Dependence of the aboveground CO2 exchange rate on tree size in field-grown hinoki cypress (Chamaecyparis obtusa)

The CO₂ exchange of the aboveground parts for five different-sized 17-year-old (as of 1991) hinoki cypress (Chamaecyparis obtusa) trees growing in the field was non-destructively measured over one year, using an open CO₂ exchange system. The CO₂ exchange of individual trees decreased with decreasing tree sizes, such as aboveground phytomass, leaf mass and leaf area. However, the CO₂ exchange abruptly decreased near the smallest-suppressed sample tree. The size dependence was well described by a generalized power function. The annual gross photosynthesis of individual trees was proportional to the square root of leaf mass or leaf area. The dependence of CO₂ exchange on annual phytomass increment was described by a simple power function with an exponent value less than unity, suggesting that CO₂ exchange per unit of phytomass increment was lower in larger-sized trees than in smaller-sized trees. The mean photosynthetic activity of a tree, i.e., gross photosynthesis per unit of leaf area, slightly increased to its highest value with decreasing leaf area and then decreased abruptly near the smallest sample tree. The maximum value of mean photosynthetic activity was estimated to be 2.85 kg CO₂ m⁻² year⁻¹ for a leaf area of 1.56 m² tree⁻¹. The ratio of mean photosynthetic activity to the maximum photosynthetic activity was the highest in an intermediate tree and decreased gradually toward larger-sized trees by ca. 60% and also decreased toward the smallest suppressed tree by ca. 35%.     

Photosynthetic and biomass allocation responses of Liquidambar styraciflua (Hamamelidaceae) to vine competition

This 2-year field study examined stomatal conductance, photosynthesis, and biomass allocation of Liquidambar styraciflua saplings in response to below- and aboveground competition with the vines Lonicera japonica and Parthenocissus quinquefolia. Vine competition did not affect stomatal conductance of the host trees. The leaf photosynthetic capacity and photosynthetic nitrogen-use efficiency were significantly reduced by root competition with vines, either singly or in combination with aboveground competition, early in the second growing season. However, such differences disappeared by the end of the second growing season. Trees competing below ground with vines also had lower allocation to leaves compared with steins. Aboveground competition with vines resulted in reduced photosynthetic capacity per unit leaf area, but not per unit leaf weight, in trees. No correlation was found between single leaf photosynthetic capacity and tree growth. In contrast, a high positive correlation existed between allocation to leaves and diameter growth. Results from this study suggest that allocation patterns are more affected than leaf photosynthesis in trees competing with vines.     

A test of the hydraulic limitation hypothesis in fast-growing Eucalyptus saligna

The hydraulic limitation hypothesis proposes that (1) reduced growth in taller trees is caused by decreased photosynthesis resulting from a decrease in hydraulic conductance promoted by a longer root‐to‐leaf flow path, and (2) this mechanism reduces stand productivity after canopy closure. This hypothesis was tested by comparing the physiology of 7 m (1 year) and 26 m (5 year) Eucalyptus saligna plantations where above‐ground productivity for the 26 m trees was approximately 69% of that for the 7 m trees, and water and nutrients were not limiting. The study compared whole tree physiology [water flux (Q_l), average crown conductance (G_T), crown hydraulic conductance per unit leaf area (K_L), carbon isotope discrimination (δ¹³C)] and leaf physiology under light saturation (leaf water potential at the canopy top (Ψ_LEAF), photosynthetic capacity (A_max), and photosynthesis (A) and stomatal conductance (g_s). K_L was 50% lower in the taller trees, but whole tree Q_l and G_T were the same for the 7 m and 26 m trees. Photosynthetic capacity was the same for leaves at the canopy top, but δ¹³C was ?1.8‰ lower for the 26 m trees. A and g_s were either lower in the taller trees or equal, depending on sampling date. The taller trees maintained 0.8 MPa lower Ψ_LEAF during the day and had 2.6‐times higher sapwood area per unit leaf area; these factors compensated for the effects of increased height and gravitational potential in the taller trees to maintain higher G_T. The hydraulic limitation hypothesis (as originally stated) failed to explain the sharp decline in net primary productivity after canopy closure in this study. The effects of increased height appear to be a universal hydraulic problem for trees, but compensation mitigated these effects and maintained Q_l and G_T in the present study. Compensation may induce other problems (such as lower Ψ_LEAF or higher respiratory costs) that could reduce carbon gain or shift carbon allocation, and future studies of hydraulic limitation should consider compensation and associated carbon costs. In this study, the combination of similar G_T and lower δ¹³C for the 26 m trees suggests that total crown photosynthesis was lower for the 26 m trees, perhaps a result of the lower Ψ_LEAF.     

