Photosynthesis of a scots pine shoot: Test of a shoot photosynthesis model in a direct radiation field

The photosynthetic light curves of eight Scots pine (Pinussylvestris L.) shoots, each with a different structure, were measured in a direct radiation field at different positions relative to the radiation beam. Using a model of shoot geometry, the irradiance distributions on the needle surface area of the same shoots were simulated in each position. Using measurements of shoot photosynthesis, the photosynthetic light curve for a needle surface area element (a small area on the needle surface) was estimated indirectly for each shoot. Shoot photosynthesis was then calculated from the estimated photosynthetic light response curve of a needle surface area element and the irradiance distribution on the surface area of needles. Calculated rates for shoot photosynthesis agreed well with the measured rates. The mean irradiance on the needle surface area was the major component causing variation in the light response of a shoot irradiated from different directions, but the shape of the irradiance distribution on the needle surface area had a considerably smaller effect.     

Integration of a nonlinear function in a changing environment: Estimating photosynthesis using mean and variance of radiation

If the mean (integral) of a convex or concave function is estimated by evaluating the response function at the mean of the environmental variable, the estimate is necessarily biased. This study computes from empirical radiation data the errors obtained when mean photosynthesis is estimated using mean radiation. Furthermore, three different methods of estimating mean photosynthesis are compared in a case where the second power of the irradiance is also integrated and the variance is thus known. The smallest errors are obtained when the irradiance distribution is approximated by a two-point distribution: the bias is reduced to one-tenth and the root mean square error to one-third compared to the situation when only mean radiation is used. The results indicate that, if accurate estimates are needed for integrals of nonlinear responses, the second power of radiation or any other fluctuating environmental variable should also be measured.     

The distribution of leaf area,radiation, photosynthesis and transpiration in a Shamouti orange hedgerow orchard. Part II. Photosynthesis,transpiration, and the effect of row shape and direction

The influence of the distribution of radiation in an orange canopy on transpiration and photosynthesis was examined by developing a model of these processes.The leaf energy balance, microclimate relationships and climatic data are combined with radiation, leaf conductance, and leaf carbon uptake models to simulate orchard photosynthesis and transpiration over 2 days. Calculated hourly values of transpiration showed good agreement with measured values of sap flow in the orange orchard.Calculated carbon uptake during the six summer months was 22 kg CO₂ per tree; however, experimental estimates of annual dry matter production yield 55 kg CO₂ per tree. The calculated figure is therefore considerably in error and indicates that present information used in carbon balance modeling of Citrus is inadequate. Even so, it is shown that radiation levels deep in the canopy, where a significant amount of leaf area and transpiration is located, are too low for significant carbon uptake to occur.As an example of the usefulness of the model, the distributions of photosynthesis, transpiration and photosynthetic radiation were simulated in hedgerow canopies of three different shapes following current pruning practices in Israel. The distribution of foliage inside the given hedgerow cross-section was calculated based on the relationship of average measured foliage density to calculated diffuse photosynthetic irradiance in the canopy. The simulation was run for rows oriented north-south and east-west and for climatic conditions of midsummer. The results of the simulation indicated that: (a) The highest photosynthesis in citrus orchards is obtained by covering the largest ground areas possible with a thick canopy, i.e., maximum leaf area index (LAI). Under such conditions most photosynthesis occurs in the upper 1 m of the canopy. (b) Although rows with slanted walls do not have the highest photosynthesis, they allow more light penetration into the canopy and have productive regions on the periphery of the canopy at all heights within the orchard. (c) Whereas row orientation has little influence on total photosynthesis of the orchard, a N-S orientation allows more light penetration into rows with slanted walls and/or wide inter-row alleys, thus reducing spatial variation in the computed photosynthesis. (d) Water use of vertically pruned citrus orchards can be decreased significantly without seriously affecting photosynthesis by reducing canopy height to as low as 3 m.     

