RGB 및 식물 형질 및 생리 적 연구에 대 한 스펙트럼 루트 이미징: 실험적인 체제 및 프로토콜을 이미징

요약

실험 프로토콜은 RGB와 hyperspectral 화상 식물 루트 시스템을 성장 하는 토양의 평가 대 한 제공 됩니다. RGB 이미지 시간 시리즈 hyperspectral에서 chemometric 정보 검색의 조합 공장 루트 역학에 대 한 통찰력을 최적화 합니다.

초록

식물 루트 역학의 더 나은 이해 농업 시스템의 리소스 사용 효율을 개선 하 고 작물 재배 환경 스트레스에 대 한 저항 증가 필수적 이다. 실험 프로토콜은 RGB 및 루트 시스템의 hyperspectral 화상 진 찰에 대 한 제공 됩니다. 접근은 식물 자연 토양에서 완전 개발된 루트 시스템을 관찰 하는 더 긴 시간을 통해 성장 rhizoboxes를 사용 합니다. 실험 설정 됩니다 rhizobox 식물 물 스트레스를 평가 하 고 뿌리의 역할을 공부 하 고 궁 행. RGB 이미지 설치 시간이 지남에 루트 개발의 저렴 하 고 빠른 정량화에 대 한 설명 합니다. Hyperspectral 영상 RGB 색상 기반 임계 처리에 비해 토양 배경에서 루트 분할을 향상 시킵니다. Hyperspectral 화상의 특정 힘은 기능 이해에 대 한 루트 토양 시스템에 chemometric 정보 수집 이다. 이 고해상도 물 콘텐츠 매핑 보여 줍니다. 그러나 스펙트럼 이미징은 이미지 수집, 처리 및 분석 RGB 방식에 비해 더 복잡 합니다. 두 방법의 조합에는 루트 시스템의 포괄적인 평가 최적화할 수 있습니다. 루트 및 지상 특성을 통합 하는 응용 프로그램 예제는 식물 형질 및 식물 생리 연구의 맥락에 대 한 제공 됩니다. 루트 영상의 추가 개선 및 확장 스펙트럼 데이터에서 루트 영역 속성에 유추 이미지 분석 방법의 다른 광원을 사용 하 여 더 나은 조명 RGB 이미지 품질을 최적화 하 여 얻을 수 있습니다.

서문

뿌리의 저장 등의 식물에 대 한 몇 가지 필수 기능을 제공 동화 한다, 토양, 및 통풍 관 지구 식물의 앵커리지와 물과 영양소¹의 전송. 진화 관점에서의 루트 축 형성 땅 식물²의 기원에 대 한 기본 전제 조건으로 간주 됩니다. 이 중요 한 역할에도 불구 하 고 역사적으로 뿌리 생물학 연구에만 한계 위치를 점령 했습니다. 그러나 더 최근에,, 거기는 증가 그림1에서 입증 공장 루트 시스템에 과학적인 관심사.

그림 1: 루트 식물 과학에 있는 연구의 관련성.
루트의 수는 지난 년간 SCI 저널에 모든 게시 된 식물 연구의 연구를 관련. Scopus 키워드 "공장" 및 "공장 및 루트"를 사용 하 여에서 결과 검색 합니다. 이 그림의 더 큰 버전을 보려면 여기를 클릭 하십시오.

두 가지 주요 이유는 루트 연구의 최근 발전 기초를 가설 수 있습니다. 첫째, 지상파 식물 글로벌 변경³결과로 더 자주 환경 스트레스에 노출 됩니다. 농업 작물 생산의 맥락에서 그것은 세계적으로 약 30%의 농업 지역 물과 인^4,5에 의해 제한 됩니다 추정. 수확량의 스트레스 감소는 세계적으로 낮은 50 %rainfed 농업 생태계⁶에 대 한 잠재적인 생산성의 추정 되어 상당한 수익률 격차에 대 한 주요 이유. 낮은 리소스 가용성, 외이 가난한 리소스 사용 효율, 즉 부족 한 수 식물의 사용 가능한 리소스⁷악용 관련이 있습니다. 이 다른 생태계에 부정적인 영향을 미칠 수 있는 질산염 등 모바일 자원의 손실에 결과. 예를 들어 현재 글로벌 질소 이용 효율 47⁸추정 됩니다. 더 나은 자원 사용 효율성 향상 된 관리 방법 및 재배 이므로 모두 높은 중요성의 성장 환경 지속 가능성 뿐만 아니라 농업 출력의 지속. 이 컨텍스트 식물에서 뿌리 향상 된 작물 및 자르기 시스템^9,10에 대 한 주요 대상으로 간주 됩니다.

식물 뿌리에 최근 관심사에 대 한 두 번째 중요 한 배경 측정 방법에 기술적 진보 이다. 루트 방법을 두 가지 주요 과제에 의해 제한 오래 있다: 그들은 주로¹¹, 세척 하 여 정량화에 대 한 격리 해야 했다 토양에서 성장 하는 식물에서 뿌리의 측정에 대 한 그로 인하여 방해 루트 축의 건축 배열. 발굴을 사용 하 여 제자리에 루트 관찰 함으로써 토양에서 뿌리의 자연 스러운 위치를 보존 하는 방법 식물 설명¹²사용 되 고 있다. 아직도 그들은 매우 시간이 걸리는 고 따라서 비교 구조 기능 루트 시스템 분석의 처리량 요구 사항을 충족 하지 않습니다. 다른 한편으로 높은 처리량 방법 루트 건축 측정 했다 주로 수행 인공 미디어에 묘 종 식물¹³ 식물의 자연 성장과 환경 추정 의심¹⁴가.

