Response of Ulmus americana seedlings to varying nitrogen and water status. 1 Photosynthesis and growth

Well-watered American elm (Ulmus americana L.) seedlings responded to increased nitrate availability with increased leaf nitrogen (N) concentration and photosynthetic rate, larger and more numerous leaves, greater total growth and greater proportional allocation of carbon to shoot than root. Plasticity of growth and carbon allocation were greater than plasticity of N concentration and photosynthetic capacity. For a given N availability, allocation of N per unit leaf area was positively correlated with dry mass per unit leaf area (specific leaf mass), but these relationships differed with N availability. Rates of net photosynthesis and leaf conductance declined logarithmically with decreasing predawn water status. Increased water stress resulted in a greater relative decline in net photosynthesis and leaf conductance for high-N than low-N plants.     

碧玉杨叶形态结构与生理特性对干旱的响应

【目的】以群众杨为对照,探讨干旱胁迫下碧玉杨的叶形态、结构和光合、水分生理特性的关系及其影响植株生物量积累的内在成因,进一步揭示碧玉杨和群众杨的抗旱机制,为区域造林绿化杨树的抗旱生理研究和品种推广提供参考。【方法】以碧玉杨与和群众杨1年生扦插苗为研究对象,应用盆栽控水法模拟干旱条件,对比分析不同干旱胁迫程度下2个品种的叶面积、叶脉密度、叶氮素利用效率、光合特性及叶水分输导能力的响应,及其对植株生物量和抗旱能力的影响。【结果】干旱胁迫程度加剧,2个杨树品种的叶脉密度(LVD)和单位干质量叶氮含量(N_mass)上升,碧玉杨的LVD增幅大、N_mass增幅小;净光合速率(P_n)、植株总光合速率(PTL)、最大净光合速率(P_nmax)、光合日累积量、蒸腾日累积量、叶导水率(K_leaf)、水分利用效率(WUE)、光合氮利用效率(PNUE)、叶面积(LA)以及植株生物量(TB)均下降,碧玉杨的降幅均小。相较群众杨而言,碧玉杨的LA、LVD、PNUE、TB和日均耗水量在干旱胁迫前后均高,P_n、K_leaf、WUE和N_mass在干旱胁迫前后均低,PTL和Tr日累积在中度和重度干旱胁迫下高,P_nmax、P_n日累积和Tr在重度干旱胁迫下高。PTL较Pn与TB相关关系更紧密,PNUE较N_mass与TB相关关系更紧密。【结论】干旱胁迫下碧玉杨的叶形态结构与生理特性呈现出低效水分利用、高效叶形态结构调整、高效氮素利用和高效光合固碳的平衡制约关系。2个品种的抗旱机制不同,抗旱能力强的碧玉杨其叶结构调整更高效,抗旱能力弱的群众杨其光合特性对干旱更敏感,叶结构与抗旱性关联最大,其次是光合特性。     

Photosynthetic capacity in relation to nitrogen in the canopy of a Quercus robur, Fraxinus angustifolia and Tilia cordata flood plain forest

We measured gas exchange and various leaf parameters of ash (Fraxinus angustifolia Vahl.) and oak (Quercus robur L.) in the high canopy and of lime (Tilia cordata Mill.) in the lower canopy of a planted, 120-year-old floodplain forest in southern Moravia, Czech Republic. The high-canopy leaves of F. angustifolia and Q. robur had nitrogen concentrations on a leaf area basis (N(area)) that were twice those of low-canopy leaves of T. cordata. Upper-canopy leaves of F. angustifolia had a photosynthetic rate at light saturation (A(max)) of about 16 micromol CO2 m(-2) s(-1), whereas A(max) of the upper-canopy foliage of Q. robur achieved only about two thirds of this value. Contrary to previous investigations of photosynthetic performance in monospecific stands, leaves of the uppermost branches of T. cordata at 15-m height had the highest A(max) and transpiration rate among the species studied. Water-use efficiency (WUE) was low in T. cordata at 15-m canopy height, whereas WUE was significantly higher for Q. robur leaves at 27-m height than for the other species. Leaves of T. cordata at 15-m height showed the strongest relationship between A(max) and N(area) (R2 = 0.90) followed by F. angustifolia (R2 = 0.69). The strong correlation between photosynthesis and nitrogen concentration in T. cordata at 15 m, together with the steep regression slope for the A(max):N(area) relationship, indicated that nitrogen allocation to the photosynthetic apparatus resulted in high nitrogen-use efficiency of light-saturated photosynthesis (PNUE). Despite differences in PNUE among species, PNUE was fairly constant for leaves sampled from the same canopy position, suggesting that single-leaf parameters are matched to optimize PNUE for prevailing light conditions. High PNUE in T. cordata at 15 m partially compensated for the species' subordinate position in the canopy, and may be an important mechanism for its coexistence in highly structured vegetation.     

