Leaf Internal Structure Research Articles

Analyzing the spectral variability of tropical tree species using hyperspectral feature selection and leaf optical modeling

Hyperspectral remote sensing can provide information about species richness over large areas and may be useful for species discrimination in tropical environments. Here, we analyze the main sources of variability in leaf spectral signatures of tropical trees and examine the potential of spectroscopic reflectance measurements (450 to 2450 nm) for tree species discrimination. We assess within- and among-species spectral variability and perform a feature selection procedure to identify wavebands in which the species most differ from each other. We assess the discriminative power of these wavebands by calculating a separability index and then classifying the species. Finally, leaf chemical and structural parameters of each species are retrieved by inversion of the leaf optical model PROSPECT-5. Among-species spectral variability is almost five times greater than within-species spectral variability. The feature selection procedure reveals that wavebands, where species most differ, are located at the visible, red edge, and shortwave infrared regions. Classification of the species using these wavebands reaches 96% overall accuracy. Leaf chemical and structural properties retrieve by model inversion show that differences in leaf pigment concentrations and leaf internal structure are the most important parameters controlling the spectral variability of species. These parameters also contribute to the variation in red edge position among species.

Environmental and genetic variation in leaf anatomy among populations ofAndropogon gerardii(Poaceae) along a precipitation gradient

Phenotypes of two Andropogon gerardii subspecies, big bluestem and sand bluestem, vary throughout the prairie ecosystem of North America. This study sought to determine the role of genetics and environment in driving adaptive variation of leaf structure in big bluestem and sand bluestem. • Four populations of big bluestem and one population of sand bluestem were planted in common gardens at four sites across a precipitation gradient from western Kansas to southern Illinois. Internal leaf structure and trichome density of A. gerardii were examined by light microscopy to separate genetic and environmentally controlled traits. Leaf thickness, midrib thickness, bulliform cells, interveinal distance, vein size, and trichome density were quantified. • At all planting sites, sand bluestem and the xeric population of A. gerardii had thicker leaves and fewer bulliform cells compared with mesic populations. Environment and genetic source population were both influential for leaf anatomy. Leaves from plants grown in mesic sites (Carbondale, Illinois and Manhattan, Kansas) had thicker midribs, larger veins, fewer trichomes, and a greater proportion of bulliform cells compared to plants grown in drier sites (Colby and Hays, Kansas). • Water availability has driven adaptive variation in leaf structure in populations of A. gerardii, particularly between sand bluestem and big bluestem. Genetically based differences in leaves of A. gerardii indicate adaptive variation and evolutionary forces differentiating sand bluestem from big bluestem. Environmental responses of A. gerardii leaves suggest an ability to adjust to drought, even in populations adapted to mesic home environments.

Seasonal and diurnal leaf orientation, bifacial sunlight incidence, and leaf structure in the sand dune herb Hydrocotyle bonariensis

A conceptual model has been proposed whereby leaf orientation and resulting sunlight exposure dictate functional leaf structure. Specifically, the model states that this relationship is driven by the absolute amount and ratio of incident sunlight on adaxial and abaxial leaf surfaces. To test this model, the relationships between corresponding values of leaf orientation and incident sunlight on both leaf surfaces were measured for the sand dune herb Hydrocotyle bonariensis over a growth season, along with examination of leaf structure. For mature leaves, leaf angle from horizontal and azimuth angle significantly increased over the growing season, indicating diurnal midday avoidance and seasonal maximization of incident sunlight. Consequently, seasonal changes in leaf orientation resulted in an overall decrease in midday sunlight incidence on the adaxial surface and a slight shift in the daily occurrence of peak abaxial incidence. Adaxial surfaces received three to four times more sunlight than abaxial surfaces, and leaf cross-sections revealed relatively thick (564 μm) leaves with multiple adaxial palisade layers and stomata on both leaf surfaces, as predicted by the conceptual model and measured ratio of incident sunlight on adaxial and abaxial leaf surfaces. These data provide further evidence of the relationship between leaf orientation and resulting absolute levels of sunlight incidence on both leaf surfaces, as well as their ratio, and corresponding differences in internal and external leaf structure.