Two-dimensional solar radiation interception model for hedgerow fruit trees

A two-dimensional, hourly or daily time step model was developed, which takes canopy characteristics and row orientation into account to simulate solar radiation interception in hedgerow orchards. In order to determine the spatial and temporal distribution of soil irradiance across the tree row, the canopy path length through which the radiation must travel to reach a certain point on the soil surface is calculated. The model assumes leaves to be uniformly distributed within an ellipsoid, and radiation penetrating the canopy is attenuated according to Beer’s law. Beam or direct radiation and diffuse radiation for the PAR (photosynthetically active radiation) and NIR (near-infrared radiation) wavebands are calculated separately, as they interact differently with the canopy. The attenuation of beam radiation by the canopy is strongly dependent on canopy dimensions and architecture, zenith and azimuth angle, as well as row orientation. Radiation can penetrate neighbouring rows, so two rows on either side of the simulated row are considered. Validation of the model was carried out for a wide range of conditions (crops, row orientation, canopy density, tree size and shape). Field measurements included solar radiation, soil irradiance at different distances from the tree row with tube solarimeters, leaf area density, as well as canopy size and row orientation. Model predictions of soil irradiance were excellent in orchards with symmetrical and elliptical canopies having a uniform leaf distribution. In orchards where the canopy was non-symmetric and/or had non-uniform leaf distribution, errors in predictions of solar radiation transmittance occurred. As a result of these discrepancies, the overall MAE was 40% of the average measured value of radiant transmittance over the whole day.     

Ultraviolet-B radiation in a row-crop canopy: an extended 1-D model

A decrease in stratospheric ozone may result in a serious threat to plants, since biologically active short-wavelength ultraviolet-B (UV-B 280–320 nm) radiation will increase even with a relatively small decrease in ozone. Numerous investigations have demonstrated that the effect of UV-B enhancements on plants includes reduction in grain yield, alteration in species competition, susceptibility to disease, and changes in plant structure and pigmentation. To determine the physiological effects on plants of any increases in UV-B radiation, the irradiances at the potential sensitive plant surface need to be known. A number of radiative transfer models exist but because of the importance of sky diffuse radiation to the global UV-B irradiance, models designed to estimate photosynthetically active radiation or total solar radiation may not accurately model the UV-B. This paper compares spatially and temporally averaged measurements of the UV-B canopy transmittance of a relatively dense maize canopy (sky view: 0.27°) to the estimations of two one-dimensional models differing mainly in the handling of sky radiance. The model that considered the distribution of sky radiance tended to underestimate the canopy transmittance, the model that assumed an isotropic sky radiance distribution tended to overestimate the canopy transmittance. However, the assumption concerning the sky radiance distribution accounted for only about 0.01 of the model error. Consequently, the sky radiance distribution is probably not important in modeling such dense crop canopies. The model that overestimated transmittance and had the generally larger errors, a modified Meyers model, used the assumption of uniform leaf angle distribution, whereas in the other model, designated the UVRT model, leaf angle distributions were estimated by sample measurements. Generally this model would be satisfactory in describing the statistically average UV-B irradiance conditions in the canopy. This model may also be applied to other dense plant canopies including forests.     