Effect of nitrogen levels on photosynthetic parameters,morphological and chemical characters of saplings and trees in a temperate forest

Exploring the response differences of leaf physiology parameters to enhanced nitrogen deposition between saplings and trees is vital for predicting the variations of terrestrial ecosystem structure and function under future global climate change. In this study, the ecophysiological parameters of saplings and trees of Fraxinus mandshurica Rupr. were measured at different levels of nitrogen addition in a temperate forest. The results show that ecophysiological parameters maximum net photosynthetic rate(P_(max)), apparent quantum efficiency(a), dark respiration(R_d), light saturation point(L_(sp)), photosynthetic nitrogen use efficiency(PNUE),specific leaf area(SLA)and stomatal conductance under saturated light intensity(G_(smax)) were higher in saplings than in trees. These physiological parameters and not N_(leaf)(leaf nitrogen content)led to relatively lower P_(max) and R_d in trees. For both saplings and trees, low and median nitrogen addition(23 and 46 kg ha~(-1)a~(-1)) resulted in significant increases in Pmax, Rd, Lsp, Chl, PNUE, SLA and Gsmax. These parameters tended to decline under high additions of nitrogen(69 kg ha~(-1)a~(-1)),whereas Nleaf was always enhanced with increasing nitrogen. Variations in Pmax and Rd with increasing nitrogen were attributed to variations in the strongly related parameters of, Lsp, Chl, PNUE, SLA and Gsmax. Overall, the response sensitivity of physiological parameters to enhanced nitrogen levels was lower in trees compared with saplings.     

Photosynthetic nitrogen-use efficiency in evergreen broad-leaved woody species coexisting in a warm-temperate forest

Photosynthetic nitrogen-use efficiency (PNUE, photosynthetic capacity per unit leaf nitrogen) varies among species from different habitats and correlates with several ecological characteristics such as leaf life span and leaf mass per area. We investigated eight evergreen broad-leaved woody species with different leaf life spans that coexist in a warm-temperate forest. We determined photosynthetic capacity at ambient CO(2) concentration in saturated light, nitrogen concentration, and the concentration of ribulose-1,5-bisphosphate carboxylase (RuBPCase), a key enzyme of photosynthesis and the largest sink of nitrogen in leaves. Each species showed a strong correlation between photosynthetic capacity and RuBPCase concentration, and between RuBPCase concentration and nitrogen concentration. Photosynthetic capacity of leaves decreased with increasing leaf life span, whereas PNUE did not correlate significantly with leaf life span. There was a twofold variation in PNUE among species. This relatively small variation in PNUE is consistent with the argument that species that coexist in a single habitat maintain a similar PNUE. The two components of PNUE-photosynthetic rate per unit RuBPCase and RuBPCase per unit leaf nitrogen-were not significantly correlated with other leaf characteristics such as leaf life span and leaf mass per area. We conclude that differences in PNUE are relatively small among coexisting species and that differences in absolute amounts of photosynthetic proteins lead to differences in photosynthetic productivity among species.     