루트 연구의 최근 붐 이미징 방법¹⁵에 사전에 긴밀 하 게 연결 된다. 접근 루트 연구에서를 이미징 세 가지 유형으로 대략 그룹화 될 수 있습니다. 우선 CT 및 MRI¹⁶같은 고해상도 3D 방법이 있다. 이러한 메서드는 가뭄 유도 xylem 폐색전증¹⁷와 같은 흙과 식물 뿌리의 상호 작용 과정 연구에 가장 적합 한. 일반적으로 그들은 그들은 상세한 관측을 허용 하는 비교적 작은 샘플에 적용 됩니다. 다른 크기의 냄비와 잘 루트 이미징에 대 한 CT와 MRI의 비교는¹⁸에 제공 됩니다. 둘째, 높은 처리량 이미징 방법^19,20있다. 이러한 메서드는 대부분에 따라 일반적인 2D RGB 영상 인공 미디어 (젤, 발 아 종이)에 성장 하는 뿌리의 고대비 뿌리와 배경 사이의 비교적 간단한 해 부를 허용 하는 곳. 그들은 다른 작물 genotypes 표준화 된 인공 성장 조건¹³에서 종묘 루트 특성 중 높은 처리량 비교를 위해 적합 하다. Rhizobox 메서드는이 두 가지 방법 사이: 그들은 더 긴 기간 동안 토양에 성장 하는 뿌리의 2D 이미지를 사용 하 고 중간 처리량^21,22. (2D) 루트 이미징에 대 한 최근 도전 캡처 또한 구조²³의 설명 외에도 루트 기능 지표입니다.

현재 신문에서 우리는 rhizobox 성장 (i)는 저렴 하 고 간단한 주문 품 RGB 이미징 설치 및 (ii)는 더 복잡 한 적외선 이미징 설치 프로그램을 사용 하 여 루트 시스템 이미징을 위한 실험 프로토콜 제시. 이러한 두 설정에서 예제 결과 표시 되며 식물 형질 및 식물 생리 연구의 맥락에서 논의.

프로토콜

1. Rhizoboxes for plant growth

NOTE: The experimental system uses rhizoboxes to grow plants for root imaging. First the design of the boxes and the substrate used are described, and then details on the filling procedure are given.

1. Design of rhizoboxes
   1. Create rhizoboxes (Figure 2) with a back plate and side frames made of grey PVC with 15 mm strength. Use a box size of 300 mm x 1000 mm. For the front window, use 6 mm mineral glass which is attached to the PVC frame by metal rails being screwed into the side walls.
   2. Create three holes on the bottom frame to allow drainage of excess water. These holes can be optionally closed by plastic screws.
   3. Before filling, adapt the inner diameter of the rhizobox (between 10 mm and 30 mm) by inserting PC multiwall sheets. An inner space of 10 mm is recommended for most crops to reduce the weight (rhizobox weight without soil is 13.2 kg) of the entire system.

Figure 2: Rhizobox experimental system and its components.
Left figure shows the dimensions of a rhizobox and right figure its single components, being a grey PVC back plate with side frame, a front mineral glass, multiwall sheets for variable inner diameter and metal angles to fix the front glass to the back compartment. Please click here to view a larger version of this figure.

2. Substrate
   1. Fill the rhizoboxes with field soil (for this experiment: silt loam top soil from a calcareous chernozem) sieved to 2 mm particle size.
   2. Open the rhizoboxes for filling the substrate into the inner compartment (back plate with side frames) in horizontal position using pre-wetted substrate. Fill horizontally to avoid layering and segregation between fine and coarse particles which occurs when filling in vertical position by pouring the substrate through the upper opening.
   3. Pre-wet the substrate before filling. Depending on the type of substrate (particularly on its silt and clay content), do not exceed a water content of 0.12-0.18 cm³*cm^-3 to avoid smearing and structure degradation. Add the difference between pre-mixing and target water content after filling the substrate into the rhizoboxes.
      NOTE: Filling with (oven-)dry substrate and subsequent addition of the entire water is not recommended as is can result in strong settlement of the substrate and formation of large cracks.
3. Step by step filling example
   1. Define the target water content. Here it is initially set at 80% plant available water (PAW) where plants do not suffer from any water shortage.
   2. Determine field capacity (FC) and permanent wilting point (PWP) of the substrate. Here obtain FC using a PVC tube of equivalent height (100 cm) as the rhizoboxes.
      1. Close the tube at the bottom with a stubble having small drainage holes, add 1 cm of gravel to avoid closure of the holes from finer substrate and fill the substrate to the same bulk density as used for the rhizoboxes (1.3 g cm^-3).
      2. Saturate the tube with water until drainage occurs and leave it for two days to equilibrate (the resulting water content is per definition equivalent to field capacity), while covering the upper opening with cling-film to avoid evaporation. The field capacity value achieved for the soil in this experiment was 0.357 cm³ cm^-3.
         NOTE: Water content at PWP has to be known in advance from standard soil physical methods (e.g. pressure plate measurements²⁴) or from texture based pedotranfer functions²⁵. Here it is equal 0.12 cm³ cm^-3 for the soil used.
      3. When measuring FC by pressure plate extraction too, take the water content at a matrix potential of h=-100 hPa and not h=-330 hPa in order to correspond to the rhizobox geometry (height of 100 cm = 100 hPa).
      4. Calculate the water content (WC) at 80% PAW: WC (cm³ cm^-3) = 0.80 (FC-PWP) + PWP. For the hydraulic limits of the soil used here this gives a volumetric water content at 80% PAW of 0.31 cm³ cm^-3.
      5. Calculate the water amount for a rhizobox volume of 2850 cm³ (30 cm width, 1 cm inner space, 95 cm in height, with top 5 cm keeping free of substrate for watering). This gives a water volume of 883.5 cm³ equal 883.5 g for the density of water being 1.0 g cm^-3 at 20 °C.
   3. Define the bulk density (d_b) to fill the rhizoboxes. Here, set at 1.3 g cm^-3 corresponding to values typically found in agricultural field soils. The amount of dry substrate required to fill a rhizobox volume of 2850 cm³ at this d_b equals 3705 g of dry soil.
   4. Pre-wet the dry soil to a gravimetric water content of 0.108 g g^-1 (equal to a volumetric water content of 0.14 cm³ cm^-3) by adding 400 g of water for 3705 g of dry soil and mix it gently to obtain a homogeneous water distribution. Manually disrupt larger aggregates to keep the particle size ≤ 2 mm.
   5. Fill the pre-wetted soil into the opened rhizoboxes and compact it gently using a polystyrene sheet (30 x 10 x 1.5 cm) to cover the inner volume of the box, thereby resulting in a homogeneous d_b of 1.3 g cm^-3.
   6. Add the remaining amount of water (483.2 g) to achieve the target water content of 0.31 cm³ cm^-3 by spraying onto the surface with a spray bottle. Ensure small drop size to avoid surface structure degradation and homogeneous wetting. Keep the box on a balance during spraying to monitor the amount of water actually added to substrate.
   7. Let the water redistribute for 10 minutes and then press the glass onto the surface and fix it with the side metal rails. The average final weight of rhizoboxes with wetted substrate was 17818 ± 68 g (13230 g rhizobox weight + 3705 g dry soil + 883 g water).
      NOTE: The homogeneous water content filled in the horizontal boxes will redistribute when the boxes are set in their final position according to the resulting potential gradient. This is a physical process in all plant growth pots according to their geometry (height) and experimenters should be conscious on their pot hydraulics²⁶.