Photosynthesis Regulation by Glucohexaose Through Redox Changes in Cucumis sativus

To investigate the effects of glucohexaose (P6) on cucumber, leaf CO₂ assimilation, chlorophyll fluorescence parameters, chlorophyll content, and carbohydrate metabolism were examined in cucumber plants. The net photosynthetic rate (P _n ) of cucumber leaves was enhanced after being treated with 10 μg mL^?1 P6. The increase was correlated with increases in transpiration rate (E) and stomatal conductance (G _s), whereas the intercellular CO₂ concentration (C _i) was not different from the control plants. Chlorophyll content, absorption of light energy per unit area (ABS/CS), capture of light energy per unit area (TR_o/CS), quantum yield of electron transport per unit area (ET_o/CS), maximum photochemical efficiency of PSII (φP _o), quantum yield of photosynthetic institution electron transfer (φE _o), probability of other electron acceptors that captured exciton-transferred electrons to the electronic chain which exceeds QA (ψ _o), number of reaction centers per unit leaf area (RC/CS_o), and the performance index on absorption basis (PI_ABS) were improved, but heat dissipation per unit area (DI_o/CS) and maximum quantum yield of non-chemical quenching (φD _o) were reduced. In addition, increases in sucrose, soluble sugars, and starch contents were observed in P6-treated plants. However, H₂O₂ scavenger (DMTU) or NADPH oxidase inhibitor (DPI) pretreatment significantly abolished the effect of P6 on photosynthesis. The results demonstrated that ROS played a critical role in P6-induced photosynthesis. The increase in chlorophyll content together with efficient light absorption, transmission, and conversion in P6-treated plants is important for increasing photosynthesis.     

Coordinated changes in photosynthesis,water relations and leaf nutritional traits of canopy trees along a precipitation gradient in lowland tropical forest

We investigated leaf physiological traits of dominant canopy trees in four lowland Panamanian forests with contrasting mean annual precipitation (1,800, 2,300, 3,100 and 3,500 mm). There was near complete turn-over of dominant canopy tree species among sites, resulting in greater dominance of evergreen species with long-lived leaves as precipitation increased. Mean structural and physiological traits changed along this gradient as predicted by cost–benefit theories of leaf life span. Nitrogen content per unit mass (N_mass) and light- and CO₂-saturated photosynthetic rates per unit mass (P_mass) of upper canopy leaves decreased with annual precipitation, and these changes were partially explained by increasing leaf thickness and decreasing specific leaf area (SLA). Comparison of 1,800 mm and 3,100 mm sites, where canopy access was available through the use of construction cranes, revealed an association among extended leaf longevity, greater structural defense, higher midday leaf water potential, and lower P_mass, N_mass, and SLA at wetter sites. Shorter leaf life spans and more enriched foliar ¹⁵N values in drier sites suggest greater resorption and re-metabolism of leaf N in drier forest. Greater dominance of short-lived leaves with relatively high P_mass in drier sites reflects a strategy to maximize photosynthesis when water is available and to minimize water loss and respiration costs during rainless periods. Overall, our study links coordinated change in leaf functional traits that affect productivity and nutrient cycling to seasonality in lowland tropical forests.     