Ontogenetic transition of leaf physiology and anatomy from seedlings to mature trees of a rain forest pioneer tree, Macaranga gigantea

In a field study, we compared anatomy and diurnal gas exchange and chlorophyll fluorescence in sunlit mature leaves of Macaranga gigantea (Reichb. f. and Zoll.) Muell. seedlings, saplings, an adult tree and suckers originating from stumps. We tested the hypothesis that the pattern of resource use shifts across various life stages with ontogenetic changes in leaf anatomy and physiology. Among leaves of different developmental stages, seedling leaves were the smallest and thinnest, whereas adult tree leaves were the largest and thickest, and the air space within the lamina was largest in seedling leaves and smallest in adult tree leaves. Photosynthetic nitrogen-use efficiency (PNUE) was higher in seedling and sapling leaves than in adult tree leaves. Mean PNUE in seedling leaves was 1.6 times that in adult tree leaves. Nevertheless, among the developmental stages, net photosynthetic rate (Pn) per unit leaf area was lowest in seedling leaves because they have the lowest nitrogen (N) content per unit leaf area. In situ water vapor stomatal conductance (g(s) at a given leaf-to-air vapor pressure deficit was highest in sapling leaves, suggesting that they have a high hydraulic efficiency per unit leaf area. Among developmental stages, intrinsic water-use efficiency (Pn/g(s)) and photochemical capacity of photosystem II were lowest in seedling leaves. Sapling leaves had the highest N concentration and Pn per unit dry mass and the highest g(s), indicating that the gradual transition from the seedling stage to the sapling stage is accompanied by an accumulation of N in plant bodies and the development of hydraulic systems to counteract unfavorable environmental stresses. The properties of adult tree leaves (low PNUE, high carbon:N ratio, small and dense cells and thick lamina) indicate that, during the transition from the sapling stage to the adult tree stage, the priority of resource use in leaves gradually shifts from enhancement of photosynthetic performance to defense against herbivory and mechanical damage. Leaf morphology and physiology were coordinated with the differences in resource use at each life stage.     

Quercus species differ in water and nutrient characteristics in a resource-limited fall-line sandhill habitat

We compared co-occurring mature Quercus laevis Walt. (turkey oak), Q. margaretta Ashe (sand post oak) and Q. incana Bartr. (bluejack oak) trees growing in resource-limited sandhill habitats of the southeastern United States for water and nutrient characteristics. The Quercus spp. differed in their distribution along soil water and nutrient gradients, and in their access to and use of water, even though the study year was wetter than average with no mid-season drought. Quercus laevis had the greatest access to soil water (least negative pre-dawn water potential, psi(pd)) and the most conservative water-use strategy based on its relatively low stomatal conductance (g(s)), high instantaneous water-use efficiency (WUE), least negative midday water potential (psy(md)) and high leaf specific hydraulic conductance (K(L)). Quercus margaretta had the least conservative water-use characteristics, exhibiting relatively high g(s), low instantaneous WUE, most negative psi(md), and low K(L). Quercus margaretta also had a low photosynthetic nitrogen-use efficiency (PNUE), but a high leaf phosphorus concentration. Quercus incana had the poorest access to soil water, but intermediate water-use characteristics and leaf nutrient characteristics more similar to those of Q. laevis. There were no species differences for photosynthesis (A), leaf nitrogen on an area basis, or seasonally integrated WUE (delta13C). Both A and g(s) were positively correlated for each species, but A and g(s) were generally not correlated with psi(pd), psi(md) or delta psi(pd-md). Although we found differences in resource use and resource status among these sandhill Quercus spp., the results are consistent with the interpretation that they are generally drought avoiders. Quercus laevis may have an advantage on xeric ridges because of its greater ability to access soil water and use it more conservatively compared with the other Quercus spp.     

Variation in nitrogen supply changes water-use efficiency of Pseudotsuga menziesii and Populus x euroamericana; a comparison of three approaches to determine water-use efficiency