2. Climate room setup

1. Equip the climate room (Figure 3) with 8 LED lamps providing homogeneous illumination of 450 μmol m^-2 s^-1 with spectral peaks at 440 (blue) and 660 (red) nm for optimum plant growth.

Figure 3: Climate chamber with rhizoboxes for stress experiment.
(A) Left view of entire chamber with LED illumination, weather station and PC (here for logging leaf hygrometers); right view with a close-up of the metal frame holding rhizoboxes in 45 ° inclination and wooden plates used to shied rhizobox glass window against light. (B) Stress experiment with sugar beet combining four stages with stress due to different atmospheric demand (high/low) and soil water availability (high/low). Green bars of mean stomata conductance give an indication of plant stress response. Please click here to view a larger version of this figure.

2. Set the ambient parameters according to plant/experimental needs. Here, use 14 hours light and 10 hours dark for illumination. During plant establishment and before stress treatments started, set the temperature to 20° C during day and 15° C during night and keep relative humidity at 50 ± 8%.
3. Put the rhizoboxes at an inclination of 45° using an adequate metal framework. This maximizes root growth towards the glass surface due to gravitropism.
4. Cover the glass window by a wooden plate to keep the root zone in dark and avoid algae growth due to light penetrating through the glass surface.

3. Sugar beet example setup and treatments

1. Pre-germinate sugar beet seeds on wet filter paper for three days at 20 °C in an incubator until the radicle emerges. This ensures seeding with viable plants.
   NOTE: Pre-germination is not required for plants with high germination vigor, thereby avoiding the risk of damaging the radicle at seeding. For sugar beet seeds with a thick pericarp, however the risk of non-viable seeds is high and pre-germination significantly accelerates the emergence of the radicle compared to direct seeding into soil.
2. Drill a small hole to about 1.5 cm soil depth in the middle of the rhizobox with a screwdriver, position one seed in it using tweezers with the radicle oriented downwards and next to the glass window (this improves the initial visibility) and gently cover it with soil.
3. Add a 0.5 cm layer of fine gravel (2-4 mm) on top of soil to protect the soil aggregates from slaking during irrigation and reduce evaporation losses. To facilitate emergence, keep the soil surface free of gravel where the seed has been positioned.
4. Add 10 g of water to enhance establishment.
5. During establishment and early growth until experimental stress treatments start, irrigate the rhizoboxes every 2-4 days to keep the initial moisture content of 80% PAW.
   1. Determine the amount of irrigation water required by weighing the rhizoboxes and adding water until achieving the initial weight of each individual box. For manual irrigation use a pipette to avoid surface structure degradation during watering.
6. Arrange the rhizoboxes according to the established design in the climate room. The protocol reported here is based on an experiment with six sugar beet cultivars in five replicates in a completely randomized design (CRD).
   1. Reposition rhizoboxes each time when taking them out of the metal holder for weighing and watering. This avoids any effects of residual (light) inhomogeneity inside the climate room.
7. Define the time for onset of stress treatments and the type of stress. The following settings (Figure 4) are used here.
   1. Start measurements at BBCH 15 (five leaves unfolded) with roots covering around 75% of the rhizobox depth and the canopy being sufficiently developed for measurements at the leaves. Keep each stage for at least three days to ensure adaptation to the new settings and make measurement.
   2. For the non-stress observations keep the initial settings with optimum soil moisture (80% PAW) and ambient conditions (20° C/15° C temperature, 50 ± 8% rH) giving a daytime vapor pressure deficit (VPD) of 1.28 ± 0.1 kPa (cf. Figure 3 C).
   3. Raise the atmospheric demand to a daytime VPD of 2.45 ± 0.4 kPa by increasing the temperature to 27°C/20°C and decreasing rH to 35%, while keeping soil moisture at 80% PAW.
   4. Subsequently dry down the rhizoboxes to 40% PAW equivalent to a water content of 0.215 cm³ cm^-3 by withholding irrigation. Reset the initial ambient conditions with low atmospheric demand (VPD of 1.28 kPa).
   5. Combine stresses increasing the atmospheric demand to a VPD of 2.45 kPa and keep soil moisture at 40% PAW.