Leaf photosynthetic traits scale with hydraulic conductivity and wood density in Panamanian forest canopy trees

We investigated how water transport capacity, wood density and wood anatomy were related to leaf photosynthetic traits in two lowland forests in Panama. Leaf-specific hydraulic conductivity (k_L) of upper branches was positively correlated with maximum rates of net CO₂ assimilation per unit leaf area (A_area) and stomatal conductance (g_s) across 20 species of canopy trees. Maximum k_L showed stronger correlation with A_area than initial k_L suggesting that allocation to photosynthetic potential is proportional to maximum water transport capacity. Terminal branch k_L was negatively correlated with A_area/g_s and positively correlated with photosynthesis per unit N, indicating a trade-off of efficient use of water against efficient use of N in photosynthesis as water transport efficiency varied. Specific hydraulic conductivity calculated from xylem anatomical characteristics (k_theoretical) was positively related to A_area and k_L, consistent with relationships among physiological measurements. Branch wood density was negatively correlated with wood water storage at saturation, k_L, A_area, net CO₂ assimilation per unit leaf mass (A_mass), and minimum leaf water potential measured on covered leaves, suggesting that wood density constrains physiological function to specific operating ranges. Kinetic and static indices of branch water transport capacity thus exhibit considerable co-ordination with allocation to potential carbon gain. Our results indicate that understanding tree hydraulic architecture provides added insights to comparisons of leaf level measurements among species, and links photosynthetic allocation patterns with branch hydraulic processes.     

Short- and long-term responses to seasonal drought in ponderosa pines growing at different plantation densities in Patagonia, South America

Trees drought responses could be developed in the short- or in the long-term, aiming at sustaining carbon fixation and water use efficiency (WUE). The objective of this study was to examine short- and long-term adjustments occurring in different size Pinus ponderosa Dougl. ex P. & C. Laws trees in response to seasonal drought when they are growing under different competition level. The following variables were studied: branch and stem hydraulic conductivity, canopy and stomatal conductance (gc, gs), transpiration (E), photosynthesis (A _max), wood δ¹³C (as a proxy of intrinsic WUE), leaf to sapwood area ratio (A _L:A _s) and growth in the biggest (B) and the smallest (S) trees of high (H) and low (L) density stands. A _L:A _s was positively correlated with tree size and negatively correlated with competition level, increasing leaf hydraulic conductance in H trees. Accordingly, higher gc and E per unit A _L were found in H than in L trees when soil water availability was high, but decreased abruptly during dry periods. BL trees maintained stable gc and E values even during the summer drought. The functional adjustments observed in H trees allow them to maintain their hydraulic integrity (no apparent k _s losses), but their stem and leaf growth were severely affected by drought events. _iWUE was similar between all tree groups in a wet season, whereas it significantly decreased in SH trees in a dry season suggesting that when radiation and water are co-limiting gas exchange, functional adjustments not only affect absolute growth, but also WUE.     

Altitudinal Change in the Photosynthetic Capacity of Tropical Trees: A Case Study from Ecuador and a Pantropical Literature Analysis

In tropical mountains, trees are the dominant life form from sea level to above 4,000-m altitude under highly variable thermal conditions (range of mean annual temperatures: <8 to >28°C). How light-saturated net photosynthesis of tropical trees adapts to variation in temperature, atmospheric CO₂ concentration, and further environmental factors, that change along elevation gradients, is not precisely known. With gas exchange measurements in mature trees, we determined light-saturated net photosynthesis at ambient temperature (T) and [CO₂] (A _sat) of 40 tree species from 21 families in tropical mountain forests at 1000-, 2000-, and 3000-m elevation in southern Ecuador. We tested the hypothesis that stand-level averages of A _sat and leaf dark respiration (R _D) per leaf area remain constant with elevation. Stand-level means of A _sat were 8.8, 11.3, and 7.2?μmol?CO₂?m^?2?s^?1; those of R _D 0.8, 0.6, and 0.7?μmol?CO₂?m^?2?s^?1 at 1000-, 2000-, and 3000-m elevation, respectively, with no significant altitudinal trend. We obtained coefficients of among-species variation in A _sat and R _D of 20–53% (n?=?10–16 tree species per stand). Examining our data in the context of a pan-tropical A _sat data base for mature tropical trees (c. 170 species from 18 sites at variable elevation) revealed that area-based A _sat decreases in tropical mountains by, on average, 1.3?μmol?CO₂?m^?2?s^?1?per?km altitude increase (or by 0.2?μmol?CO₂?m^?2?s^?1 per K temperature decrease). The A _sat decrease occurred despite an increase in leaf mass per area with altitude. Local geological and soil fertility conditions and related foliar N and P concentrations considerably influenced the altitudinal A _sat patterns. We conclude that elevation is an important influencing factor of the photosynthetic activity of tropical trees. Lowered A _sat together with a reduced stand leaf area decrease canopy C gain with elevation in tropical mountains.     