Differences in leaf traits, leaf internal structure, and spectral reflectance between two communities of lianas and trees: Implications for remote sensing in tropical environments

Increasing dominance of lianas in many tropical forests is considered a fingerprint of global environmental change. Despite the key role they play in ecosystem functioning, lianas remain one of the least studied life forms in tropical environments. This paper contrasts leaf traits and spectral properties (400–1100 nm) of liana and tree communities from a tropical dry forest and a tropical rainforest in Panama, Central America. Differences between lianas and tree leaf traits were analyzed using spectroscopy, leaf histology and pigment extractions. Results from this study indicate that many of the biochemical, structural, and optical properties of lianas and trees are different in the dry forest site but not in rainforest sites. In the dry forest site, liana leaves exhibited significantly lower chlorophyll and carotenoid contents and were thinner than the leaves of their host trees. Specific leaf area, dry to fresh mass ratio, and mean water content of liana leaves were significantly higher when compared with tree leaves. The differences observed in the tropical dry forest site indicate that lianas may have a higher rate of resource acquisition and usage, whereas trees tend to conserve acquired resources. We suggest that our results may be indicative of the presence of a liana syndrome related to water availability and thus best exhibited in tropical dry forests. Our findings have important implications for using remote sensing to accurately map the distribution of liana communities at regional scales and for the continued expansion of lianas in tropical environments as a result of global change.

Leaf xylem embolism, detected acoustically and by cryo‐SEM, corresponds to decreases in leaf hydraulic conductance in four evergreen species

Hydraulic conductance of leaves (K(leaf)) typically decreases with increasing water stress. However, the extent to which the decrease in K(leaf) is due to xylem cavitation, conduit deformation or changes in the extra-xylary pathway is unclear. We measured K(leaf) concurrently with ultrasonic acoustic emission (UAE) in dehydrating leaves of two vessel-bearing and two tracheid-bearing species to determine whether declining K(leaf) was associated with an accumulation of cavitation events. In addition, images of leaf internal structure were captured using cryo-scanning electron microscopy, which allowed detection of empty versus full and also deformed conduits. Overall, K(leaf) decreased as leaf water potentials (Psi(L)) became more negative. Values of K(leaf) corresponding to bulk leaf turgor loss points ranged from 13 to 45% of their maximum. Additionally, Psi(L) corresponding to a 50% loss in conductivity and 50% accumulated UAE ranged from -1.5 to -2.4 MPa and from -1.1 to -2.8 MPa, respectively, across species. Decreases in K(leaf) were closely associated with accumulated UAE and the percentage of empty conduits. The mean amplitude of UAEs was tightly correlated with mean conduit diameter (R(2) = 0.94, P = 0.018). These results suggest that water stress-induced decreases in K(leaf) in these species are directly related to xylem embolism.

Predicting vegetation water content in wheat using normalized difference water indices derived from ground measurements

Vegetation water content (VWC) is an important variable for both agriculture and forest fire management. Remote sensing technology offers an instantaneous and non-destructive method for VWC assessment provided we can relate in situ measurements of VWC to spectral reflectance in a reliable way. In this paper, based on radiative transfer models, three new normalized difference water indices (NDWI) are proposed for VWC [fuel moisture content (FMC), and equivalent water thickness (EWT)] estimation, taking both leaf internal structure and dry matter content into account. Reflectance at 1,200, 1,450 and 1,940 nm were selected and normalized with reflectance at 860 nm to establish three water indices, NDWI(1200), NDWI(1450) and NDWI(1940). Good correlations were observed between FMC (R (2) = 0.65-0.80) and EWT (both at the leaf scale, R (2) = 0.75-0.81 for EWT(L) and at the canopy scale, R (2) = 0.80-0.83 for EWT(C)) at various stages of wheat crop development.