4. Root imaging methods

1. Combine imaging methods to make use of their respective advantages and depending on the information targeted.
   1. Apply RGB imaging in the VIS range to track root growth, architecture and morphology over time which implicitly requires frequent measurement. Advantages of RGB imaging are (i) low costs, (ii) rapid image acquisition, (iii) low requirements of hard-disk space (image size: 48 MB) and (iv) high resolution (3648 x 5472 pixels).
   2. Use hyperspectral imaging (HSI) in the NIR range when chemometric features of root and soil are required. Advantages are (i) spectral features for segmentation between roots and soil background and (ii) access to physico-chemical system properties (e.g. soil water content, root age). Disadvantages are (i) higher scanning time (about 16 minutes per rhizobox), (ii) large size of datasets (13.7 GB per rhizobox image) and (iii) lower resolution of the NIR camera (320 x 256 Pixel), and (iv) higher complexity of data analysis.
2. RGB root imaging
   1. Use an imaging box (Figure 4) that shields from ambient light and fixes the camera position consisting of a metal frame with a width of 1 m and a height of 1 m with side walls lined with pressboards. At the front end, fix the camera at two positions at a distance of 80 cm from the rhizobox.
      1. Attach a tapeline on the rhizobox frame with transparent adhesive tape and position the rhizobox in the holder of the imaging box.
      2. Illuminate the rhizobox using four 24 W fluorescent light tubes attached at a distance of 80 cm from the rhizobox. Also, mount four 15 W UV tubes at 20 cm from the rhizobox as alternative illumination making use of root auto-fluorescence in case of low contrast between root and (bright colored) substrate background.
      3. Take two images (top and bottom position) to cover the upper and the lower half of a rhizobox with an overlap of about 3 cm.
   2. Acquire RGB images with a digital single-lens reflex camera which is fixed by quick release plates on the respective positions of the imaging box.
      1. Apply the following settings when using the fluorescent light tubes. Adapt these example settings for any changes in dimensions and illumination as well as the camera model.
         1. Turn off autofocus and stabilizer on the camera objective. Set camera to manual mode.
         2. Set ISO speed to 500. Set shutter speed to 13. Set Aperture to 5.6.
         3. Turn off the mirror lock. Set the white balance to Auto White Balance.
      2. Use the following settings for illumination with UV tubes:
         1. Turn off autofocus and stabilizer on the camera objective. Set camera in manual mode.
         2. Set ISO speed to 1000. Set shutter speed to 13. Set Aperture to 5.6.
         3. Turn off the mirror lock. Set the white balance to Fluorescence.
   3. Merge the RGB images from top and bottom of the rhizobox into a single image a photo editor (e.g., Adobe Photoshop). Use the tapeline at the side frame of the rhizoboxes and control overlapping objects (root and soil features).
      1. Copy the two separate images, each with pixels size of 3648 x 5472, into a new file of size 3648 x 10944 pixels and white background.
      2. Reduce the layer opacity for one image to 60% and align overlapping parts of the images (tapeline, objects). Thereafter restore layer opacity to 100%.
      3. Based on the ruler on the image add two red lines of exactly 1 cm to the top of the image where no roots are present. These lines are later used at image analysis to scale the image to correct length dimensions.
      4. Merge all layers and remove parts of the image outside the soil filled window with the cropping tool.
      5. Save the image as tiff-file for further analysis.
      6. In case of images with UV illumination, reduce colors to greyscale before saving using the toolbar Adjustments-Black & White, and selecting the predefined High Contrast Blue filter.
   4. For segmentation between roots and soil background as well as for quantification of root traits of interest (e.g. length, surface, diameter, branching) use any root analysis software (see www.plant-image-analysis.org for available tools). Here WinRhizo is used.
      1. Open the rhizobox tiff-image the software.
      2. Calibrate the length scale of the image using the scaling bars added to the image.
      3. Select Based on Color in the Analysis menu where selecting Root & Background Distinction.
      4. Define a calibration file with color classes corresponding to roots and soil (background). For the example images used here (cf. Figure 8) three root color classes (old laterals, young laterals, tap root) and three soil color classes are defined.
      5. For the greyscale images (UV-illumination) select (i) Based on grey levels, and (ii) Pale Root on Black Background in the Root & Background Distinction menu and use a local Lagarde intensity threshold for segmentation.
      6. Open a data file where results are saved.
      7. Run the analysis and subsequently control whether there are regions (e.g. at the edges) which are mismatched. In this case define an exclusion region and restart the analysis. For roots not classified, add additional color classes and restart the analysis. For elements wrongly classified as roots, activate/increase the Debris & Rough Edges filtering options.

Figure 4: Imaging box to acquire RGB rhizoboxes pictures.
Left view of front side where rhizoboxes are attached for imaging with light sources inside; right view of backside where the camera is mounted. Please click here to view a larger version of this figure.

