO(2) may, in part, be related to their ability to adapt to low irradiance. Such a relationship has implications for altered C balance and nitrogen use efficiency of understory seedlings.     

Physiological traits of two Populus x euramericana clones,Luisa Avanzo and Dorskamp,during a water stress and re-watering cycle

We compared responses to drought and re-watering of greenhouse-grown cuttings of Populus x euramericana (Dode) Guinier clones, Luisa Avanzo and Dorskamp. Total leaf area, leaf number, leaf area increment and stomatal conductance were evaluated periodically during a 29-day drought period and for 16 days after re-watering. Soil water content and predawn leaf water potential (Psi(wp)) were measured on Days 29 and 45. On the same days, relative water content (RWC), specific leaf area (SLA), nitrogen, chlorophyll, soluble sugars, total phenols, flavanols and antioxidant activity were determined for leaves taken from the bottom to the top of each cutting. Leaves of Luisa Avanzo cuttings grew more rapidly than leaves of Dorskamp and exhibited higher SLA, but lower concentrations of nitrogen, chlorophyll and soluble sugars and lower antioxidant activity per unit area. On Day 29, after withholding water, both clones had closed their stomata, reduced rates of leaf growth, and lower Psi(wp) and RWC; however, the clones differed in their responses to soil water depletion. Compared to Dorskamp, Luisa Avanzo closed its stomata earlier and maintained higher Psi(wp), but lower RWC and leaf sugar concentrations. Antioxidant activity of leaf methanolic extracts decreased in response to water stress only in Luisa Avanzo. Leaf physiology and its modulation by water stress were age dependent in Luisa Avanzo.