A rapid, whole‐tissue determination of the functional fraction of PSII after photoinhibition of leaves based on flash‐induced P700 redox kinetics

Assaying the number of functional PSII complexes by the oxygen yield from leaf tissue per saturating, single-turnover flash, assuming that each functional PSII evolves one oxygen molecule after four flashes, is one of the most direct methods but time-consuming. The ratio of variable to maximum Chl fluorescence yield (F(v)/F(m)) in leaves can be correlated with the oxygen yield per flash during a progressive loss of PSII activity associated with high-light stress and is rapid and non-intrusive, but suffers from being representative of chloroplasts near the measured leaf surface; consequently, the exact correlation depends on the internal leaf structure and on which leaf surface is being measured. Our results show that the average F(v)/F(m) of the adaxial and abaxial surfaces has a reasonable linear correlation with the oxygen yield per flash after varied extents of photoinactivation of PSII. However, we obtained an even better linear correlation between (1) the integrated, transient electron flow (Sigma) to P700+, the dimeric Chl cation in PSI, after superimposing a single-turnover flash on steady background far-red light and (2) the relative oxygen yield per flash. Leaves of C3 and C4 plants, woody and herbaceous species, wild-type and a Chl-b-less mutant, and monocot and dicot plants gave a single straight line, which seems to be a universal relation for predicting the relative oxygen yield per flash from Sigma. Measurement of Sigma is non-intrusive, representative of the whole leaf tissue, rapid and applicable to attached leaves; it may even be applicable in the field.

Spectral reflectance and leaf internal structure changes of barley plants due to phytoextraction of zinc and cadmium

The focus of our research is to seek spectral signatures that indicate the impact and content of heavy metals in the leaves and canopies of living plants during the process of phytoremediation. Potted plants of barley (Hordeum vulgare) were grown for 5–6 weeks before being subjected to metal treatments of Zn and Cd. Diffuse reflectance spectra (350–2500 nm) of the plant canopies were collected daily using a portable spectroradiometer throughout the treatment period. Foliar structural changes of Zn‐treated plants included a decrease in intercellular space, palisade and epidermal cell size while Cd‐treated plants displayed fewer structural changes in leaf. Spectral analysis revealed that the band ratios at 1110 nm to that at 810 nm might be used as an indicator of the accumulation of certain metals in plant shoots. Normalized Difference Vegetation Index (NDVI) and leaf‐water‐content indices examined as part of our spectral analysis were not able to distinguish plants treated with different metals. Our ratio index R1110/R810, on the other hand, correlates closely with the magnitude of leaf structural changes. This study suggests that the infrared reflectance spectrum (800–1300 nm) of plant canopy might provide a non‐intrusive monitoring method for the physiological status of plants grown on heavy metal contaminated soil.

Monitoring the effects of arsenic and chromium accumulation in Chinese brake fern (Pteris vittata)

The objectives of this study were (i) to investigate the feasibility of using spectral reflectance for monitoring As and Cr accumulation in Chinese brake fern (Pteris vitatta), and (ii) to search for spectral indices sensitive to structural changes caused by metal accumulation during the process of phytoremediation. Potted Chinese brake fern plants were exposed to As (100 and 300 ppm) and Cr (300 and 600 ppm) treatments for 22 days. The plants were then harvested and analysed for metal accumulation. Diffuse reflectance spectra (350–2500 nm) of the plant canopies were collected regularly throughout the metal treatment period using a portable spectroradiometer. Leaf reflectance is governed by leaf surface properties, internal structure, and foliar pigments and biochemical components. Leaf samples were collected and analysed for structural changes through microscopic observations. Our microscopic studies on changes of leaf structure provide insight into the physical changes that are remotely detected as changes in reflectance, and may permit extrapolation of these results to other plant species. Cr accumulation resulted in a decrease in biomass, relative water content (RWC), and changes in the internal structure of the leaf. The structural and spectral results show significant changes in Cr‐treated plants while the changes were minimal in As‐treated plants compared to untreated plants. Our spectral analysis revealed that a unique ratio index R 1110/R 810 can be used to monitor structural changes in plants due to accumulation of Cr. This index distinguished Cr‐treated plants from untreated and As‐treated plants. The Normalized Difference Vegetative Index (NDVI) distinguished stressed plants, but NDVI cannot distinguish Cr‐stressed plants from As‐stressed plants. Our results show that brake fern can accumulate significant amounts of Cr in shoots (2108 mg kg−1 dry weight), but it is not a hyperaccumulator for Cr because much higher Cr accumulation was found in roots (7686 mg kg−1 dry weight). This study suggests that the infrared reflectance spectrum (800–1300 nm) of plant canopy may provide a non‐intrusive monitoring method to access the physiological status of plants grown in heavy metal‐contaminated soil.