3. Hyperspectral root imaging
   1. Hardware setup
      1. Use a hyperspectral root imaging system (Figure 5) consisting of (i) a thermo-electrically cooled 14-bit monochrome NIR camera with a spectral range from 900 nm to 1700 nm, 320 by 256 pixels and a frame rate of 100Hz and (ii) an imaging spectrograph with a spectral range 900 nm to 2500 nm and a spectral resolution of 3.6 nm. Arrange a halogen line illumination source (four 50 W halogen spots) in a 45°/-45° geometry. Mount the imaging sensor on a two-axis positioning system. The scan window has a size of 240 x 1000 mm, i.e. 30 mm at each edge of the rhizobox are not covered by the image.
      2. Control the system by a Matlab script for (i) white and dark standard acquisition, (ii) setting of camera integration time, (iii) selecting spatial (pixel size 0.1 mm; pixel size 1.0 mm) and spectral resolution (all 222 spectral bands with a resolution of 3.6 nm; smoothed spectrum with 54 bands and a resolution of 14.8 nm), and (iv) defining the scan region on the rhizobox.
      3. Save images as SIF-files. To avoid problems during saving of large files, subdivide each scanned image stride (9 strides per rhizobox) into four segments (three of 300 mm length, one of 100 mm length) and save separately with a unique file name consisting of stride number (1 to 9) and part (1 to 4) as well as date and time (YYYY.MM.DD HH:MM:SS). An entire rhizobox scan requires a hard-disk space of 13.7 GB.
   2. Image acquisition and analysis.
      NOTE: Figure 6 shows the steps of image acquisition, segmentation and analysis.
      1. Image acquisition comprises selection of camera setting for optimum image quality and definition of scan parameters.
         1. Determine the camera integration times for the rhizobox scan and the white standard in the camera software.
            1. Open the imaging GUI and move the camera to a position of the rhizobox where roots are present.
            2. Adjust the integration time of the camera targeting a light object (i.e. root) in a way that approximately 85% of the full dynamic range of the camera is used on the histogram displayed by the software. Repeat for the white standard by moving the camera positioning system to target the white standard. Then close the camera software.
         2. Open the Matlab Imaging GUI and make all settings for the current rhizobox scan. For the data reported here, use the following settings:
            Integration time white standard: 1000
            Integration time rhizobox: 4000
            Spectral resolution: Full resolution (i.e. 222 narrow-range spectral bands)
            Full spatial resolution (pixel size of 0.1 mm)
            1. Acquire the dark and white standards before each imaging run, e.g. once a day. The dark standard represents the camera noise, while the white standard gives the maximum reflectivity. These data are required for image normalization during pre-processing.
            2. Define whether the entire rhizobox or only part of it is scanned. For the present case entire rhizoboxes are imaged. Then start the scan.
      2. Process the image with a Matlab script. Operations performed by the script are described.
         NOTE: Scripts are currently in an undocumented version and can be obtained from the corresponding author. After proper documentation, they will be available for download from the website of the corresponding author's institution (www.dnw.boku.ac.at/pb/).
         1. Compose an entire image stride from the rhizobox center (containing roots) merging the four parts of the stride.
            NOTE: At this stage it is neither necessary nor recommended to use a spectral image of an entire rhizobox (i.e. all 9 strides) as the file size will make each calculation step in Matlab very time consuming and the information contained in one central stride is sufficient for the first steps of image analysis.
         2. Normalize the image using the acquired dark and white standards and taking into account the different integration times of white standard and rhizobox scans which are saved automatically during scanning in a file.
         3. Optionally apply a smoothing filter to remove noise from the image. The script currently offers 3x3 kernel median filtering and multiple scatter correction. For the image evaluation presented here, no filters are applied.
         4. Display the image at all recorded spectral bands to obtain a first insight and decide a wavelength to be displaying for selecting regions of interest (cf. image segmentation).
      3. Perform segmentation between roots and soil background in a separate script with the following steps.
         1. Select regions of interest (ROI) for root and soil to find spectral features for segmentation. Use the freehand selection tool to mark a ROI on the image displayed at a wavelength previously identified with good contrast between roots and soil. Here, use three ROIs on the root (old and young laterals, tap root) and two ROIs in the soil (dry, wet region).
         2. Display a rectangle with the selected ROIs and the remaining part as a black mask and visualize the selected ROI image at all wavelengths.
         3. Remove all lines in the image matrix containing pixels of spectral intensity = 0 (black pixels).
         4. Fuse the root and soil ROIs into one foreground (root) and one background (soil) matrix for segmentation.
         5. Search spectral bands (intensity of single spectra or spectral ratios) providing the best separation between root foreground and soil background²⁷. Quantify the distinction between the resulting pixel histograms for root and soil using Bhattacharyya distance²⁸.
         6. Select a threshold intensity value separating the histograms.
         7. Create a binary image by applying the selected threshold to the original image. This sets all pixels having smaller intensity than the threshold to zero and those with higher intensity to one (done automatically be the script).
         8. Save the binary image as tiff-file.
      4. Open the binary image and analyze root traits. Select (i) Based on grey levels, and (ii) Pale Root on Black Background in the Root & Background Distinction menu and use global intensity threshold (the image is already binarized).
      5. For mapping the water content from the hyperspectral data acquire a calibration dataset and apply a calibration equation to a rhizobox image.
         1. Subdivide a rhizobox into 5 cm compartments using polystyrene sheets to fill them with soil (same substrate and d_b as used for the experiment) at different water contents (Figure 8).
         2. Calculate the respective amounts of water to be mixed with soil and fill the compartments (same procedure as described in 1.3. for the entire rhizobox).
         3. Scan the calibration rhizobox with the same settings as used for the planted rhizoboxes.
         4. Perform the following steps using a script.
            1. Merge the four parts of a stride from the water calibration box to one stride and normalize it with dark and white standards.
            2. Select rectangular boxes at each compartment with different water content and save them in a structure array.
            3. Determine the spectral feature that best separates the water content compartments. This is done with a search algorithm for the global maximum of the intensity differences between mean spectra of adjacent water contents.
            4. Calculate the mean spectral intensity value for this feature for each water content compartment of the calibration rhizobox.
            5. Fit a regression equation through the directly measured water content and the respective spectral quantifier.
            6. Apply the regression equation to each pixel not classified as root on a rhizobox image and for the spectral feature (band) determined above which best relates to water content.