We studied the effects of three nitrogen (N) supply rates (low, intermediate and high) on Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings and poplar clone "I-214" (Populus x euroamericana (Dole) Guinier) cuttings growing in mini-stands. Our specific objectives were to: (1) evaluate the effects of N supply on water-use efficiency (WUE) and biomass production; (2) determine if N affects WUE through control of carbon assimilation rates or through stomatal control of water loss; and (3) compare three methods of estimating WUE: one short-term method (WUE(i), based on gas exchange measurements) and two long-term methods (WUE(T), based on the ratio between biomass production and transpired water, and Delta, based on leaf carbon isotope discrimination tested as a proxy of WUE). In both species, biomass production, WUE(i) and WUE(T) increased with increasing N supply, but there was no effect of N supply on either transpiration or stomatal conductance and Delta was negatively related to leaf N concentration. Plots of Delta versus both WUE(i) and WUE(T) revealed negative trends, but the regression between WUE(i) and Delta was significant only for Douglas-fir, and the regression between WUE(T) and Delta was significant only for poplar. Thus, the mechanisms underlying the response of WUE to N supply were mainly related to a positive effect of N supply on photosynthetic rates. The data confirm that carbon isotope discrimination may be a useful proxy of WUE. The finding that N availability enhances both biomass production and WUE may have practical implications in regions where these factors impose constraints on forest productivity.     

Leaf growth, photosynthesis and tissue water relations of greenhouse-grown Eucalyptus marginata seedlings in response to water deficits

Leaf growth, rate of leaf photosynthesis and tissue water relations of shoots of Eucalyptus marginata Donn ex Sm. (jarrah) seedlings were studied during a soil drying and rewatering cycle in a greenhouse experiment. Rates of leaf growth and photosynthesis were sensitive to water deficits. The rate of leaf growth decreased linearly with predawn leaf water potential to reach zero at -1.5 MPa. Rate of leaf growth did not recover completely within the first three days after rewatering. Midday photosynthetic rates declined to 40% of those of well-watered seedlings at a predawn leaf water potential of -1.0 MPa and reached zero at -2.2 MPa. Photosynthetic rate recovered rapidly following rewatering and almost fully recovered by the second day after rewatering. All tissue water relations parameters, except the bulk modulus of elasticity, changed significantly as the soil dried and recovered completely by the third day after rewatering. Changes in osmotic pressure at full turgor of 0.4 MPa indicated considerable capacity for osmotic adjustment. However, because there was little osmotic adjustment until predawn leaf water potential fell below -1.5 MPa, this capacity would not have enhanced seedling growth, although it may have increased seedling survival. The sensitivity of photosynthesis and relative water content to water deficits suggests that greenhouse-grown E. marginata seedlings behave like mesophytic plants, even though E. marginata seedlings naturally grow in a drought-prone environment.     

Effects of foliar nitrogen concentration on photosynthesis and water use efficiency in Douglas-fir

Leaf-level physiological processes were studied in Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) to determine whether apparent increases in stand-level water use efficiency (WUE) observed in response to nitrogen (N) fertilization were attributable to foliar N effects on carbon fixation rates or on stomatal control of water loss. Photosynthesis and transpiration were measured at different light intensities and ambient CO(2) molar fractions and comparisons were made between current-year shoots with average foliar N concentrations of 1.58% (High-N) and 1.25% (Low-N). Photosynthetic rates and foliar N concentrations were positively correlated. In response to light, photosynthesis and stomatal conductance were closely coupled and a similar coupling was observed in response to different ambient CO(2) concentrations. Partitioning the photosynthetic responses into mesophyll and stomatal components indicated that foliar N altered mesophyll conductance but not stomatal control of water loss. High-N shoots had significantly greater rates of photosynthesis and transpiration than Low-N shoots and, as a result, instantaneous WUE did not differ significantly between High-N and Low-N shoots.     

Tolerance of Japanese larch to drought is modified by nitrogen and water regimes during cultivation of container seedlings

Improving drought tolerance of container seedlings of Japanese larch is of high importance to afforestation. We hypothesized that adequate nitrogen (N) and limited water supply would increase the tolerance of container seedlings to water-deficit stress, circumventing photoinhibition, by means of (i) enhanced photosynthetic capacity with higher leaf N and (ii) decreased water loss from leaves with lower biomass allocation into aboveground parts. Container seedlings of Japanese larch were grown under the treatment combinations of adequate (+?N: 300 mg N container^?1) or limited (??N: 150 mg N container^?1) N and adequate (+?W: daily irrigation) or limited (??W: twice-a-week irrigation) water. Then, seedlings were subjected to a progressive drought treatment. Higher leaf N was observed in container seedlings grown under?+?N and???W. During progressive drought, lower stomatal conductance and net photosynthetic rate were observed in leaves with higher leaf N at a given predawn leaf water potential. Furthermore, the maximum efficiency of PSII photochemistry (F_v/F_m) was lower in leaves with higher leaf N, suggesting that higher leaf N might impair intrinsic tolerance to drought at the leaf level contrary to expectations. Conversely,???N and???W seedlings with lower shoot biomass delayed soil drying as a whole-plant response via a reduction in leaf transpiration, leading to delayed photoinhibition as indicated by a decline in F_v/F_m. To circumvent stress at the initial stage of water deficit, lower leaf N via limited N regime and smaller shoot biomass driven by limited N and water regimes would be important.