Field crop responses to ultraviolet-B radiation: a review

This paper provides an overview of existing literature on the ultraviolet-B (UV-B) radiation effects on field crops. Earlier reviews on field crop responses to UV-B considered few physiological processes or crops. For this review, we easily located about 129 studies on 35 crop species published since 1975. Here, we report the effects of UV-B radiation on visual symptoms, leaf ultrastructure and anatomy, photosynthetic pigments, UV-B absorbing compounds, photosynthesis, growth and development, yield, genotypic differences, and finally, interactions of UV-B with abiotic and biotic factors of crop plants. Experiments conducted in glasshouses, in closed and open top chambers, and under field conditions, with varying source (solar or artificial) and intensity of photosynthetically active radiation (PAR, 50–1800 μmol m⁻² s⁻¹) and UV-B (0–50 kJ m⁻² per day) are included. It is easy to conduct experiments that purport to evaluate the effects of projected UV-B intensities on crop species by providing supplemental irradiance with lamps or by reducing UV-B with filters; however, it is very difficult to simulate UV-B irradiance spectral changes that are likely to occur in nature. Collated results for each process are presented as percent change from control along with the experimental conditions in tabular format. Many of the studies showed dramatic effects of UV-B radiation, but under conditions with supplemental UV-B irradiance that was higher than would ever occur outside experimental conditions or in which the longer wavelengths in the PAR and UV-A, which moderate UV-B effects, were greatly reduced. Only 25 of the studies reviewed used experimental conditions and supplemental UV-B irradiance that approached realism. However, unrealistic the experimental conditions might be, an increase in understanding of basic plant physiology was gained from most of the studies.Visual symptoms consisting of chlorotic or necrotic patches on leaves exposed to UV-B were not unique. Both vegetative and reproductive morphology were altered by UV-B radiation. Leaf anatomy was altered due to changes in thickness of epidermal, palisade, and mesophyll layers. Enhanced UV-B generally decreased chlorophyll content (10–70%), whereas it increased UV-B absorbing compounds (10–300%) in many crops. Decrease in photosynthesis (3–90%), particularly at higher UV-B doses, was due to both direct (effect on photosystem) and indirect (decrease in pigments and leaf area) effects. The decreases in chlorophyll pigments and photosynthesis resulted in lower biomass and yield of most crop plants. Genotypes of crop species exhibited variability in leaf wax layer thickness, loss of chlorophyll, and increase in phenolics as mechanisms of tolerance to enhanced UV-B radiation resulting in changes in biomass/yield. Results from the few studies on interaction of UV-B with other abiotic and biotic factors did not lead to useful conclusions. Studies are needed to quantify the effects of UV-B radiation on crops in order to develop dose response functions that can facilitate development of dynamic simulation models for use in UV-B and other environmental impact assessments.     

Effects of grazing and rainfall variability on root and shoot decomposition in a semi-arid grassland

Overgrazing increasingly affects large areas of Inner Mongolian semi-arid grasslands. Consequences for ecosystem functions and, in particular, for the decomposition as a key process of ecosystem carbon (C) and nitrogen (N) cycling are still unclear.We studied the effects of grazing on shoot and root decomposition with the litter bag method in a long-term grazing exclosure (UG79), a moderate winter grazed (WG) and a long-term heavily grazed site (HG). We separated the effects of local environmental factors and litter quality as altered by grazing. Growing seasons of average and very low precipitation allowed us to study the effect of inter annual rainfall variability on decomposition.Grazing-induced differences in environmental factors of the three studied grassland sites had no effect on decay rates of shoot and root dry mass. Also differences in litter quality among the grazing sites were not reflected by root decomposition dynamics. The accelerated shoot decay at site HG could not clearly be linked to litter quality parameters. Shoot decay rates were more or less constant, even under very dry conditions. This indicates the possibility of photodegradation (solar UV-B radiation) to control aboveground decomposition in this semi-arid ecosystem. By selecting the best predictors of root decomposition from regression analysis, we found that soil water content was the best parameter explaining the dynamics.Net N immobilization was generally not detected during the decay process of shoot and root. It is likely, when root decomposition is strongly reduced in dry periods, shoot decomposition becomes relatively more important for nutrient cycling. A separate analysis of shoot and root decay dynamics is required in order to describe C and N cycling in this semi-arid grassland. The grazing impact on C and N fluxes through decomposition of plant material likely exhibits a strong interaction with seasonal rainfall pattern.     

利用3D模型模拟天空与叶面散射对玉米冠层截光率的影响

该文利用并行蒙特卡罗光线跟踪模型定量分析了天空散射与叶片散射对玉米冠层截光率的影响.模拟试验表明:相同辐射强度时,冠层截获的光强与受剑照射的叶片在天空散射时都比直接辐射时多;当太阳高度角小于60°时,冠层在阴大截获的总光强小于晴大,但至少60%的叶片在阴大截获的光强大于晴大时截获的光强;叶片散射对冠层光分布的影响与波长...     

