Physiological interpretation of a hyperspectral time series in a citrus orchard

Hyperspectral remote sensing for monitoring horticultural production systems requires the understanding of how plant physiology, canopy structure, management and solar elevation affect the retrieved canopy reflectance during different stages of the phenological cycle. Hence, the objective of this study was to set up and to interpret a hyperspectral time series for a mature and healthy citrus orchard in the Western Cape province of South Africa considering these effects. Based on the remotely sensed data, biophysical parameters at the canopy level were derived and related to known observed physiological and phenological changes at the leaf level and to orchard management. Fractions of mature fruit, flowers, and sunburnt leaves were considered, and indices related to canopy structure chlorophyll content and canopy water status were calculated.Results revealed small cover fractions of mature fruit, flowers and sunburnt leaves of respectively 2.1%, 3.1% and 7.0%, but the high spectral contrast between flowers and leaves allowed a successful classification of flowering intensity. Furthermore, it was shown that canopy level time series of vegetation indices were sensitive to changes in solar elevation and soil reflectance which could be reduced by applying an empirical soil line correction for the most affected indices. Most trends in vegetation indices at the canopy level could be explained by a combination of changes at the leaf level (chlorophyll, carotenoids, dry matter), changes in canopy structure (leaf area index and leaf angle distribution) and changes in cover fractions of vegetative flushes, flowers and sunburnt leaves. The transformed chlorophyll absorption ratio index over the optimised soil adjusted vegetation index (MCARI/OSAVI) was best related to leaf level trends in chlorophyll content. Seasonal changes in the photochemical reflectance index (PRI) were linked to inverse changes in the carotenoid-to-chlorophyll ratio. Canopy structure indices (the modified triangular vegetation index or MTVI2 and the standardized leaf area index determining index or sLAIDI) were sensitive to changes in leaf area index, average leaf angle as well to management interactions (pruning and harvest). Canopy water status was highly impacted during the spring flush due to expanding leaves that concealed trends in the underlying mature leaves. Seasonal trends in soil and weeds reflectance were related to changes in volumetric soil water content and to the earlier and reduced growth period of non-irrigated weeds.     

Seasonal variation in canopy reflectance and its application to determine the water status and water use by citrus trees in the Western Cape, South Africa

This study describes the diurnal and seasonal dynamics of the canopy reflectance, water use and water status of Midknight Valencia citrus trees under semi-arid conditions. Hyperspectral canopy reflectance data was collected on 30 trees at monthly intervals over a period of 16 months in a commercial orchard in South Africa. The mean canopy reflectance in the wavelength range 350-2500 nm followed a clear seasonal trend influenced by environmental conditions and tree phenology. Mean monthly reflectance peaked in summer (∼22%) while the lowest value (∼15%) was reached in winter with the seasonal changes in the sun's position accounting for a significant proportion of the variations. A sensitivity analysis of a Penman-Monteith transpiration model showed that water use by individual trees changed by up to 13% when the canopy reflectance was varied over the seasonal range of measured values. This suggested that the seasonal changes in tree water use influenced the seasonal trend of the canopy reflectance. Thus monitoring the canopy reflectance of citrus trees could offer information on the tree water status. To test this, sap flow data of water uptake and loss by the trees were compared with the canopy spectra. Sap flow data showed a heavy reliance by the citrus trees on the internally stored water with up to 25% of the daily total transpiration withdrawn from the trees’ internal water storage pools when soil water was limited. This depletion of internally stored water, and hence the change in tree water status, was detected using spectral indices based on the first order derivatives of the canopy reflectance centered at two and, at most, four spectral bands. We conclude that even if citrus trees are evergreen, their canopy reflectance changes significantly throughout the year with a considerable impact on tree energy balance and water use. In addition, the contribution of the internally stored water to daily transpiration is a possible indicator of drought stress for citrus trees detectable from changes in canopy reflectance and it has potential applications in irrigation scheduling using canopy level spectral information.     