Figure 5: Hyperspectral root scanner.
The main components of the scanner are indicated. The small picture shows the camera during imaging of a rhizobox. Please click here to view a larger version of this figure.

Figure 6: Steps in hyperspectral root imaging.
Hyperspectral root imaging consists of three mains steps being (i) image acquisition, (ii) image segmentation and (iii) analysis of the spectral data. Please click here to view a larger version of this figure.

Figure 7: Rhizobox for water calibration.
The rhizobox contains compartments with substrate at different water content which are subdivided by polystyrene sheets. Germination paper at the dry compartments ensures that soil particles do not rinse into neighboring compartments. Please click here to view a larger version of this figure.

5. Application examples

NOTE: Quantitative root information is applied in the context of plant phenotyping (cultivar comparison) and for plant physiological research. The following aboveground data are reported to exemplify these cases.

1. Leaf area: Measure leaf area non-destructively at selected stages during the experiment via the length and width of leaves as a proxy. Alternatively canopy images can be used²⁹.
   1. At the end of the experiment measure leaf length and width together with area of clipped leaves using a leaf area meter. Calibrate the non-destructive method applying a regression equation to the data pairs.
2. Dry matter: At the end of the experiment, measure aboveground dry matter by clipping the plants with scissors and dry them for 24 hours at 105 °C in an oven.
3. Stomata conductance: Measure stomata conductance with a leaf porometer. Before measurement, keep the device for at least one hour in the climate chamber to allow sensors equilibrate with ambient conditions and calibrated the device each time when ambient settings in the climate chamber are changed. Take measurements from at least three leaves per plant.

결과

Example results are presented for root segmentation based on RGB and HS imaging. For spectral imaging an example of high resolution water mapping is provided. Finally results are shown that demonstrate the scientific context where image based root data are applied.

RGB based root measurement

Figure 8 shows an RGB root image time series of sugar beet cultivar Ferrara. The images reveal some artefacts from inhomogeneous illumination of the rhizoboxes, with brighter areas along the left side and different brightness at the overlapping area between top and bottom images.

Figure 8: Root growth time series from the RGB imaging.
Pictures show the sugar beet cultivar Ferrara at different days after sowing (DAS). The images show some artefacts due to non-homogeneous illumination at the left side of the image and between top and bottom images. Scale bars, 2 cm. Please click here to view a larger version of this figure.

Figure 9 provides details on root segmentation based on color thresholding for cultivar Ferrara at day after sowing (DAS) 35. As a reference (Figure 9A), a binary image is used where all roots were manually tracked with a Graphic Tablet. The time required for manual tracking of the entire, fully developed, dense sugar beet root system was around four hours. Figure 9B gives a detailed view on a selected area at the top of the image where old lateral roots are present. Here several root axes are not classified by the color threshold. At the bottom (Figure 9C) on the contrary, where white young roots are predominant, the color based segmentation properly classifies all root axes. The binarized root system (Figure 9D) shows a black area at the left side from the illumination artefact which was defined as exclusion region before running quantitative analysis. Figure 9E shows the corresponding pixel histograms of selected features (roots vs. soil) for the red channel of the RGB image from Ferrara at DAS 35. The root pixels (blue color) clearly show three peaks corresponding to bright young laterals, dark old laterals and tap root. The overlap between the old laterals and the soil background is very strong, leading to unclassified root axes (cf. Figure 9B).

Figure 9: Root segmentation using a color threshold.
(A) Manually segmented root system using a Graphic Tablet, (B) area with poorly segmented old root axes in the top and (C) properly segmented young axes in the bottom of the image, and (D) binary image obtained from color based thresholding. (E) Pixel histograms for selected features of the RGB image. Roots are represented by the blue bars with different root types indicated; soil is represented by the red bars. Scale bars in A and D, 2 cm; scale bars in B and C, 1 cm. Please click here to view a larger version of this figure.

The resulting total visible root length quantified for the manually segmented reference image is 1534.1 cm, while the automatized, color based segmentation gives a total root length of 1427.6 cm.

Greyscale images from UV-illumination do not provide an advantage in the case shown here and performed worse compared to color thresholding (root length: 1679.7 cm). Old roots could not be segmented, and there was more noise in the image, probably due to lower light intensity of the UV lamps. However, in case of young roots with high auto-fluorescence and a bright background substrate, UV-illumination can still be an option as shown by an image obtained from another experiment where sand was used as background substrate (Figure 10).

Figure 10: UV illumination to visualize roots on bright background.
Example from a durum wheat root system growing in a rhizobox filled with quartz sand. The rhizobox is imaged with illumination for (A) UV light and (B) fluorescent (day) light. Scale bars, 2 cm. Please click here to view a larger version of this figure.

HSI based root measurements

Figure 11 provides the mean spectra for three root ROIs (old and young lateral, tap root) and two soil ROIs (top and bottom of rhizobox).

Figure 11: Mean spectra of root and soil.
Spectra from regions of interest (ROIs) on the root (three root types) and in the soil (top and bottom of the rhizobox). The ROIs are selected to determine an optimum segmentation criterion between root and soil. Please click here to view a larger version of this figure.