Daily photosynthesis totals calculated from solar radiation histograms

Histograms of the number of hours in the day in which the photosynthetic photon flux density (PPFD) of solar radiation was within 5 preset bands, were constructed from readings made with a miniature battery-operated millivolt histogram recorder, connected to a photon sensor. The number of histograms showing a bimodal distribution was much fewer than was expected from a review of the literature, even in a site shaded by trees. This was attributed to smoothing of the data over a whole day, compared with published data which were mostly taken near noon. Bimodal distributions were found in a greenhouse under clear or partly cloudy conditions.The histograms were combined with published data for the instantaneous gross photosynthetic responses to PPFD of two bermudagrass swards, one of low leaf area index (LAI) and highly non-linear response, and the other of high LAI and more linear response. The daily photosynthesis totals of the swards were calculated by summing the products of the number of hours accumulated in each band by the photosynthetic rate in the center of the band.Because of the non-linearity of response, the photosynthesis total in the greenhouse was 75–82% of that calculated for an open site on a clear day, while the daily photon total was only 70% of the open site value. Also, for the same daily photon total, the daily photosynthesis total of each sward on the open site was greater on a day of light clouds (moderate PPFD) than on a day of sunshine alternating with heavy clouds (bimodal histogram of PPFDs). The difference between the two days was greater for the sward with the more non-linear response. These interactions between non-linearity of photosynthetic response and type of radiation histogram were in the expected direction, however, the differences were not very large (+ 10% for the sites and responses studied).The histogram recorder provides an inexpensive and convenient means of obtaining solar radiation data that can be used to calculate daily photosynthesis totals, either from experimental photosynthesis data or from canopy photosynthesis models. Because the response of photosynthesis is non-linear, linear integrals of solar irradiance do not provide sufficient information for such photosynthesis studies.     

油菜光合生产模拟模型

Photosynthetic production is a major determinant of final yield in crop plants. A simulation model was developed for canopy photosynthesis and dry matter accumulation in oilseed rape (Brassica napus L.) based on the ecophysiological processes and using a three-layer radiation balance scheme for calculating the radiation interception and absorption by the layers of flowers, pods, and leaves within the canopy. Gaussian integration method was used to calculate photosynthesis of the pod and leaf layers, and the daily total canopy photosynthesis was determined by the sum of photosynthesis from the two layers of green organs. The effects of physiological age, temperature, nitrogen, and water deficit on maximum photosynthetic rate were quantified. Maintenance and growth respiration were estimated to determine net photosynthetic production. Partition index of the shoot in relation to physiological development time was used to calculate shoot dry matter from plant biomass and shoot biomass loss because of freezing was quantified by temperature effectiveness. Testing of the model for dynamic dry matter accumulation through field experiments of different genotypes, sowing dates, and nitrogen levels showed good fit between the observed and simulated data, with an average root mean square error of 10.9% for shoot dry matter. Thus, the present model appears to be reliable for the prediction of photosynthetic production in oilseed rape.     

Total reflected solar radiation calculated from multi-band sensor data

Reflected solar radiation is a significant term in the net radiation equation and is the one most strongly affected by surface conditions. The evaluation of net radiation over a heterogeneous area requires a detailed knowledge of the areal distribution of the reflected solar radiation. Remote sensing offers a means to obtain this areal distribution, provided that the total reflected solar spectrum can be estimated from discrete band multispectral radiometric data. A radiative transfer model was used to calculate the irradiance at the earth's surface for a number of atmospheric scattering and adsorption conditions. Response functions of two multiband radiometers were used to obtain the partial spectrum/total spectrum (P/T) ratio for each radiometer at each atmospheric condition. It was found that the P/T ratio was essentially independent of atmospheric scattering and only mildly dependent on water vapor absorption. Reflectance spectral distributions for 14 different surface conditions (bare soil to full green canopy) were used along with the irradiance data to determine the P/T ratio for reflected solar radiation. Multispectral data, with the appropriate P/T ratio, were used to calculate the total incoming radiation and the total reflected radiation from a wheat canopy. The calculated data differed from wide band pyranometer data by about 5%. It was concluded that both total incoming and reflected solar radiation can be evaluated from multispectral radiometric data. This development is a step towards regional net radiation maps, and possibly regional evapotranspiration maps.     

