Leaf Spatial Distribution Research Articles

Wilted cucumber plants infected by Fusarium oxysporum f. sp. cucumerinum do not suffer from water shortage.

Fusarium wilt is primarily a soil-borne disease and results in yield loss and quality decline in cucumber (Cucumis sativus). The main symptom of fusarium wilt is the wilting of entire plant, which could be caused by a fungal toxin(s) or blockage of water transport. To investigate whether this wilt arises from water shortage, the physiological responses of hydroponically grown cucumber plants subjected to water stress using polyethylene glycol (PEG, 6000) were compared with those of plants infected with Fusarium oxysporum f. sp. cucumerinum (FOC). Parameters reflecting plant water status were measured 8d after the start of treatment. Leaf gas exchange parameters and temperature were measured with a LI-COR portable open photosynthesis system and by thermal imaging. Chlorophyll fluorescence and chloroplast structures were assessed by imaging pulse amplitude-modulated fluorometry and transmission electron microscopy, respectively. Cucumber water balance was altered after FOC infection, with decreased water absorption and hydraulic conductivity. However, the responses of cucumber leaves to FOC and PEG differed in leaf regions. Under water stress, measures of lipid peroxidation (malondialdehyde) and chlorophyll fluorescence indicated that the leaf edge was more seriously injured, with a higher leaf temperature and disrupted leaf water status compared with the centre. Here, abscisic acid (ABA) and proline were negatively correlated with water potential. In contrast, under FOC infection, membrane damage and a higher temperature were observed in the leaf centre while ABA and proline did not vary with water potential. Cytologically, FOC-infected cucumber leaves exhibited circular chloroplasts and swelled starch grains in the leaf centre, in which they again differed from PEG-stressed cucumber leaves. This study illustrates the non-causal relationship between fusarium wilt and water transport blockage. Although leaf wilt occurred in both water stress and FOC infection, the physiological responses were different, especially in leaf spatial distribution.

Foliage clumping index of main vegetation types in Daxing'an Mountains, Northeast China

The foliage clumping index quantifies the cluster degree of the leaf spatial distribution under random canopy. It is of comparable importance for establishment of ecological models. MODIS BRDF model parameter products (MCD43A1 data) and land cover types (MCD12Q1 data) were used in this study to simulate the reflectivity of the hot spots and dark spots, and calculate the normalized difference between hotspot and darkspot (NDHD) based on the Ross-Li semi-empirical model. Least square method was then used to simulate the relationship between NDHD and the foliage clumping index and foliage clumping index products of 500-m resolution in August 2014 were retrieved. Measurements of the foliage clumping index in Daxing'an Mountains were conducted by using the TRAC (Tracing Radiation and Architecture of Canopies) sampling instrument for mo-del validation and analysis. Results showed that it was a feasible algorithm to retrieve clumping index from MCD43A1 product with the correlation of simulated data and the measured data of significance (R2=0.8879). The MODIS near infrared wave band was more sensitive than that on red band to foliage clumping index change. With the increase of the solar zenith angle, the clumping index retrieved by Ross-Li model had a linear increase (R2=0.9699), which indicated that the foliage clumping index related to the solar zenith angle.

Revisiting the vegetation hot spot modeling: Case of Poisson/Binomial leaf distributions

The accuracy of spaceborne/airborne sensor measurements in the solar domain keeps increasing over time. High resolution, multi-directional and hyperspectral image acquisitions start to be abundant. With regard to the multi-angular remote sensing data, the hot spot, i.e. the exact backscattering direction of direct sunlight together with its neighboring directions, is of special interest. Accurate hot spot models have to be used to adequately simulate the hot spot signature and to allow reliable inversion of multi-angular data. In this paper, we propose a physical hot spot model (Leaf Spatial Distribution based Model, LSDM) assuming that for a given point inside the vegetation to be sunlit (respectively, observed) it should be located within a cylinder free from leaf centers. The cylinder is oriented to the sun (respectively, sensor) direction. Assuming a leaf random, regular or clumped spatial distributions, the gap probabilities in the sun and sensor directions are expressed as a function of these cylinder volumes. Based on the same hypothesis, the bidirectional gap probability is estimated as a function of the total volume of the two cylinders. The evaluation of the needed common volume of two cylinders having different radii is reduced to calculation of some elliptic integrals. Finally, the hot spot signature is estimated based on the bidirectional gap probability distribution. Different model versions with different leaf spatial distribution functions are compared. Particularly, it is shown that compared to the random distribution, the regular (the clumped, respectively) distribution increases (decreases, respectively) the reflectance due to single scattering contribution from foliage. The proposed model is validated using the ROMC web-based tool and its better performance compared to the Semi-Discrete Model and Kuusk's model is confirmed.