Novel tissue preparation method and cell-specific marker for laser microdissection of Arabidopsis mature leaf

Laser microdissection (LMD) is a powerful tool to isolate pure cell populations from heterogeneous tissues. This system has been successfully used for animal research; however, the reports of its application to plant tissues remain limited. One of the challenges of LMD for plant material is the tissue preparation. Although cryosectioning is commonly used for animal tissues, this is not a desirable method for fragile plant material with large central vacuoles. While paraffin preparation provides high histological quality and stability, the procedure is highly time consuming and may result in degradation of molecules of interest. In addition, conventional fixation and paraffin preparation methods do not preserve the structural integrity of very delicate plant tissues such as mature Arabidopsis thaliana leaves. Here, we used the rapid microwave paraffin preparation method with no fixative for preparation of Arabidopsis leaf tissue for LMD. This method resulted in Arabidopsis leaf sections with excellent preservation of leaf internal structure as evidenced by well-defined vascular bundles, phloem, and chloroplasts, and expanded and rounded epidermal cells. RNA extracted from leaf epidermal and mesophyll cells was of sufficient yield and specificity to use in downstream applications such as microarray analysis of the amplified mRNA. We employed the mesophyll cell-specific molecular marker, chloroplastic carbonic anhydrase, and developed an epidermal cell-specific marker, the very-long-chain fatty acid-condensing enzyme, CUT1, to assess specificity of harvested Arabidopsis leaf cell types by reverse transcription polymerase chain reaction. The described method is also likely to be superior for the preparation of other fragile botanical tissue for LMD and downstream applications.

Estimating winter wheat plant water content using red edge parameters

Remote sensing of plant water content is difficult because the absorption band sensitive to foliar liquid water is also sensitive to the atmospheric vapour. A method using non-water-absorption spectral parameters to evaluate plant water content (PWC) would be valuable. In our experiment, canopy spectra of 48 winter wheat treatments with different varieties, different fertilization and irrigation levels were measured by an ASD FieldSpec FR spectrometer in six different growth stages from erecting stage to milking stage, and the PWCs of the related wheat plant samples were also measured. Significant positive coefficients of correlation were observed between PWC and spectral reflectance in 740–930 nm region in all of the six different growth stages, which indicates that the NIR spectral reflectance increases due to the effect of PWC on the leaf internal structure. This mechanism also affects the red edge spectrum in 680–740 nm region. The spectral reflectance increases more rapidly and the red edge becomes steeper if PWC is higher. The coefficients of correlation between PWC and red edge width, derived from the inverted-Gaussian model, are significant at the 0·999 confidence-level, which is more reliable than WI and NDWI, and the statistical models for PWC based on red edge width were set up in all the six different growth stages. In addition, LAI and canopy chlorophyll density (CCD) are also related to red edge parameter, such as red edge position and red edge width. It seems that PWC plays a more important role in red edge width than LAI and CCD due to the effect of PWC on the leaf internal structure, and that CCD plays a more important role in red edge position than LAI and PWC.

Effects of elevated atmospheric CO 2 and temperature on leaf optical properties in Acersaccharum

Elevated partial pressures of atmospheric carbon dioxide, similar to numerous causes of plant stress, may alter leaf pigmentation and structure and thus would be expected to alter leaf optical properties. Hypotheses that elevated CO 2 pressure and air temperature would alter leaf optical properties were tested for sugar maple ( Acer saccharum) in the middle of its fourth growing season under treatment. The saplings had been growing since 1994 in open-top chambers and partial shade at Oak Ridge, Tennessee under the following treatments: (1) ambient CO 2 pressure and air temperature (control); (2) CO 2 pressure approximately 30 Pa above ambient; (3) air temperatures 3°C above ambient; and (4) elevated CO 2 and air temperature. Under elevated CO 2 or temperature, spectral reflectance, transmittance and absorptance in the visible spectrum (400–720 nm) tended to change in patterns that generally are associated with chlorosis, with maximum differences from the control near 700 nm. However, these changes were not significant at P=0.05. Although reflectance, transmittance and absorptance at 700 nm correlated strongly with leaf chlorophyll concentration, variability in chlorophyll concentration was greater within than among treatments. The lack of treatment effects on pigmentation explained the non-significant change in optical properties in the visible spectrum. Optical properties in the near-infrared (721–850 nm) were similarly unresponsive to treatment with the exception of an increased absorptance throughout the 739–850 nm range in leaves that developed under elevated air temperature alone. This response might have resulted from effects of air temperature on leaf internal structure.

