During this period the plants were not illuminated. Development of the total leaf area of plants was derived from daily counts of leaflets per plant and determination of leaf area of individual leaflets at harvest (average leaflet area of lupin was.07 cm 2 ). Tomography reconstruction and image processing were described by carminati. Based on the large field of view, identification of roots was done following a segmentation protocol: first we applied a diffusion filter based on total variation (Rudin. Then a pseudomedian filter was applied to separate the structures (roots and gaps) from the soil matrix background. The difference between the filtered image and the original was then used to identify the roots via a region growing algorithm.
Measuring Plant, transpiration with a data logger
We carried out local tomography of the upper part (2 7 cm depth) and lower part (12 17 cm depth) of the cylinder with a field of view of 5 cm 5 cm and voxel side length.1. In local tomography, the field of view is smaller than the sample size and the reconstruction is focused on a subsample, resulting in a higher spatial resolution. This is possible write as long as the region outside the field of view has no macroscopic structure. For the local tomography we decreased the energy to 135 kv, to improve contrast within the image. Consequently, we increased the current to 1,600 μa and the exposure time to 360. The used ct set-up was a compromise between spatial resolution and field of view, that should be large enough to image a sufficient portion of tap root and laterals. We considered that a spatial resolution.1 mm was sufficient to visualize the gap around a tap root with a diameter of approximately 2. However, a better resolution would be preferable for a detailed analysis of the gaps around lateral roots with diameter smaller than.5. The samples were scanned by ct during the drying cycle and after irrigation. One complete scanning procedure took approximately.
Throughout the experiment each column was placed on a roles balance and the weight was continuously recorded (10 min interval). From these data whole plant transpiration was calculated. The four samples, named Lupin i-iv, were scanned using a x-ray micro-ct scanner (X-tex hmx 225). Our ct device was a cabinet model where the columns could be placed vertically during scanning. The source was a 5 μm focal spot source. X-ray imaging was performed at two levels of spatial resolution and field of view. First we recorded the entire sample with a large field of view.4.4 cm, voxel side length.32 mm, x-ray energy of 170 kv, current of 180 μa and exposure time of 400. Then specific locations of the sample were imaged at higher resolution.
One seed per column was placed in the soil at 1 cm depth. The soil surface was covered with a layer of coarse quartz gravel (2 5 mm) to reduce evaporation from the surface. Liquid write fertilizer (7-3-6 Terrasan GmbH, containing 7 n,.3 p, 5 K) was diluted 1:100 and 100 ml of diluted solution per column was applied as the initial fuller irrigation water. Columns were watered by capillary rise from the bottom. At the bottom of the columns a water table (soil matric potential 00 kpa) was maintained with deionized water until 10 days after planting. Then water supply was discontinued and plants were only irrigated again when they showed severe wilting symptoms. Plants were grown under controlled conditions (23 Cday/23 C night; 65 relative humidity; 14 h photoperiod with 350 μmolm 2 s 1 ) in a climate chamber. Microtensiometers as described by vetterlein. (1993) were inserted at depths of 5 cm and.
The soil was sieved to 2 mm and then poured into the cylinders through two sieves placed at a 10-cm distance from each other. This filling procedure was chosen to obtain a homogeneous packing (Glass. The soil bulk 3 density was.450.045 gcm. The soil hydraulic properties were reported by carminati. The water retention curve of this soil is plotted as supplementary information in the online version (Figure S2). Seeds of Lupinus albus. (Feodora) were surface sterilized for 10 min in 10 h 2 O 2 and soaked for 1 h in saturated caso 4 solution.
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Nobel and cui (1992) solved this difficulty by separately measuring each component of the soil-plant continuum and then store piecing them together as a flow in series. However, in such a way, rhizosphere processes and gradients in water potential towards the root, which are expected to become increasingly important as soil dries, cannot be included. Additionally, in Nobel and cui (1992) root shrinking and swelling were measured for excised roots and all root parts were assumed to shrink uniformly. The present study aimed to extend the results of Carminati. (2009) in which soil water status was monitored simply by weighing the columns once a day. Here, we intend to relate gap dynamics with transpiration rate and soil water potential more accurately. To this end, we combined the information from tomography with measurements of transpiration rate, water content and soil water potential during one drying and wetting cycle.
The specific questions posed are: at what soil water potentials do gaps form? Where do they form along the root systems -. Tap root versus laterals? And most importantly, do gaps limit helping transpiration rate and the plant water balance? Materials and methods four cylinders of 8-cm diameter and 20-cm height were filled with sandy soil collected from the field site hühnerwasser (Germany). The soil consisted of 92 sand, 5 silt and 3 clay.