     

Water relations and gas exchange of Acer saccharum seedlings in contrasting natural light and water regimes

Field measurements were made of leaf photosynthesis (A), stomatal conductance (g) and leaf water relations for sugar maple (Acer saccharum Marsh.) seedlings growing in a forest understory, small gap or large clearing habitat in southwestern Wisconsin, USA. Predawn water status, leaf gas exchange and plasticity in field and laboratory water relations characteristics were compared among contrasting light environments in a wet year (1987) and a dry year (1988) to evaluate possible interactions between light and water availability in these habitats. Leaf water potentials (Psi(leaf)) at predawn and midday were lower for clearing than gap or understory seedlings. Acclimation of tissue osmotic potentials to light environment was observed among habitats but did not occur within any of the habitats in response to prolonged drought. During a summer drought in 1988, decreases in daily maximum g (g(max)) and maximum A (A(max)) in clearing seedlings were correlated with predawn Psi(leaf), which reached a seasonal minimum of -2.0 MPa. Under well-watered conditions, diurnal fluctuations in Psi(leaf) of up to 2.0 MPa in clearing seedlings occurred along with large midday depressions of A and g. In a wet year, strong stomatal responses to leaf-to-air vapor pressure difference (VPD) in sunny habitats were observed over nine diurnal courses of gas exchange measurements on seedlings in a gap and a clearing. Increasing stomatal limitations to photosynthesis appeared to be responsible for the reduction in A at high VPD for clearing seedlings. In understory seedlings, however, low water-use efficiency and development of leaf water deficits in sunflecks was related to reduced stomatal limitations to photosynthesis relative to seedlings in sunny habitats. Predawn Psi(leaf) and VPD appear to be important factors limiting carbon assimilation in sugar maple seedlings in light-saturating irradiances, primarily through stomatal closure. The overall results are consistent with the idea that sugar maple seedlings exhibit "conservative" water use patterns and have low drought tolerance. Leaf water relations and patterns of water use should be considered in studies of acclimation and species photosynthetic performance in contrasting light environments.     