Canopy Apparent Photosynthetic Characteristics and Yield of Two Spike-Type Wheat Cultivars in Response to Row Spacing under High Plant Density

In northern China, large-spike wheat (Triticum aestivum L) is considered to have significant potential for increasing yields due to its greater single-plant productivity despite its lower percentage of effective tillers, and increasing the plant density is an effective means of achieving a higher grain yield. However, with increases in plant density, the amount of solar radiation intercepted by lower strata leaves is decreased and the rate of leaf senescence is accelerated. Row spacing can be manipulated to optimize the plant spatial distribution under high plant density, therefore improving light conditions within the canopy. Consequently, field experiments were conducted from 2010 to 2012 to investigate whether changes in row spacing under high plant density led to differences in canopy apparent photosynthesis (CAP), individual leaf photosynthesis and grain yield. Two different spike-type winter wheat cultivars, Jimai22 (a small-spike cultivar as a control cultivar) and Wennong6 (a large-spike cultivar), were grown at a constant plant density of 3,600,000 plants ha^–1 (a relatively higher plant density) over a wide range of row spacing as follows: 5-cm row spacing (R₀), 15-cm row spacing (R₁), 25-cm conventional row spacing (R₂), and 35-cm row spacing (R₃). The two-year investigations revealed that increased row spacing exhibited a significantly higher light transmission ratio (LT), which improved light conditions within the canopy; however, excessive light leakage losses in R₂ and R₃ treatments were not favorable to improved irradiation energy utilization efficiency. Aboveground biomass accumulation was influenced by row spacing. Two spike-type wheat accumulated greater biomass under 15-cm row spacing compared to other row spacing treatments, although a markedly improved photosynthetic rate (P_N), effective quantum yield of photosystem II (Φ_PSII) and maximal efficiency of photosystem II photochemistry (F_v/F_m) in the penultimate and third leaves were observed in R₂ and R₃ treatments. At the same time, a longer duration of CAP and green leaf area was maintained in R₁ during grain filling. Compared with conventional row spacing, Wennong6 in R₁ treatment obtained 21.0% and 19.1% higher grain yield in 2011 and 2012, respectively, while for Jimai22 it increased by 11.3% and 11.4%, respectively. A close association of yield with CAP and LAI at mid-grain filling was observed. In conclusion, for the tested growing conditions, decreasing the row spacing to an optimal distance (15 cm) maintained a longer duration of LAI and CAP during grain filling, made a better coordination of group and individual leaf photosynthesis, and accumulated higher aboveground biomass, leading to a greater grain yield. In addition, Wennong6 had a more rational canopy architecture than Jimai22 (improved LT and higher LAI) and CAP under 15-cm row spacing, leading to a higher grain yield, which indicated that the large-spike type cultivar has the potential to obtain higher yields by increasing plant density through optimum row spacing allocation (15 cm).     

No evidence that elevated CO2 gives tropical lianas an advantage over tropical trees

Recent studies indicate that lianas are increasing in size and abundance relative to trees in neotropical forests. As a result, forest dynamics and carbon balance may be altered through liana‐induced suppression of tree growth and increases in tree mortality. Increasing atmospheric CO₂ is hypothesized to be responsible for the increase in neotropical lianas, yet no study has directly compared the relative response of tropical lianas and trees to elevated CO₂. We explicitly tested whether tropical lianas had a larger response to elevated CO₂ than co‐occurring tropical trees and whether seasonal drought alters the response of either growth form. In two experiments conducted in central Panama, one spanning both wet and dry seasons and one restricted to the dry season, we grew liana (n = 12) and tree (n = 10) species in open‐top growth chambers maintained at ambient or twice‐ambient CO₂ levels. Seedlings of eight individuals (four lianas, four trees) were grown in the ground in each chamber for at least 3 months during each season. We found that both liana and tree seedlings had a significant and positive response to elevated CO₂ (in biomass, leaf area, leaf mass per area, and photosynthesis), but that the relative response to elevated CO₂ for all variables was not significantly greater for lianas than trees regardless of the season. The lack of differences in the relative response between growth forms does not support the hypothesis that elevated CO₂ is responsible for increasing liana size and abundance across the neotropics.     