Chemical and sensory quantification of geosmin and 2-methylisoborneol in rainbow trout (Oncorhynchus mykiss) from recirculated aquacultures in relation to concentrations in basin water

Globally, aquaculture systems with water recirculation experience increasing problems with microbial taste and odor compounds (TOCs) such as geosmin and 2-methylisoborneol (MIB). This study investigated the content of geosmin and MIB in water and the flesh of 200 rainbow trouts from eight recirculated aquaculture systems in Denmark. TOC content in the fish flesh was measured by a dynamic headspace extraction method and was evaluated by a sensory panel. The results showed significant correlations between TOC content in water and fish and between chemical analysis and sensory perception. When geosmin exceeded 20 ng/L in the water, 96% of the fish had an intense muddy flavor, but below 10 ng geosmin/L, 18% of the fish (only 3% in special depuration ponds) had an intense muddy flavor. The results indicate that TOC levels <10 ng/L will ensure that a negligible portion of the fish obtains an unpalatable taste and flavor due to TOCs.     

Insular pest control within a metapopulation context

Introduced pests threaten many species and their control is generally beneficial for conservation, particularly on islands where complete eradication is possible. Unfortunately on ‘nearshore’ islands neighbouring source populations exist and unaided reinvasion is likely. Pest control programmes at these sites thus require a metapopulation context to adequately manage movements between source and sink populations. We investigated the ecology of introduced ship rats (Rattus rattus) on a nearshore island, and gene flow with adjacent mainland populations, in order to understand the metapopulation dynamics and relative levels of pest control required within the landscape. We sampled the entire population by trapping (n = 30), achieving eradication, and found a low rat density (3.2 ha⁻¹) indicative of a sink population. Seed and other plant material constituted the major dietary component of rats. Despite its proximity to mainland source populations, the island population was genetically distinct with reduced allelic diversity caused by a recent reinvasion founder effect. Genetic analyses also detected recent migrants between the populations. In contrast, two mainland populations separated by a similar distance displayed complete genetic mixing. The small water gap therefore provides a sufficient barrier to lower the migration rate to the island and delay reinvasion, which nonetheless eventually happens. In order to maintain nearshore islands pest-free, conservation management will require a metapopulation approach simultaneously focusing on both island and source population pest control.     

Growth and Metal Accumulation of Geyer and Mountain Willow Grown in Topsoil versus Amended Mine Tailings

Willows (Salix spp.) are an integral component in the restoration of wetland plant communities that have been impacted by the fluvial deposition of mine tailings. A greenhouse study was conducted to compare growth and metal uptake of Geyer (S. geyeriana) and mountain (S. monticola) willow grown in topsoil versus lime and biosolids amended mine tailings. Biomass, leader length, and tissue metal contents were measured after four months growth. Above and belowground biomass and leader length of Geyer willow were greater for plants grown in topsoil compared to amended mine tailings. However, soil type did not affect mountain willow growth. Analysis for five metals yielded complex results for the two willow species and soil types. As compared to mountain willow, Geyer had greater concentrations of Mn and Pb in aboveground tissues, and Cu in senesced leaves and bark-less leaders when grown in tailings; mountain willow leaves contained greater levels of Cd than Geyer when grown in tailings. Both willow species had foliar Cd levels which were above livestock toxicity tolerance values. Based on growth characteristics, mountain willow appeared better suited for restoration of mine tailings compared to Geyer willow. However, because of the high Cd uptake by both willow species, care should be taken in restoration efforts where wildlife and domestic livestock are likely to browse on the willows.     