山鸡椒水分及氮素利用效率性别特异性动态

【目的】以雌雄异株山鸡椒为研究材料,分析雌雄植株在生殖生长过程中水分及氮素利用策略的性别特异性动态变化规律,以期为雌雄异株植物资源分配动态变化机制提供理论依据。【方法】测定雌雄植株在开花后105～165天共5个时期叶片养分含量(碳含量、氮含量、碳氮比)、稳定碳同位素组成(δ¹³C)、稳定氮同位素组成(δ¹⁵N),并分析其动态变化规律。【结果】1)雌雄株叶片δ¹³C平均值分别为-29. 38‰和-28. 08‰。性别和发育时期对雌雄株叶片δ¹³C值影响均显著,5个时期雌株δ¹³C值均显著低于雄株;且随发育进程雌雄植株δ¹³C值均不断下降。2)雌雄株叶片δ¹⁵N平均值分别为1. 90‰和2. 95‰。性别和发育时期对雌雄株叶片δ¹⁵N值均有极显著影响,开花后105～150天(即雌株果实中精油及柠檬醛含量快速积累期)雌株δ¹⁵N值显著低于雄株;随着发育进程,雌雄株叶片δ¹⁵N值均表现出双峰变化趋势,双峰出现在开花后105天和135天,雌株叶片δ¹⁵N在开花后120天达到最低值,雄株叶片在开花后165天时达到最低值。3)雌雄株叶片碳含量平均值分别为49. 44%和49. 28%。开花后105天和120天,雌株叶片C含量高于雄株叶片C含量,开花后135～165天(果实精油快速积累期到稳定期),雌株叶片C含量低于雄株。雄株叶片C含量随发育进程显著升高,而雌株叶片C含量在不同时期无显著差异。4)雌雄株叶片N含量平均值分别为1. 71%和1. 51%。性别对N含量影响显著,雄株叶片N含量均低于雌株(除开花后135天)。5)雌雄株叶片C/N平均值分别为29. 15和33. 72。性别对叶片碳氮比影响显著,雌株C/N值在不同发育时期均小于雄株,但发育时期对其无显著影响;随着发育时期推进,雄株C/N值下降程度高于雌株。6)雌雄植株叶片δ¹³C与N含量以及δ¹³C与δ¹⁵N之间均无显著相关性。【结论】山鸡椒雌雄植株在养分含量、水分及氮素利用策略存在差异,且在开花后105～165天表现出动态变化规律。雌株水分利用效率低于雄株,从果实精油及柠檬醛含量快速积累期到稳定期,雌株水分利用效率不断下降;雌株氮利用低于雄株,且随着植株发育,雌雄株叶片氮素利用效率均表现出双峰变化趋势;雄株叶片氮含量低于雌株,分配更多的氮素至花芽以保证花粉形成;果实精油快速积累期到稳定期,雌株叶片碳含量低于雄株,为果实及种子的形成提供更多的碳元素。     

Co-optimal distribution of leaf nitrogen and hydraulic conductance in plant canopies

Leaf properties vary significantly within plant canopies, due to the strong gradient in light availability through the canopy, and the need for plants to use resources efficiently. At high light, photosynthesis is maximized when leaves have a high nitrogen content and water supply, whereas at low light leaves have a lower requirement for both nitrogen and water. Studies of the distribution of leaf nitrogen (N) within canopies have shown that, if water supply is ignored, the optimal distribution is that where N is proportional to light, but that the gradient of N in real canopies is shallower than the optimal distribution. We extend this work by considering the optimal co-allocation of nitrogen and water supply within plant canopies. We developed a simple 'toy' two-leaf canopy model and optimized the distribution of N and hydraulic conductance (K) between the two leaves. We asked whether hydraulic constraints to water supply can explain shallow N gradients in canopies. We found that the optimal N distribution within plant canopies is proportional to the light distribution only if hydraulic conductance, K, is also optimally distributed. The optimal distribution of K is that where K and N are both proportional to incident light, such that optimal K is highest to the upper canopy. If the plant is constrained in its ability to construct higher K to sun-exposed leaves, the optimal N distribution does not follow the gradient in light within canopies, but instead follows a shallower gradient. We therefore hypothesize that measured deviations from the predicted optimal distribution of N could be explained by constraints on the distribution of K within canopies. Further empirical research is required on the extent to which plants can construct optimal K distributions, and whether shallow within-canopy N distributions can be explained by sub-optimal K distributions.     

Gas exchange during a soil drying cycle in seedlings of four black walnut (Juglans nigra L.) families

Photosynthetic and stomatal responses to a soil drying cycle were examined in half-sib seedlings of four walnut (Juglans nigra L.) families. Well-watered seedlings of an Iowa seed source had significantly higher rates of net photosynthesis than seedlings from New York or Michigan sources. This superior photosynthetic potential was associated with both greater stomatal conductance and mesophyll capacity for CO(2) fixation. In a drying soil, net photosynthesis and leaf conductance to water vapor of all families declined substantially, even under mild water stress. These responses were more strongly related to soil water status, as estimated by predawn leaf water potential, than to leaf water potential at the time of gas exchange measurement. There were no differences among families in the pattern of gas exchange response to developing water stress; however, families differed in capacity for recovery of gas exchange from water stress following rehydration. Sensitivity of photosynthesis of black walnut seedlings to water stress may be associated with poor growth and survival of this species in xeric habitats.     