Carbon balance of young birch trees grown in ambient and elevated atmospheric CO2 concentrations

We constructed a carbon budget for young birch trees grown in ambient and elevated CO₂ concentrations over their fourth year of growth. The annual total of net leaf photosynthesis was 110% more in elevated CO₂ than in ambient CO₂. However, the trees in elevated CO₂ grew only 59% more biomass than the trees in ambient CO₂ over the year. Modelling studies showed that larger loss of carbon from fine-root production and growth of the root-associated mycorrhiza by the trees in elevated CO₂ probably accounted for all the remaining difference in net photosynthesis between the two treatments. Our modelling also showed that the fraction of net photosynthate consumed by respiration of nonleaf tissue was similar in the two CO₂ treatments, and was 26% and 24% for trees in ambient and elevated CO₂, respectively. Trees in elevated CO₂ had 43% more leaves, and produced 110% more net photosynthate than trees in ambient CO₂, even though the maximum rate of carboxylation per unit leaf nitrogen decreased by 21%. Sensitivity studies showed that down-regulation reduced the annual net photosynthetic production of the trees in elevated CO₂ by only 6%. Direct effects of higher CO₂ on photosynthesis and greater leaf area of the trees in elevated CO₂ increased the net photosynthesis of the trees by 68% and 60%, respectively; and together accounted for most of the difference in net photosynthesis between the two treatments.     

Effects of polyploidy on photosynthesis

In polyploid plants the photosynthetic rate per cell is correlated with the amount of DNA per cell. The photosynthetic rate per unit leaf area is the product of the rate per cell times the number of photosynthetic cells per unit area. Therefore, the photosynthetic rate per unit leaf area will increase if there is a less than proportional increase in cell volume at higher ploidal levels, or if cell packing is altered to allow more cells per unit leaf area. In autopolyploids (Medicago sativa, C₃ species, and Pennisetum americanum, C₄ species) there is a doubling of photosynthesis per cell and of cell volume in the tetraploid compared to the diploid. However, there is a proportional decrease in number of cells per unit leaf area with this increase in ploidy such that the rate of photosynthesis per leaf area does not change. There is more diversity in the relationship between ploidal level (gene dosage) and photosynthetic rates per unit leaf area in allopolyploids. This is likely to reflect the effects of natural selection on leaf anatomy, and novel genetic interactions from contributed genomes which can occur with allopolyploidy. In allopolyploid wheat (C₃ species) a higher cell volume per unit DNA at the higher ploidal level is negatively correlated with photosynthesis rate per unit leaf area. Although photosynthesis per cell increases with ploidy, photosynthesis per leaf area decreases, being lowest in the allohexaploid, cultivated bread wheat (Triticum aestivum). Alternatively, doubling of photosynthetic rate per cell with doubling of DNA, with apparent natural selection for decreased cell volume per unit DNA, results in higher rates of photosynthesis per leaf area in octaploid compared to tetraploid Panicum virgatum (C₄) which may be a case of allopolyploidy. Similar responses probably occur in Festuca arundinacea. Therefore, in some systems anatomical factors affecting photosynthesis are also affected by ploidal level. It is important to evaluate that component as well as determining the effect on biochemical processes. Current information on polyploidy and photosynthesis in several species is discussed with respect to anatomy, biochemistry and bases for expressing photosynthetic rates.Abbreviations Chl chlorophyll - RuBPC ribulose-1,5-bisphosphate carboxylase     

Optimal photosynthetic use of light by tropical tree crowns achieved by adjustment of individual leaf angles and nitrogen content

Background and Aims

Theory for optimal allocation of foliar nitrogen (ONA) predicts that both nitrogen concentration and photosynthetic capacity will scale linearly with gradients of insolation within plant canopies. ONA is expected to allow plants to efficiently use both light and nitrogen. However, empirical data generally do not exhibit perfect ONA, and light-use optimization per se is little explored. The aim was to examine to what degree partitioning of nitrogen or light is optimized in the crowns of three tropical canopy tree species.