Forage yield and crude protein of interseeded legume‐bermudagrass mixtures as affected by phosphorus fertilizer 1

Bermudagrass (Cynodon dactylon L.) is a warm season perennial that is well adapted in the southern Great Plains. It is one of the region's most important forage crops used for livestock production, and is commonly grown without legume interseeding. Recent research has investigated ways of improving the quality and quantity of this forage. The objectives of this study were to determine the effect of interseeded legumes and phosphorus (P) fertilizer on bermudagrass pasture forage yield and crude protein content. One experiment was initiated in 1993 in eastern Oklahoma in an established bermudagrass pasture. Red clover (Trifolium pratense L.), ladino clover (Trifolium repens L.), and two varieties of alfalfa (Medicago sativah), ’alfagraze’ and'common’, were interseeded by hand into an established stand of bermudagrass. The effect of P on forage yield and crude protein was evaluated using a 30‐kg P ha^‐1 rate applied at establishment versus no applied P. Forage yield was collected three times throughout the growing season each year from 1994 through 1997. When both alfalfa varieties were interseeded into a bermudagrass pasture without applying additional P fertilizer, forage yields for the legume‐grass mixtures decreased below those obtained from the monoculture bermudagrass in the first year of the stand. The alfalfa variety ‘alfagraze’ interseeded into established bermudagrass decreased total forage yield over the entire 4‐yr study. Interseeded red clover and ladino clover increased crude protein of the forage compared with monoculture bermudagrass the first two years of the study, with red clover continuing to increase crude protein in the fourth year. However, when 30 kg P ha^‐1 was applied to the bermudagrass prior to establishment of the legumes, no change in yield or protein was observed for both alfalfa varieties’ interseeding treatments versus the unfertilized mixtures. Although forage yield may not be increased, interseeding legumes into established bermudagrass could provide an efficient way to improve pasture crude protein without the use of inorganic fertilizers. However, if alfalfa ('common’ or ‘alfagraze') is interseeded, additional P may need to be applied at legume establishment to prevent possible yield decreases.     

Seasonal Changes of Mineral Nutrients in Fruit and Leaves of ‘Newhall’ and ‘Skagg's Bonanza’ Navel Oranges

In southern Jiangxi province of China, ‘Newhall’(Citrus sinensis Osbeck) navel orange presented a conspicuous symptom of boron (B) deficiency in mature leaves, whereas B deficiency symptoms were not manifested on ‘Skagg's Bonanza’(C. sinensis Osbeck) navel orange. In this study, changes in concentrations of B, calcium (Ca), potassium (K), magnesium (Mg), manganese (Mn), and zinc (Zn) were comparatively investigated in the structural parts of the fruit (rind and pulp) and leaves (old leaves from last season and spring-flush leaves from current year) of ‘Newhall’ and ‘Skagg's Bonanza’ navel orange during the growing season. Two peaks of B concentrations were observed in the rind of the two cultivars during fruitlet growth and fruit enlargement, respectively. Boron concentrations were relatively high in the rind during fruitlet growth, and then decreased in both rind and pulp, whereas, during middle and late fruit enlargement significant increases were found for B in both rind and pulp of the two cultivars. Boron concentrations in old leaves of ‘Newhall’ decreased progressively and remained relatively low, whereas that of ‘Skagg's Bonanza’ was relatively high and changed slightly as the season progresses. Both Ca and K concentrations were above the critical threshold values, while their dynamics were reverse to that of B in fruit and leaves during certain times. Old leaf Mg concentrations of samples at 140 days after full bloom from the two cultivars and spring-flush leaves from ‘Newhall’ were below the threshold limit for sufficiency. In addition, Mg in old leaves was much lower from ‘Newhall’ than from ‘Skagg's Bonanza’. Spring-flush leaf concentrations of Mn and Zn and Mn concentrations in old leaves from ‘Newhall’ were relatively lacking during middle and late season, which accelerated the occurrence of B deficiency symptoms on mature leaves of ‘Newhall.’     