Diffuse PAR irradiance under clear skies in complex alpine terrain

The diffuse component of photosynthetically active radiation (PAR) is important for models that calculate carbon dioxide uptake in photosynthesis by vegetation. Few PAR measurements are usually available, especially in complex terrain. Thus, extrapolation based on models becomes the primary means for deriving diffuse PAR in complex terrain. Most models treat the diffuse PAR sky distribution isotropically to avoid complex calculations. However, this leads to errors in diffuse PAR estimation due to the anisotropic nature of PAR sky radiance distribution. This paper describes a model for estimating diffuse PAR at any given point within complex terrain from measurements at a reference site, by taking into account both the anisotropic nature of sky radiance distribution and topographic effects. The model has been validated using two different sites, Kederbichl, and Bartholomä in the National Park Berchtesgaden, Germany, with Schönau as the reference site. The results revealed, that by replacing the isotropic treatment with PAR sky radiance distribution modified from the sky radiance distribution function in the model MURAC, the simulation accuracy is greatly improved. The R² of measured to modelled diffuse PAR reached 0.64 and 0.65 at Kederbichl and Bartholomä sites, respectively, compared to the isotropic treatment of 0.14 and 0.33. The normalized root mean square error (NRMSE) also improved from 0.47 to 0.23 and 0.40 to 0.26 for the Kederbichl and Bartholomä sites. An analysis of the ratio of diffuse radiation on slope to that on the horizontal (R_d) with solar zenith angle and the Linke turbidity factor indicated that R_d varies over time. Moreover, the variations of R_d are quite different at the above two sites, suggesting that R_d varies spatially as well. Overall, R_d in complex terrain is jointly controlled by diffuse sky radiance distribution and topography. Different combinations of topographic and atmospheric conditions will alter time dependent patterns in R_d. This casts doubt on the commonly held view that R_d can be assumed to be constant for radiation models.     

Modelling species competition in mixtures of perennial sow-thistle and spring barley based on shoot radiation use efficiency

Abstract

Perennial sow-thistle (Sonchus arvensis L.) may be a serious weed in organic and conventional farming. To assess the effects of radiation acquisition and resource allocation on competitive ability, S. arvensis was grown together with spring barley (Hordeum distichon L.) in six mixtures in a replacement series with initial above-ground biomass proportions of S. arvensis ranging from 2% to 96%. A one-season experiment was made outdoors in boxes in Uppsala, Sweden, at a low level of nitrogen supply (5 g N m^?2). The study tested the predictability of shoot biomass of each species based on two principal assumptions: (i) growth model parameters derived from species in monocultures could be applied in mixtures, and (ii) radiation in the mixed stand was partitioned between species proportional to their leaf area. Calibration of two parameters, for scaling of shoot radiation use efficiency and radiation partitioning respectively, were the base for the evaluation. When the coefficients were close to unity, which was the case for all mixtures dominated by barley, and for one of the mixtures with high proportion of S. arvensis in the early season, observed and predicted shoot biomass coincided well. For the evenly composed mixtures, total shoot biomass was underestimated (the scaling coefficient of shoot radiation use efficiency was>1), whereas the relative composition among species was predicted well. In the late season the principal model assumptions were not applicable to S. arvensis, likely due to increasing root allocation not accounted for in the model. Sonchus arvensis in mixtures with high proportions was planted early in relation to sowing of barley, which resulted in a comparably late development stage of S. arvensis. Consequently the relation between species development stages varied with species composition suggesting a need to introduce effects of differences in development stage into the model.     