Methods

Instantaneous photosynthetic photon flux density (PPFD) incident on the adaxial surface of individual leaves was measured along vertical PPFD gradients in tree canopies at a frequency of 0·5 Hz over 9–17 d, and summed to obtain the average daily integral of PPFD for each leaf to characterize its insolation regime. Also measured were leaf N per area (N_area), leaf mass per area (LMA), the cosine of leaf inclination and the parameters of the photosynthetic light response curve [photosynthetic capacity (A_max), dark respiration (R_d), apparent quantum yield (ϕ) and curvature (θ)]. The instantaneous PPFD measurements and light response curves were used to estimate leaf daily photosynthesis (A_daily) for each leaf.

Key Results

Leaf N_area and A_max changed as a hyperbolic asymptotic function of the PPFD regime, not the linear relationship predicted by ONA. Despite this suboptimal nitrogen partitioning among leaves, A_daily did increase linearly with PPFD regime through co-ordinated adjustments in both leaf angle and physiology along canopy gradients in insolation, exhibiting a strong convergence among the three species.

Conclusions

The results suggest that canopy tree leaves in this tropical forest optimize photosynthetic use of PPFD rather than N per se. Tropical tree canopies then can be considered simple ‘big-leaves’ in which all constituent ‘small leaves’ use PPFD with the same photosynthetic efficiency.Key words: Optimal resource allocation, nitrogen, photosynthetic capacity, leaf mass per area, tropical trees, radiation use efficiency, scaling, leaf angle, canopy architecture, big leaf model     

Carbon dioxide diffusion across stomata and mesophyll and photo-biochemical processes as affected by growth CO2 and phosphorus nutrition in cotton

Nutrients such as phosphorus may exert a major control over plant response to rising atmospheric carbon dioxide concentration (CO₂), which is projected to double by the end of the 21st century. Elevated CO₂ may overcome the diffusional limitations to photosynthesis posed by stomata and mesophyll and alter the photo-biochemical limitations resulting from phosphorus deficiency. To evaluate these ideas, cotton (Gossypium hirsutum) was grown in controlled environment growth chambers with three levels of phosphate (Pi) supply (0.2, 0.05 and 0.01 mM) and two levels of CO₂ concentration (ambient 400 and elevated 800 μmol mol⁻¹) under optimum temperature and irrigation. Phosphate deficiency drastically inhibited photosynthetic characteristics and decreased cotton growth for both CO₂ treatments. Under Pi stress, an apparent limitation to the photosynthetic potential was evident by CO₂ diffusion through stomata and mesophyll, impairment of photosystem functioning and inhibition of biochemical process including the carboxylation efficiency of ribulose-1,5-bisphosphate carboxylase/oxyganase and the rate of ribulose-1,5-bisphosphate regeneration. The diffusional limitation posed by mesophyll was up to 58% greater than the limitation due to stomatal conductance (g_s) under Pi stress. As expected, elevated CO₂ reduced these diffusional limitations to photosynthesis across Pi levels; however, it failed to reduce the photo-biochemical limitations to photosynthesis in phosphorus deficient plants. Acclimation/down regulation of photosynthetic capacity was evident under elevated CO₂ across Pi treatments. Despite a decrease in phosphorus, nitrogen and chlorophyll concentrations in leaf tissue and reduced stomatal conductance at elevated CO₂, the rate of photosynthesis per unit leaf area when measured at the growth CO₂ concentration tended to be higher for all except the lowest Pi treatment. Nevertheless, plant biomass increased at elevated CO₂ across Pi nutrition with taller plants, increased leaf number and larger leaf area.     