It is evident that the tap and young lateral roots differ substantially from the background in intensity of most spectral bands. For the old laterals the intensity differences are much lower. A feature that can be inferred visually is the different slope of the spectrum around water absorption region (1450 nm). Here the slope of root spectra is higher compared to soil spectra. Furthermore a change of tap and young lateral spectra in the region around 1100 nm can be identified that does not occur in the old laterals.

Figure 12A shows the result from the search algorithm identifying a spectral ratio with strongest foreground-background contrast. The ratio of spectra at 1476 nm to 1076 nm provides the best separation between roots and soil. The resulting histogram of root foreground and soil background pixels is shown in Figure 12B. Although there is some overlap, most pixels are clearly separated from the soil background. Fitting a bimodal Gaussian curve through the histogram and using Bhattacharyya distance for quantification, a value of 7.80 is obtained. A value higher 3.0 indicates strong image contrast allowing reliable separation²⁸.

Figure 12: Difference in reflectance between root foreground and soil background for different spectral band ratios and pixel histogram at spectral ratio used for segmentation.
(A) Bright colors (yellow) show high contrast between foreground and background, dark colors (blue) show low contrast. The first 15 bands have been removed because of noise. The red lines indicate the band ratio with highest contrast. (B) Pixel histogram of roots (blue) and soil (red) at segmentation spectral ratio. Blue bars represent the root and red bars the soil. The intensity value corresponds to the ratio of spectral band 160 to spectral band 49. Please click here to view a larger version of this figure.

The binary image (Figure 13) is created by applying a global intensity threshold of the identified spectral ratio at a value of 1.008 calculated from the histogram distance²⁷. Analysis of root length of this image gives a total length of 1557.3 cm which represents an error of only 1.5% compared to the manually tracked reference image.

Figure 13: Binary image of the root system of sugar beet cultivar Ferrara.
The image is obtained by applying a global spectral threshold. Scale bar (bottom left corner), 2 cm. Please click here to view a larger version of this figure.

Although root segmentation has improved using spectral information compared to color based information, the main intention of HS imaging is analysis of chemometric image properties. This is exemplified via mapping the water content of a rhizobox image.

Figure 14A shows the mean spectra of the compartments in the calibration rhizobox (cf. Figure 7) filled with soil of different water content. The shape of spectra is similar between the compartments, i.e. here a spectral ratio does not necessarily provide a more stable classification criterion. Thus intensity at a single spectral band (1680 nm), where the average difference between adjacent water contents is maximized, is identified as best separating criterion. The resulting pixel histograms for this spectral wavelength are shown in Figure 14B.

Figure 14: Spectral features for water content calibration.
(A) Mean spectra of nine water compartments from the calibration rhizobox with different water contents; (B) Pixel histograms for the water compartments at band 216 where average distance between neighboring compartment is maximum. Please click here to view a larger version of this figure.

The relation of the average pixel intensity at 1680 nm and the measured water content is shown in Figure 15.

Figure 15: Relation of spectral reflectance and volumetric water content.
The figure shows data pairs of measured water content and spectral reflectance with empirical curves (linear and exponential) fit to the data excluding the highest water contents (red triangles). Please click here to view a larger version of this figure.

Differentiation of higher water contents from spectral intensity becomes difficult. A significant regression (either linear or exponential) with high R² can be fit to water contents up to around 0.30 cm³ cm^-3. Wetter soil conditions cannot be reliably predicted by the intensity value. Similar behavior of an exponential relation between reflectivity and water content with a decreasing response to water contents higher 0.30 cm³ cm^-3 was also found in other studies³⁰.

A rhizobox image with fine mapping of water content is shown in Figure 16. Four aspects have to be remarked. First, a region of lower water content can be seen in the rooted parts of the rhizobox. Second, strongest depletion is concentrated in the vicinity to single root axes. Third, depletion zones also occur where no root axes are visible on the surface, indicating regions where roots are hidden in soil. Fourth, water mapping without further image-processing results in a patchy appearance due to the aggregated soil. This can indicate inhomogeneous water content distribution at the aggregate scale, but also surface morphology effect on image quality. Chemometric image-processing techniques are an option to overcome such morphological effects in spectral images³¹, but are not implemented so far in the Matlab scripts used here.

Figure 16: Water content mapping on a rhizobox.
The dark blue colours represent regions of high water content, green to red areas show regions with low water content. The plant root is overlaid on the image in black. Scale bar (bottom left corner), 2 cm. Please click here to view a larger version of this figure.

Application examples

Figure 17 relates quantitative root traits from image analysis with aboveground measurements.

Figure 17: Typical application examples for root data.
(A) and (B) show root information used for aboveground-belowground plant characterization in a phenotyping context. (A) represents root growth from the sugar beet cultivar Ferrara, (B) compares six rhizobox grown sugar beet cultivars using leaf-to-root area ratio (data from one replicate). (C) and (D) are functional relations between traits as found in plant physiological research. (C) shows the influence of leaf-to-root area ratio on dry matter production and (D) the relation of root surface area to stomata conductance. Please click here to view a larger version of this figure.

Figure 17A and 17B are relevant for phenotyping focusing on comprehensive aboveground and belowground plant characterization. Figure 17A shows root growth of sugar beet cultivar Ferrara (cf. Figure 8 for images). Expansion of the root system indicates the capacity of a cultivar to explore the soil volume in a given time span of the vegetation period. Figure 17B shows leaf-to-root surface area ratio of six sugar beet cultivars, providing a descriptor for the balance between plant supply (root) and demand (leaf).