Functional and genetic characterization of gas exchange and intrinsic water use efficiency in a full-sib family of Pinus pinaster Ait. in response to drought

Drought is an important environmental factor in Mediterranean ecosystems affecting seedling recruitment, productivity or susceptibility to fires and pathogens. Studying water use efficiency in these environments is crucial due to its adaptive value allowing trees to cope with low water availability. We studied the phenotypic variability and genetic control of intrinsic water use efficiency (WUE(i)) and related traits in a full-sib family of Pinus pinaster under drought imposition. We detected significant differences in WUE(i) between clones of the same family and moderate heritability estimates that indicate some degree of genetic control over this trait. Stomatal conductance to water vapor was the trait most affected by drought imposition and it showed the strongest influence in WUE(i). Stomatal conductance to water vapor and specific leaf area (SLA) were the traits with highest heritabilities and they showed a significant genetic correlation with WUE(i), suggesting that selection of needles with low SLA values will improve WUE(i) in this species by reducing water losses through stomatal control.     

Effects of enhanced ultraviolet-B radiation on water use efficiency, stomatal conductance, leaf nitrogen content and morphological characteristics of Spiraea pubescens in a warm-temperate deciduous broad-leaved forest

Spiraea pubescens, a common shrub in the warm-temperate deciduous forest zone which is distributed in the Dongling Mountain area of Beijing, was exposed to ambient and enhanced ultraviolet-B (UV-B, 280–320 nm) radiation by artificially supplying a daily dose of 9.4 kJ/m² for three growing seasons, a level that simulated a 17% depletion in stratospheric ozone. The objective of this study was to explore the effects of long-term UV-B enhancement on stomatal conductance, leaf tissue δ ¹³C, leaf water content, and leaf area. Particular attention was paid to the effects of UV-B radiation on water use efficiency (WUE) and leaf total nitrogen content. Enhanced UV-B radiation significantly reduced leaf area (50.1%) but increased leaf total nitrogen content (102%). These changes were associated with a decrease in stomatal conductance (16.1%) and intercellular CO₂ concentration/ air CO₂ concentration (C _i /C _a) (4.0%), and an increase in leaf tissue δ ¹³C (20.5‰), leaf water content (3.1%), specific leaf weight (SLW) (5.2%) and WUE (4.1%). The effects of UV-B on the plant were greatly affected by the water content of the deep soil (30–40 cm). During the dry season, differences in the stomatal conductance, δ ¹³C, and WUE between the control and UV-B treated shrubs were very small; whereas, differences became much greater when soil water stress disappeared. Furthermore, the effects of UV-B became much less significant as the treatment period progressed over the three growing seasons. Correlation analysis showed that enhanced UV-B radiation decreased the strength of the correlation between soil water content and leaf water content, δ ¹³C, C _i/C _a, stomatal conductance, with the exception of WUE that had a significant correlation coefficient with soil water content. These results suggest that WUE would become more sensitive to soil water variation due to UV-B radiation. Based on this experiment, it was found that enhanced UV-B radiation had much more significant effects on morphological traits and growth of S. pubescens than hydro-physiological characteristics. __________ Translated from Journal of Plant Ecology, 2006, 30(1): 47–56 [译自: 植物生态学报]     

Seasonal changes in photosynthesis, nitrogen content and nitrogen partitioning in Lindera umbellata leaves grown in high or low irradiance

Seasonal changes in photosynthetic capacity, leaf nitrogen (N) content and N partitioning were studied from before leaf maturation (spring) until death (autumn) in high- and low-light-exposed leaves of a deciduous shrub, Lindera umbellata var. membranacea (Maxim.) Momiyama growing in a natural forest in northeast Japan. In spring, light-saturated photosynthetic rate (Pmax) was low despite high leaf N and Rubisco contents, indicating that the photosynthetic apparatus was not yet functionally developed. Rubisco seemed to be only partially active. In summer and autumn, Pmax per unit leaf N increased and changes in Pmax were correlated with changes in leaf N and two photosynthetic components, Rubisco and chlorophyll. Changes in these components paralleled the changes in leaf N. During leaf senescence, about 70% of leaf N was resorbed. Metabolic proteins that accounted for the majority of leaf N in summer were highly degradable and more than sufficient to explain the high N-resorption efficiency. Structural proteins represented only a small part of leaf N and were relatively resistant to degradation and thus contributed little to N resorption. Leaf N partitioning between metabolic and structural proteins determined the amount of retranslocatable N, but did not strictly determine the N content of a dead leaf or N-resorption efficiency.     