Today, recent advances in X-ray computer tomography allow visualization of roots in soil in samples large enough to accommodate three-dimensional root growth at the spatial resolution required to observe gaps at the root-soil interface. By using local tomography and zooming inside the sample, carminati. (2009) were able to monitor the formation of gaps at the interface between roots of a lupin in a sandy soil with a spatial resolution.1. Higher resolution (pixel size.4 μm) was achieved by Aravena. They studied soil compaction around roots by means of synchrotron X-ray microtomography. While the presence of gaps at some root locations is well documented, the effect of gaps on water uptake is not well understood.
Do gaps limit root water uptake? Few studies have simultaneously measured gap formation, transpiration rate, and soil and plant water status. Important work has been performed by nobel and co-workers (Nobel and cui 1992; Northand Nobel 1997). Nobel and cui (1992) measured the hydraulic conductance of soil and roots of a desert succulent and they compared them to the conductance of an air-filled gap, assuming water flow only via the vapor phase. 5 they showed that root conductivity was limiting when soil was wet, soil conductivity was limiting when soil was dry, and, in an intermediate soil moisture regime, the gap was the limiting factor for the water flow to roots, which is in agreement with the. The reason why there are so few studies of gap effects on water flow is the difficulty of measuring simultaneously and at the required spatial resolution gap dynamics, soil matric potential, soil conductivity and xylem water potential.
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In this study we focus book on one specific aspect of root-soil properties, the formation of gaps at the rootsoil interface. Root shrinkage and consequent loss of contact with the soil were observed by huck. (1970) in experiments with roots growing in soil along a glass plate. Such experiments are prone to artifacts, in particular regarding resumes soil structure. An alternative approach is based on resin-impregnation and thinsectioning (Veen. 1992; North and Nobel 1997). However, this method does not allow the observation of the temporal evolution of the gaps.
2009 to measurements of water content (Young 1995 structure (Whalley. 2005 wettability (Read. 2003 and mechanical stability (Czarnes. 2000) of the rhizosphere compared to bulk soil; to in-situ infiltration in business the rhizosphere (Hallet. 2003 and to recent observations of unexpected water dynamics in the rhizosphere (Carminati. These studies show several controversies, for example in whether the rhizosphere has increased or decreased water content and conductivity compared to the bulk soil. One conclusion that can be drawn is that the rhizosphere has dynamic and complex characteristics that are not understood sufficiently at present.
In a recent review, Draye. (2010) discussed the relative importance of soil and root hydraulic properties for water availability to plants. Referring to the classic paper by passioura (1980 they stated that when the soil is wet it has little influence on water availability, while when the soil is dry it controls water uptake. When the soil is neither too wet nor too dry, water uptake depends on both soil and plant properties. In this range of soil conditions, the root-soil interface may play an important role. Distinct and unique properties of the root-soil interface have often been invoked in the literature. We 2 refer to several recent reviews on the specific physical, chemical, biological properties of the rhizosphere (Gregory 2006; Hinsinger.
Transpiration rate was monitored by continuously weighing the columns and gas exchange measurements. Responsible Editor: Peter. Electronic supplementary fuller material The online version of this article (doi: /s ) contains supplementary material, which is available to authorized users. Carminati division of soil Hydrology, georg-August Universität Göttingen, büsgenweg 2, göttingen, germany. Vogel soil Physics Department, helmholtz centre for Environmental Research - ufz, halle, germany results Transpiration started to decrease at soil matric potential between 5 kpa and 10 kpa. Air-filled gaps appeared along tap roots between 0 10 kpa and 0 20 kpa. As decreased below 40 kpa, roots further shrank and gaps expanded.1.35. Gaps around lateral roots were smaller, but a higher resolution is required to estimate their size. Conclusions Gaps formed after the transpiration rate decreased.
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1 doi /s regular article do roots mind the gap? Vogel Received: / Accepted: The author(s) This article is published with open access at m Abstract Aims roots need to be in good contact with the soil to take up water and nutrients. However, when the soil dries and roots shrink, air-filled gaps form at the rootsoil interface. Do gaps actually limit presentation the root water uptake, or do they form after water flow in soil is already limiting? Methods four white lupins were grown in cylinders of 20 cm height and 8 cm diameter. The dynamics of root and soil structure were recorded using X-ray ct at regular intervals during one drying/wetting cycle. Tensiometers were inserted at 5 and 18 cm depth to measure soil matric potential.