Ontogenetic differences in mesophyll structure and chlorophyll distribution in Eucalyptus globulus ssp. globulus (Myrtaceae)

Mesophyll structure has been associated with the photosynthetic performance of leaves via the regulation of internal light and CO(2) profiles. Differences in mesophyll structure and chlorophyll distribution within three ontogenetically different leaf types of Eucalyptus globulus ssp. globulus were investigated. Juvenile leaves are blue-grey in color, dorsiventral (adaxial palisade layer only), hypostomatous, and approximately horizontal in orientation. In contrast, adult leaves are dark green in color, isobilateral (adaxial and abaxial palisade), amphistomatous, and nearly vertical in orientation. The transitional leaf type has structural features that appear intermediate between the juvenile and adult leaves. The ratio of mesophyll cell surface area per unit leaf surface area (A(mes)/A) of juvenile leaves was maximum at the base of a single, adaxial palisade layer and declined through the spongy mesophyll. Chlorophyll a + b content showed a coincident pattern, while the chlorophyll a:b ratio declined linearly from the adaxial to abaxial epidermis. In comparison, the mesophyll of adult leaves had a bimodal distribution of A(mes)/A, with maxima occurring beneath both the adaxial and abaxial surfaces within the first layer of multiple palisade layers. The distribution of chlorophyll a + b content had a similar pattern, although the maximum ratio of chlorophyll a:b occurred immediately beneath the adaxial and abaxial epidermis. The matching distributions of A(mes)/A and chlorophyll provide further evidence that mesophyll structure may act to influence photosynthetic performance. These changes in internal leaf structure at different life stages of E. globulus may be an adaptation for increased xeromorphy under increasing light exposure experienced from the seedling to adult tree, similar to the characteristics reported for different species according to sunlight exposure and water availability within their native habitats.

Spectral reflectance of dehydrating leaves: Measurements and modelling

Remotely sensed measurements at optical wavelengths may provide information on crop water status and increase the accuracy of crop production forecasts. Previous research has shown that canopy spectral response to water stress is attributable to change in leaf water content, canopy structure and soil moisture. This experiment was designed to study leaf spectral response resulting from changes in leaf water content and to evaluate the use of a radiative transfer model for predicting the spectral behaviour of the leaf. The difference between measured and modelled reflectance increased as leaf water content decreased and it was hypothesized that this may be due to a change in leaf internal structure that was unaccounted for by the model.

Remote sensing of crop parameters with a polarized, frequency-doubled Nd:YAG laser

Polarized laser remote-sensing measurements that correlate the yield, the normalized difference vegetation index, and the leaf area index with the depolarized backscattered radiation from corn plots grown with eight different nitrogen fertilization dosages are presented. A polarized Nd:YAG laser emitting at 1064 and 532 nm is used. Depolarization increased significantly with increasing fertilization at the infrared wavelength, and there was a decrease in the depolarization at the green wavelength. The depolarization spectral difference index, defined as the absolute difference in the depolarization at the two wavelengths, is introduced as a parameter that is an indicator of the condition of the internal leaf structure.

A contribution to the anatomy and surface details of leaves in the genus Bulbine (Asphodelaceae) in Africa

Investigations of the structural details of leaf surfaces (of all species) and the internal leaf structure (of seven species) of African Bulbine Wolf revealed several interesting characters. Structures of diagnostic importance include striae, papillae, stomata, chlorenchyma and vascular tissues. The information accumulated is of particular value in demarcating species, but no correlation between these characters and infra-generic groupings was found.