Maturation in Larch : II. Effects of Age on Photosynthesis and Gene Expression in Developing Foliage

The effect of maturation on the morphological and photosynthetic characteristics, as well as the expression of two genes involved in photosynthesis in the developing, current year foliage of Eastern larch (Larix laricina [Du Roi]) is described. These effects were observed on foliage during the third growing season after grafting of scions from trees of different ages onto 2 year old rootstock. Specific leaf weight (gram dry weight per square meter), leaf cross-sectional area (per square millimeter), and chlorophyll content (milligram per gram dry weight) all increase with increasing age in long shoot foliage from both indoor- and outdoor-grown trees. Net photosynthesis (NPS) (mole of CO₂ per square millimeter per second) increases with age on indoor- but not outdoor-grown trees. NPS also increases with increased chlorophyll content, but outdoor-grown scions of all ages had higher chlorophyll content, and chlorophyll does not appear to be limiting for NPS outdoors. To extend these studies of maturation-related differences in foliar morphology and physiology to the molecular genetic level, sequences were cloned from the cab and rbsS gene families of larch. Both cab and rbcS gene families are expressed in foliage but not in roots, and they are expressed in light-grown seedlings of larch but only at very low levels in dark-grown seedlings (~2% of light-grown seedlings). Steady-state cab mRNA levels are relatively higher (~40%) in newly expanding short shoot foliage from juvenile plants compared to mature plants. Unlike cab, the expression of the rbcS gene family did not seem to vary with age. These data show that the maturation-related changes in morphological and physiological phenotypes are associated with changes in gene expression. No causal relationship has been established, however. Indeed, we conclude that the faster growth of juvenile scions reported previously (MS Greenwood, CA Hopper, KW Hutchison [1989] Plant Physiol 90: 406-412) is not due to increased NPS or cab expression. Long shoot foliage is the dominant foliar type on young trees and its lower specific leaf weight will permit production of more photosynthetic surface area per unit of leaf biomass.     

陕北沙地小叶杨“小老树”的水力适应性

以黄土高原"小老树"发生面积最大的树种-小叶杨为例,研究了不同水分生境下(水分相对好的沟道和干旱的梁坡片沙地,分别标记为生境A和生境B)小叶杨的生长、光合、水力学特性等,试图探讨小叶杨"小老树"对干旱生境的适应机制。结果表明:生境B小叶杨树高、地径、1 m树高处直径明显小于生境A,同时其主茎顶端枯枝长度大于生境A;生境B小叶杨叶净光合速率和气孔导度明显低于生境A。两种生境下小叶杨黎明前和正午叶水势无显著差异,生境B小叶杨正午时小枝枝干的比导水率明显低于生境A,但两种生境的比叶导水率则无显著差异,生境B小叶杨的Huber值明显大于生境A。生境B小叶杨枝干的P50(导水率损失50%时所对应的木质部水势)比生境A低约0.76 MPa左右,其气孔关闭的水势比生境A晚0.2 MPa左右,生境B小叶杨水分传输安全距离明显大于生境A。表明干旱生境下小叶杨高Huber值和低气孔导度有助于其叶水分关系维持相对稳定,低光合速率和维持大的水分传输安全距离所需的木质部碳投资增加是小叶杨形成"小老树"的重要原因。     

Seasonal differences in leaf-level physiology give lianas a competitive advantage over trees in a tropical seasonal forest

Lianas are an important component of most tropical forests, where they vary in abundance from high in seasonal forests to low in aseasonal forests. We tested the hypothesis that the physiological ability of lianas to fix carbon (and thus grow) during seasonal drought may confer a distinct advantage in seasonal tropical forests, which may explain pan-tropical liana distributions. We compared a range of leaf-level physiological attributes of 18 co-occurring liana and 16 tree species during the wet and dry seasons in a tropical seasonal forest in Xishuangbanna, China. We found that, during the wet season, lianas had significantly higher CO₂ assimilation per unit mass (A _mass), nitrogen concentration (N _mass), and δ¹³C values, and lower leaf mass per unit area (LMA) than trees, indicating that lianas have higher assimilation rates per unit leaf mass and higher integrated water-use efficiency (WUE), but lower leaf structural investments. Seasonal variation in CO₂ assimilation per unit area (A _area), phosphorus concentration per unit mass (P _mass), and photosynthetic N-use efficiency (PNUE), however, was significantly lower in lianas than in trees. For instance, mean tree A _area decreased by 30.1% from wet to dry season, compared with only 12.8% for lianas. In contrast, from the wet to dry season mean liana δ¹³C increased four times more than tree δ¹³C, with no reduction in PNUE, whereas trees had a significant reduction in PNUE. Lianas had higher A _mass than trees throughout the year, regardless of season. Collectively, our findings indicate that lianas fix more carbon and use water and nitrogen more efficiently than trees, particularly during seasonal drought, which may confer a competitive advantage to lianas during the dry season, and thus may explain their high relative abundance in seasonal tropical forests.     