Estimation of vegetation clumping index using MODIS BRDF data

The foliage clumping index quantifies the degree of the deviation of leaf spatial distribution in the canopy from the random case. It is of comparable importance for ecological models as the leaf area index for quantifying radiation interception and distribution in plant canopies. Previously, an improved angular index named normalized difference between hotspot and darkspot was proposed for retrieving the clumping index using multi-angle remote sensing data. Global maps of clumping index have been derived successfully from multi-angular Polarization and Directionality of Earth Reflectance (POLDER) data at ∼6 km resolution. In this article, we investigate whether it is feasible to derive the clumping index at 500 m resolution with the 16-day Moderate Resolution Imaging Spectroradiometer (MODIS) bidirectional reflectance distribution function model parameters product. The results are compared with an assembled set of field measurements from 63 different sites, covering five continents and diverse biomes.

Contributions of foliage distribution and leaf functions to light interception, transpiration and photosynthetic capacities in two apple cultivars at branch and tree scales

Both the spatial distribution of leaves and leaf functions affect the light interception, transpiration and photosynthetic capacities of trees, but their relative contributions have rarely been investigated. We assessed these contributions at the branch and tree scales in two apple cultivars (Malus x domestica Borkh. 'Fuji' and 'Braeburn') with contrasting architectures, by estimating their branch and tree capacities and comparing them with outputs from a radiation absorption, transpiration and photosynthesis (RATP) functional-structural plant model (FSPM). The structures of three 8-year-old trees of each cultivar were digitized to obtain 3-D representations of foliage geometry. Within-tree foliage distribution was compared with shoot demography, number of leaves per shoot and mean individual leaf area. We estimated branch and tree light interception from silhouette to total leaf area ratios (STAR), transpiration from sap flux measurements and net photosynthetic rates by the branch bag method. Based on a set of parameters we previously established for both cultivars, the outputs of the RATP model were tested against STAR values, sap fluxes and photosynthetic measurements. The RATP model was then used to virtually switch foliage distribution or leaf functions (stomatal and photosynthetic properties), or both, between cultivars and to evaluate the effects on branch and tree light interception, transpiration and photosynthetic capacities in each cultivar. 'Fuji' trees had a higher proportion of leaf area borne on long shoots, fewer leaves per unit shoot length and a larger individual leaf area than 'Braeburn' trees. This resulted in a lower leaf area density and, consequently, a higher STAR in 'Fuji' than in 'Braeburn' at both branch and tree scales. Transpiration and photosynthetic rates were significantly higher in 'Fuji' than in 'Braeburn'. Branch heterogeneity was greater in 'Braeburn' than in 'Fuji'. An analysis of the virtual switches of foliage distribution or leaf function showed that differences in leaf spatial distribution and functions had additive effects that accounted for the lower transpiration and photosynthetic rates of branches and trees of 'Braeburn' compared with 'Fuji'. Leaf distribution had a more important role at the branch scale than at the tree scale, but the leaf function effect exceeded the leaf distribution effect at both scales. Our study demonstrated the potential of FSPM to disentangle physiological differences between cultivars through in silico scenarios.

A comparison of direct and indirect methods for estimating forest canopy leaf area

Two indirect gap fraction methods for estimating leaf area index (LAI) are compared with estimates from litterfall collections in a mixed-age oak-hickory forest. One indirect method uses averaged, direct beam penetration data obtained with a moving tram. The second uses a portable light sensor system that measures diffuse light penetration for five sky sectors between zenith angles 0 and 75°. Data were collected from September 1989 to January 1990. The Poisson model and the negative binomial model of gap frequency were applied to estimate LAI from observed transmittances. With the Poisson model, an assumption of a random leaf spatial distribution contributes to an underestimation of LAI by as much as 45%; this is because leaves at this site are actually clumped at both large and small scales. The negative binomial, which requires determination of a clumping parameter, produces estimates comparable with those of the litterfall method. Both indirect techniques accurately describe temporal changes in leaf area using either the Poisson or negative binomial model. The portable system also allows easy estimation of the spatial variation in leaf area within the site or between sites, and it can be used to obtain a vertical profile of leaf area.