PRIMARY AND SECONDARY EFFECTS OF WATER CONTENT ON THE SPECTRAL REFLECTANCE OF LEAVES

Leaves of six species selected to represent a broad range in internal structure were collected in the field and studied in the laboratory to determine primary and secondary effects of water content on leaf spectral reflectance. Primary effects were those that resulted solely from the radiative properties of water. Secondary effects were those that could not be explained solely by these properties. Decreased leaf water content generally increased reflectance throughout the 400‐2,500‐nm wavelength range. For the aquatics Eichhornia crassippes and Nuphar luteum, the broadleaved trees Liquidambar styraciflua and Magnolia grandiflora, the cane‐grass Arundinaria tecta, and the needle‐leaved Pinus taeda, the sensitivity of reflectance to water content was greatest in the water absorption bands near 1,450, 1,940, and 2,500 nm. Sensitivity maxima occurred also between 400 and 720 nm, indicating secondary effects that resulted from decreased absorption by pigments. Secondary effects of water content on reflectance that were largely wavelength‐independent, together with any wavelength‐independent effects of leaf internal structure, were far less significant than primary and secondary effects resulting from decreased absorption by water and pigments, respectively.

Stereology of the Internal Structures of Leaves in Peperomia obtusifolia, P. camptotricha, and P. scandens

The internal structure of leaves from Peperomia obtusifolia, P. camptotricha, and P. scandens were analyzed with stereology to detect relationships between the internal leaf structure of each species with its diurnal cycles of gas exchange and organic acid fluctuation. In these succulent leaves, the relative volume of leaf occupied by each of three tissues (multiple epidermis and median and spongy mesophyll) varied according to the species and leaf development. In P. obtusifolia, which exhibits light-period gas exchange and little acid fluctuation, 75% of the leaf volume was multiple epidermis. In P. scandens, which exhibits dark-period gas exchange and large acid fluctuation, 77% of the leaf volume was spongy mesophyll. In P. camptotricha, which exhibits increasing dark-period gas exchange and acid fluctuation during leaf development, the multiple epidermis and spongy mesophyll were each about 48% of the leaf volume. In each species the median mesophyll was about 4% of the leaf volume. The area of cell wall exposed to the internal air space of the leaf was dominated by the spongy mesophyll in each species; the area was least in P. obtusifolia, greater in thicker leaves of P. camptotricha, and greatest in P. scandens. The relative volumes of cellular components in P. camptotricha were determined for leaves of different thickness. A high density of chloroplasts in the median mesophyll gave a low ratio of vacuole to cytoplasm, which was similar to the ratio in bundle sheath cells of many plants with C4 metabolism. The vacuole predominated in cells of the multiple epidermis and spongy mesophyll, and the cells were much larger than those in the median mesophyll. However, only the spongy mesophyll contained chloroplasts with the stroma and thylakoid volume more typical of chlorenchyma in plants with Crassulacean acid metabolism. Tissue dimorphism in cell size, chloroplast content, vacuole volume, and cell wall exposed to air space were the structural features most easily associated with the known physiological data of gas exchange and organic acid fluctuation patterns in these species of Peperomia.

Polarized and non-polarized leaf reflectances of Coleus blumei

It is proposed that, through measurement of polarized, reflected radiation, leaf reflectance can be separated into two components: a polarized component from the first surface of the leaf, and a non-polarized, diffuse component primarily from the internal leaf structure. The reflectance of three variegated portions of Coleus blumei, Benth. was measured in five wavelength bands of the visible and near-infrared spectrum with a polarization photometer. The polarized and non-polarized components of the reflectance factor and the degree of polarization were calculated. The polarized component was independent of wavelength, demonstrating that the polarized reflectance arose from the surface of the leaf. Differences in the polarized component were presumed to result from variations in surface features. The values of the non-polarized component depended on leaf pigmentation and on wavelength, demonstrating that it emanated from the internal leaf structure. The degree of polarization varied with wavelength and was greatest in regions of the spectrum where absorption occurs.

Enhanced crop discrimination using the mid-IR (1.55–1.75μm)

Improvements in crop discrimination can be realized by using mid-IR bands (1.55–1.75 μm and 2.08–2.35 μm) which are sensitive to canopy moisture content. Analyses of data from two growing seasons in Webster County, Iowa clearly indicate that corn and soybeans are highly separable in the mid-IR from early season through harvest. This contrasts sharply with visible and near-IR bands where corn and soybeans are confused throughout much of the growing season. The mid-IR temporal reflectance behavior appears to result from differences between C 4 monocot and C 3 dicot internal leaf structure. If this hypothesis holds, mid-IR observations should improve discrimination in other instances where similar differences in internal leaf structure are present.