Osmotic adjustment in leaves ofLycopersicon esculentum andL. pennellii in response to saline water irrigation

Two tomato species (Lycopersicon esculentum andL. pennellii) were grown under unheated plastic greenhouse and irrigated with 0 or 140 mM NaCl. Salinity induces a more important reduction in predawn leaf water potential (ψ_pd) inL. esculentum than inL. pennellii. In both species the osmotic adjustment was achieved by active solute accumulation. The leaf water potential at turgor loss point (ψ_tlp) seemed to be controlled by leaf osmotic potential (ψ_os). The results revealed the existence of limits to the accumulation of osmotic solutes in leaf tissues and the existence of an ontogenetic effect on the solute accumulation. In both species, but essentially inL. pennellii the inorganic solutes contribution especially Na⁺ and Cl^? accumulation to ψ_os was higher than the organic solutes. Therefore, wild species save energy more markedly.     

Partitioning of leaf nitrogen with respect to within canopy light exposure and nitrogen availability in peach (Prunus persica)

Summary Relationships between leaf nitrogen content and within canopy light exposure were studied in mature nectarine peach trees (Prunus persica cv. Fantasia) that had received 0, 112, 196, 280 or 364 kg of fertilizer nitrogen per hectare per year for the previous 3 years. The relationships between light saturated leaf CO₂ assimilation rates and leaf nitrogen concentration were also determined on trees in the highest and lowest nitrogen fertilization treatments. The slope of the linear relationship between leaf N content per unit leaf area and light exposure was similar for all nitrogen treatments but the y-intercept of the relationship increased with increasing N status. The slope of the relationship between leaf N content per unit leaf area and light saturated CO₂ assimilation rates was greater for the high N trees than the low N trees, but maximum measured leaf CO₂ assimilation rates were similar for both the high and low N treatments. A diagrammatic model of the partitioning of leaf photosynthetic capacity with respect to leaf light exposure for high and low nitrogen trees suggests that the major influence of increased N availability is an increase in the photosynthetic capacity of partially shaded leaves but not of the maximum capacity of highly exposed leaves.     

成都市沙河主要绿化树种固碳释氧和降温增湿效益

以成都市沙河植物廊道广泛应用的8种绿化植物为材料,利用LI-6400XT便携式光合测定系统进行了光合生理生态指标的测定,并对其固碳释氧与降温增湿效应进行了量化研究.结果表明:整个生长季节同类植物各季节的单位叶面积固碳释氧和 降温增湿能力表现出夏季＞秋季＞春季.日固碳释氧能力由强到弱为桂花、垂柳、香樟、黄葛树、山杜英、银杏、天竺桂、水杉,年固碳释氧能力由强到弱为垂柳、香樟、黄葛树、银杏、桂花、天竺桂、水杉、山杜英,日降温增湿效果由强到弱为垂柳、山杜英、水杉、天竺桂、黄葛树、香樟、银杏、桂花.据估算,整个沙河植物群落中乔木树种年总固碳量约为5.87×104 t,总释氧量约为4.27×104 t.根据对主要树种固碳释氧和降温增湿能力的分析表明,在树种配置时,垂柳、桂花、山杜英、香樟为优选乔木树种,而银杏的固碳释氧和降温增湿能力较弱,可作为长寿树种和观赏树种适量引种,不宜大面积绿化.