Gas exchange, leaf structure and nitrogen in contrasting successional tree species growing in open and understory sites during a drought

Seasonal ecophysiology, leaf structure and nitrogen were measured in saplings of early (Populus grandidentata Michx. and Prunus serotina J.F. Ehrh.), middle (Fraxinus americana L. and Carya tomentosa Nutt.) and late (Acer rubrum L. and Cornus florida L.) successional tree species during severe drought on adjacent open and understory sites in central Pennsylvania, USA. Area-based net photosynthesis (A) and leaf conductance to water vapor diffusion (g(wv)) varied by site and species and were highest in open growing plants and early successional species at both the open and understory sites. In response to the period of maximum drought, both sunfleck and sun leaves of the early successional species exhibited smaller decreases in A than leaves of the other species. Shaded understory leaves of all species were more susceptible to drought than sun leaves and had negative midday A values during the middle and later growing season. Shaded understory leaves also displayed a reduced photosynthetic light response during the peak drought period. Sun leaves were thicker and had a greater mass per area (LMA) and nitrogen (N) content than shaded leaves, and early and middle successional species had higher N contents and concentrations than late successional species. In both sunfleck and sun leaves, seasonal A was positively related to predawn leaf Psi, g(wv), LMA and N, and was negatively related to vapor pressure deficit, midday leaf Psi and internal CO(2). Although a significant amount of plasticity occurred in all species for most gas exchange and leaf structural parameters, middle successional species exhibited the largest degree of phenotypic plasticity between open and understory plants.     

Vertical, horizontal and azimuthal variations in leaf photosynthetic characteristics within a Fagus crenata crown in relation to light acclimation

An understanding of spatial variations in gas exchange parameters in relation to the light environment is crucial for modeling canopy photosynthesis. We measured vertical, horizontal and azimuthal (north and south) variations in photosynthetic capacity (i.e., the maximum rate of carboxylation: Vcmax), nitrogen content (N), leaf mass per area (LMA) and chlorophyll content (Chl) in relation to relative photosynthetic photon flux (rPPF) within a Fagus crenata Blume crown. The horizontal gradient of rPPF was similar in magnitude to the vertical gradient of rPPF from the upper to the lower crown. The rPPF in the north quadrant of the crown was slightly lower than in the south quadrant. Nitrogen content per area (Narea), LMA and Vcmax were strictly proportional to rPPF, irrespective of the vertical direction, horizontal direction and crown azimuth, whereas nitrogen content per dry mass, Chl per area and photosynthetic capacity per dry mass (Vm) were fairly constant. Statistical analyses separating vertical trends from horizontal and azimuthal trends indicated that, although horizontal and vertical light acclimation of leaf properties were similar, there were two significant azimuthal variations: (1) Vcmax was lower in north-facing leaves than in south-facing leaves for a given Narea, indicating low photosynthetic nitrogen-use efficiency (PNUE) of north-facing leaves; and (2) Vcmax was lower in north-facing leaves than in south-facing leaves for a given LMA, indicating low Vm of the north-facing leaves. With respect to the low PNUE of the north-facing leaves, there were no significant azimuthal variations in leaf CO2 conductance from the stomata to the carboxylation site. Biochemical analysis indicated that azimuthal variations in nitrogen allocation to ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and in nitrogen allocation between carboxylation (Rubisco and other Calvin cycle enzymes) and light harvesting machinery (Chl pigment-protein complexes) were not the main contributor to the difference in PNUE between north- and south-facing leaves. Lower specific activity of Rubisco may be responsible for the low PNUE of the north-facing leaves. Anatomical analysis indicated that not only high leaf density, which is compatible with a greater fraction of non-photosynthetic tissue, but also thick photosynthetic tissue contributed to the low Vm in the north-facing leaves. These azimuthal variations may need to be considered when modeling canopy photosynthesis based on the Narea-Vcmax or LMA-Vcmax relationship.