Modelling below-ground shoot elongation and emergence time of Sonchus arvensis shoots

To assess emergence time of shoots from roots of the perennial weed Sonchus arvensis as a function of root weight and soil temperature, we performed an experiment to which linear models were fitted. Root parts of three distinct initial weight classes were grown in pots in the dark at constant temperatures of 4, 8 and 18°C, respectively. During five harvest occasions, prior to or at shoot emergence, below-ground shoot length was measured. Root planting depth (3, 10 and 17 cm) did not influence shoot elongation rate. The below-ground shoot elongation rate for a given initial root-weight class was estimated from the observations to be constant with time, but to increase with temperature and initial root weight. By expressing shoot length for a given day as a linear function of the number of days from planting date, and elongation rate as a linear function of temperature, we calculated (1) the accumulated temperature-sum requirement for emergence, (2) emergence time for variable temperature conditions in a clay soil using soil temperature recordings at 5-cm depth for seven seasons in central Sweden and (3) the emergence time at three elevated temperature levels and initial root-weight classes. The accumulated temperature-sum requirements for below-ground shoots of S. arvensis to reach soil surface are independent of temperature regime for roots of a given initial weight but lower for heavier than lighter roots. The temperature limit for below-ground shoot elongation to occur is about 2.0–2.5°C. Between-year variations in temperature under field conditions cause larger variation to emergence time than initial root-weight differences. An average temperature increase of 3°C would cause an earlier emergence time, in the same range (about 2 weeks) as the difference between the earliest and latest year in the current weather conditions.     

化学处理对竹笋壳润湿性的影响

竹笋壳疏水性好,润湿性差,影响自身及与其他材料的复合,该文旨在通过化学处理提高竹笋壳表面润湿性。分别采用不同浓度的HCl、Na OH溶液对竹笋壳进行不同时间的处理,测定处理前后竹笋壳内外表面的接触角,分析处理后内外表面的表面自由能,并用扫描电镜(scanning electron microscope,SEM)分析处理前后表面微观结构的变化。结果表明:HCl、Na OH对竹笋壳表面处理效果较好的分别为质量分数5%HCl处理6 h和质量分数2%Na OH处理6 h。水和酚醛胶作为接触试剂时,竹笋壳内外表面接触角的排列顺序均为:未处理组质量分数5%HCl处理6 h组质量分数5%Na OH处理6 h组。质量分数5%HCl处理6 h的竹笋壳内外表面的表面自由能分别提高了1%、77.68%,质量分数2%Na OH处理6 h的竹笋壳内外表面的表面自由能分别升高58.33%、79.17%。SEM表面微观结构分析表明质量分数5%HCl处理6 h的竹笋壳内表面可明显看到气孔,外表面微乳突数量增多;质量分数2%Na OH处理6 h的竹笋壳内表面纤维方向更加清晰,外表面微乳突数量增多。综上所述,质量分数2%Na OH处理6 h的竹笋壳内外表面润湿性改善效果最好,质量分数5%HCl处理6 h和质量分数2%Na OH处理6 h对竹笋壳外面表的润湿性改进的效果相近。     

Vertical variability of spectral ratios in a mature mixed forest stand

The quality and quantity of solar radiation are crucial for growth and competition within forest ecosystems. The spectral waveband between 400 and 700 nm is mainly responsible for photosynthesis and thus for plant growth. Spectral ratios such as red/far red (R/FR) and blue/red (B/R) give important information about the light quality within stands. Changes in R/FR and B/R trigger or inhibit effects such as seed germination, stem growth, dormancy, leaf expansion and flowering. Blue light strongly influences development and growth of plants e.g. an increase of B/R may lead to higher photosynthetic rates per unit leaf area. In addition to spectral properties, a higher fraction of diffuse radiation on overcast days can be used more effectively by plant organs. In this study, the spectral composition and variability of solar radiation were analyzed for different sky conditions and solar angles in six different vertical layers of a mixed European beech and Norway spruce stand in Southern Germany. The results showed lower R/FR ratios in beech than in spruce. Spruce showed markedly higher B/R values in comparison to beech under clear sky conditions. A steep increase of the B/R ratio at the height of the sun crown in both species was observed. Cloud cover had a major effect, elevating R/FR and reducing B/R values compared to clear sky days, due to a higher fraction of diffuse, unattenuated radiation within the canopy. The penetration of blue and red light into the canopy strongly depends on both the diffuse index and solar elevation angle.     