Sensitivity of soft red winter wheat cultivars to chlorate‐induced toxicity

The use of chlorate as a nitrate analogue to screen soft red winter wheat (Triticum aestivum L.) cultivars for differences in nitrate reductase activity (NRA) was studied by adding potassium chlorate to a hydroponic nutrient solution in which wheat seedlings were growing. After 14 days, leaf symptoms indicating chlorate‐induced toxicity were rated. It was hypothesized that wheat plants which were susceptible to chlorate‐induced toxicity reduced chlorate and nitrate more rapidly than did resistant plants. In experiments testing the potential of this assay, wheat and barley (Hordeum vulgare L.) cultivars previously reported to have low NRA were less susceptible to chlorate‐induced toxicity than were cultivars reported to have high NRA. The assay was used to screen 15 soft red winter wheat cultivars for differences in sensitivity to chlorate‐induced toxicity. Variable toxic reactions were observed both among and within the cultivars. To determine whether the within‐cultivar variation was environmental or genetic, single plant selections for contrasting chlorate response were made, and bulked progeny were rescreened. In eight of 15 cultivars, the contrasting selections were different for chlorate‐induced toxic response, indicating heterogeneity for this trait within these eight cultivars. These chlorate‐selected lines may also be near‐isogenic lines for NRA. Seedling screening of wheat for chlorate response may be useful for identification of high NRA breeding lines.     

Use of spectral radiance for correcting nitrogen deficiencies and estimating soil test variability in an established bermudagrass pasture

The use of variable rate technology has become increasingly popular for applying plant nutrient elements. The most widely used method for determining variable fertilizer rates is presently based on soil testing and yield mapping. Three field studies (Bumeyville 1995, Burneyville 1996, and Ardmore 1996) were initiated in established Midland bermudagrass [Cynodon dacrylon (L) Pers.] pastures to determine the relationship between spectral radiance at specific wavelengths with forage nitrogen (N) removal and biomass, and to determine field variability of soil test parameters. Variable N (applied to 1.5 × 2.4 m subplots within 2.4 × 45.7 m main plots), fixed N and check treatments were evaluated at each location. Spectral radiance readings were taken in the red (671±6 nm), green (570±6 nm), and near infrared (NIR) (780±6 nm) wavelengths. The normalized difference vegetation index (NDVI) was calculated as NIR‐red/NIR+red. Variable N rates were applied based on NDVI. The highest fixed variable N rate was set at 224, 336, and 672 kg N ha^‐1 for Burneyville, 1995, 1996, and Ardmore, 1996, respectively. At Bumeyville, soil samples were collected in all variable rate plots (1.5 × 2.4 m) and analyzed for various soil test characteristics. NDVI, red, green, and NIR spectral radiance readings were correlated with bermudagrass forage N removal and yield. Correlation of forage yield and N removal with red, NIR, and NDVI were best with maximum forage production, however, when forage production levels were low correlation decreased dramatically for the red wavelength compared with NIR and NDVI. Forage yield and forage N removal in variable rate treatments increased when compared to the check while being equal to the half‐fixed and fixed rates where higher N rates were applied. Also, variability about the mean in variable rate plots was significantly lower than half‐fixed and fixed rates which supports adjusting N rates based on indirect NDVI measurements. Variable N rate plots reduced fertilizer inputs by 60% and produced the same yield as fixed rate plots, while fixed and half‐fixed rates did not increase N content in the forage over that of the variable rate treatment. Soil sample data collected from small consecutive plots (<4 m²) was extremely variable indicating that intense sampling would be needed if variable fertilizer application were to be based on soil test results.     

基于分形插值的三维路面重构与分析

原始精细的路面谱是研究车辆与路面耦合的重要基础,对车辆的通过性和噪声、振动及不平顺性NVH(noise,vibration and harshness)分析有重要的应用价值。为了构建与原始路面相同或相近的路面谱,利用非接触式激光路面不平度仪测量沥青路面、水泥路面、比利时路面和砂石路面的三维路面不平度;基于分形理论,采用迭代函数法重构这四种路面谱;结合路面不平度的统计特性评价指标和分形维数对四种路面的原始谱和重构谱进行评价。研究结果表明:重构前、后路面谱的平均值、标准差、峰度系数以及分形维数变化范围在±5%以内,除水泥路面的偏态系数变化范围均在±9%以内,重构路面谱与原始谱具有一致性;重构路面谱保持了原始路面的结构特性,并具有路面的细微结构。