Figures 17C and 17D give examples for functional relations of interest in physiological research. In Figure 17C leaf-to-root surface area ratio is related to dry matter formed during the experiment, indicating the predominant role of the assimilating surface as a limiting factor for dry matter accumulation. The lack of significance in spite of a comparatively high R² is related to the low number of paired data (n=6) used here. Figure 17D reveals that cultivars with higher root surface area (improved uptake) have an average higher stomata conductance over the course of the experiment. The higher root area apparently sustains water extraction, thereby prolonging stomata opening.

토론

프로토콜 루트 시스템 이미징 성장 하는 토양에 대 한 두 개의 상호 보완적인 접근을 제공 합니다. 신뢰할 수 있는 실험 결과 대 한 중요 한 단계는 관측 창에 꽉 루트-토양 접촉을 제공 하 고 공기 간격을 방지 하는 전면 유리에도 동종 기판 레이어를 보장 하는 rhizoboxes의 작성 이다. 이것의 비교적 좋은 체질된 흙을 사용 하는 주요 이유는 < 2: 골 재 사이의 공 극으로 관찰 창에서 더 높은 표면 형태에 더 큰 집계 결과. 루트 팁 탈수의 위험이 높은, 외이 물 매핑³¹에 대 한 더 복잡 한 이미지 처리 기법을 필요합니다.

프로토콜의 수정 따라서 rhizoboxes의 향상 및 빠른 작성에 초점을. 현재 시간을 작성 하는 것은 상자 당 약 30 분입니다. 또한 측 및 더 나은 RGB 이미지에 대 한 조명 동질성을 최적화 하기 위해 수정에서 영상에 대 한 두 개의 유리창와 rhizoboxes의 사용은 테스트. 추가 하드웨어 확장 또한 평면 optodes³² rhizobox 시스템으로³³ 이미징 커패시턴스의 통합을 좋습니다. 그러나 이것이 현재 업그레이드 활동 넘어입니다.

소프트웨어 수정 퓨즈 상단 및 하단 RBG 이미지³⁴자동 이미지 등록에 초점. Hyperspectral에 대 한 이미징 고급 자동된 기능 추출 SVMs³⁵ 테스트와 같은 더 민감한 감독된 대상 탐지 방법으로²⁸ 접근 한다. 그로 인하여 hyperspectral 데이터는 잠재적으로 여러 토양, rhizosphere 및 루트 속성³⁶의 평가 대 한 허용. 개발 (세미)는 것 또한 건축 특성 (분기 주파수, 각 분기) 뿐만 아니라 rhizobox 루트에 따라 이미지 루트 시스템 분석기³⁷ 형태학 계량의 수정된 된 버전 (길이, 직경, 표면)에 대 한 소프트웨어 automatized.

3D 이미징 접근에 비해 프로토콜의 주요 제한은 표면 표시 루트 및 rhizosphere 속성 제한입니다. 그러나 그것은 보이는 루트 특성 전체 루트 시스템²¹에 대 한 신뢰할 수 있는 프록시는 증명 되었습니다. Rhizobox 기술은 전통적인 파괴적인 샘플링 (세척) 동적 성장 전체 루트 시스템 특성 대 표시의 관계를 확인 하려면 이미징의 끝에와 쉽게 결합 된다. 이 관계는 종²¹중 다 수, 파괴적인 샘플링 다른 작물 종으로 어떤 새로운 형질 시리즈에 대 한 표시 특성에서 신뢰할 수 있는 유추 되도록 것이 좋습니다.

여기에 제시 된 프로토콜의 주요 장점은 현실적인 성장 조건 (토양), 일시적으로 해결 RGB 이미지와 루트 기능 (예: 물 통풍 관) hyperspectral 화상 진 찰에서 chemometric 및 rhizosphere 데이터를 통해 유추에 대 한 상대적으로 높은 잠재적인 처리량의 조합입니다. 그로 인하여 방법 유추 제한 높은 처리량 경종 및 비-토양 루트 방법¹⁴, 그것은 부분적으로 덜 실험적인 복잡성과 고급 3D 방법¹⁵에 비해 높은 처리량으로 업무 프로세스에 대 한 깊은 형질 통찰력을 허용 하는 동안 이미지를 극복 한다.

곧 실험 프로토콜 mycorrhiza 루트 시스템 개발 및 토양 구조, 질소 및 탄소 관련 된 덮개 작물 종의 형질 루트 특성에 관해서는 뿐만 아니라 콩의 기능에의 효과 연구 하는 데 사용 됩니다 사이클링.

자료

Name	Company	Catalog Number	Comments
Rhizobox	Technisches Büro für Bodenkultur	Experimental builder
LED Lamps ATUM HORTI 600	Klutronic	AHI10600F
Fluorescent light tube HiLite T5 Day	Juwel Aquarium	86324
UV light tube Eurolite 45cm slim 15 W	Conrad	593384 - 62
Canon EOS 6D	Canon Austria GmbH	8035B024
Adobe Photoshop CS5 Extended Version 12.0 x 32	Adobe Systems Software Ireland Ltd.
WinRhizo Pro v. 2013	Regent Instruments Inc.
Xeva-1.7-320 SWIR camera	Xenics	XEN-000105
Spectrograph Imspector N25E	Specim
Hyperspectral imaging scanner	Carinthian Tech Research AG	Experimental builder	Design and assemblage of Hyperspectral Imaging Scanner and software
Matlab R2106a	Mathworks		Including Toolboxes for Image Processing, Signal Processing and Statistics and Machine Learning
AP4 Poromoeter	Delta-T-Devices
LI-3100C Area Meter	LI-COR
BASF Styradur polystyrene sheets	Obi Baumarkt	9706318	Different types of polystyrene sheets or other material separating differently moistured soil can be used