UV-B辐射对设施桃结果枝15N尿素吸收、利用及分配特性的影响

研究外源UV-B辐射对设施桃二年生结果枝对15N尿素吸收、利用和分配特性的影响。以6年生设施栽培春捷毛桃（Prunus persica cv.Chunjie）为试材,设对照和补充UV-B辐射的低、中、高剂量的4个处理,在果实发育不同时期饲喂15N标记的尿素,采样进行测定并计算。UV-B辐射可以提高各器官的Ndff,但是不同强度UV-B在不同物候期中提高效果不同。果实膨大期,除二年生枝是在低剂量处理下Ndff值最高之外,其他各器官都是在中、高剂量处理下Ndff值最高;硬核期,营养器官在中或者高剂量处理下Ndff较高,果肉和果核则是在低剂量处理下效果最明显;而果实着色期各器官均是在中剂量UV-B处理下Ndff值最高。UV-B提高结果枝对15N的利用率高。果实膨大期,高剂量处理提高效果最明显,利用率达到50.28%,与对照形成极显著性差异;硬核期和果实着色期15N的利用率则是在中剂量处理下达到最高,与对照相比分别比对照提高了83.99%和105.56%,形成极显著性差异。不同物候期15N在各器官的分配率受UV-B影响差异显著。果实膨大期和着色期,叶片中15N分配率在对照处理下达到最高,分别为13.44%和8.97%,UV-B处理促进15N向其他器官的分配;硬核期,中、高剂量UV-B处理促进15N在叶片中的分配,低剂量处理降低15N在叶片中的分配率,差异均不显著。UV-B增强条件下,不同物候期,各器官对15N的吸收、利用及分配特性受其对UV-B的敏感性调控。     

Canopy architecture,irradiance distribution on leaf surfaces and consequent photosynthetic efficiencies in heterogeneous plant canopies. Part II. Results and discussion

The influence of canopy architecture on the irradiance distribution on leaf surfaces in horizontally heterogeneous canopies was investigated using a procedural radiative transfer model. Four model canopies were abstracted as plant stands of 10 rows and 20 plants per row, and the procedural model was executed in conjunction with these digitized canopies. From the resulting distribution of the penetration function, the energy incident on leaf surfaces along the normal was calculated. The irradiance distribution was characterized by formally defining three kinds of irradiance — normal, diminished and null. Gross and net photosynthetic efficiencies due to irradiance in each energy class are presented. Canopy architecture was found to significantly influence the efficiency of energy use by affecting the interception pattern of solar radiation.     

The influence of the physical attributes of a spruce shoot on momentum transfer

The drag force on fourteen three-prong spruce shoots (Picea rubens) was measured over a range of wind tunnel velocities. The boundary layer resistance to momentum transfer was found to be influenced by the shoot orientation, inclination, and flexibility. Momentum transfer was also influenced by shoot needle density; however, this influence was a function of the shoot orientation. For wind velocities below about 2.0 m s^?1, these factors did not greatly influence the momentum transfer to the shoots. For wind velocities above about 2.0 m s^?1 the momentum transfer decreased with; (1) decreasing inclination of the shoot to the airflow, (2) decreasing shoot flexibility, and (3) orientating the shoot so that the densely-needled side of the shoot faced the wind. A pair of empirical equations were derived relating momentum transfer to wind velocity.