Rhizosphere soil-water collection by immiscible displacement-centrifugation technique

Progress in determining nutrient availability in the rhizosphere is restricted by a lack of reliable and convenient methods for rhizosphere soil-water collection. A modified centrifugation method with a fluorocarbon (Fluorinert,FC-70) as an immiscible displacement liquid was developed. Our objectives were to: i) obtain an adequate soil-water volume from a small rhizosphere sample within a reasonable time; ii) collect rhizosphere soil-water at container capacity ( 90% of field capacity) to determine soluble soil ions; and iii) evaluate FC-70 as an extractant. The soil used was a Beadle clay loam (fine, montmorillonitic mesic Typic Argiustoll) with low and high levels of CaCO₃ (5 and 204g kg^-1). Soil samples from the rhizosphere of 30-days-old sordan (sorghum (Sorghum bicolor L.), sudangrass (Sorghum sudanese L.) hybrid seedlings were thin-sectioned at 1-, 2- and 3-mm from the root surface. The extraction parameters (sample size, volume of extractant, relative centrifugal force and centrifugation time) were varied to determine optimal values. We obtained adequate amounts of aqueous solutions from moist soil ( 6 g) when mixed with 2 mL of FC-70, packed into a filter unit, and centrifuged for 1 hour at 14,500 × g. The displaced soil-water was analyzed by inductively coupled plasma spectrometry. The modified centrifugation technique with FC-70 offers a reliable, rapid, safe, and contamination-free method for obtaining unaltered soil-water from the rhizosphere, at a moisture content normally found in soil.     

The Joensuu dasotrons: A new facility for studying shoot,root, and soil processes

A new, controlled, environment facility for growing trees was built at Joensuu, Finland, between 1996 and 1998. It consists of four large rooms called dasotrons, with four large root pots in each. Each room is a separate unit, with independent control of air and soil temperature, air humidity and light. The environmental variables can be controlled to simulate conditions ranging from tropical to boreal. The controller set-points can be programmed locally or through a central control system running on a PC. The floor area and height of the rooms allows us to grow small trees (up to 3.7 m height) for several growing seasons. In each dasotron, there are four cylindrical pots with a removable upper section. There are access holes in the walls of the pots for the installation of sensors and minirhizotron tubes. Each pot has a drain, with valves, at the bottom to enable the removal of excess water or the collection of percolate samples. The operation of the facility was tested during one simulated annual growing cycle. During this test period, the dasotrons worked reliably and no systematic differences were found in the environmental conditions or in the growth of Norway spruce seedlings between the dasotrons. This new facility will enable diverse physiological and ecophysiological studies to be carried out on the responses of trees to their below- and above-ground environment.     

A rootbox for quantitative observations on intact entire root systems

A rootbox is described which allows observation of an intact, entire root system. Roots are sandwiched against a plexiglass surface by a nylon mesh that is impermeable to roots, but permeable to water and nutrients. To quantify root growth non-destructively, roots of different size classes are traced onto acetate sheets using different color pens, and root lengths determined by digital image analysis.     

In-situ soil solution chemistry in an acid forest soil as influenced by growing roots of Norway spruce (Picea abies [L.] Karst.)

A new approach for non-destructive monitoring of soil solution chemistry in high spatial and temporal resolution for rhizosphere studies is presented. In a 5×10 mm grid, 30 micro suction cups (1mm) were installed in a rhizotron with Norway spruce (Picea abies [L.] Karst.) growing in low pH B-horizon soil. Roots grew through the grid, closely passing the suction cups. Soil solution composition before, during and after root passage was determined. For K⁺ and Mg²⁺ a significant decrease of soil solution concentration near root tips and elongation zones was observed, indicating a marked uptake of these elements. Mg²⁺ concentration was also significantly lowered when the root system aged, suggesting that this ion might also be taken up in older parts of the root system. No influence of growing roots was found on Na⁺-concentrations.     

Portable rhizotron and color scanner system for monitoring root development

Rhizotrons allow the examination of spatial and temporal in situ root development. Permanent rhizotron installations provide 2-D images of whole root profiles, but their immobility limits the number of soil-plant systems that can be studied. Our objectives were to develop a portable rhizotron and color scanning system for studying the development of whole root systems. Potato root development was monitored in an irrigated experiment at Othello, WA. Covered, rectangular hollow boxes with a transparent glass face were installed perpendicular to planted potato rows, and a seed piece was planted in the soil adjacent to the glass. Rooting in the hill furrow topography was measured at 2 to 4 week intervals. Images of roots growing along the glass face are captured with five scans with a portable, color scanner and a portable computer. Image thresholding discriminated roots from soil using primary color values, color intensity differences, color proportions, or overall intensity. Seasonal patterns of computed root lengths by image analysis were comparable to manual tracing. Primary roots extended to 15 cm from the seed piece prior to shoot emergence, 21 days after planting. Lateral roots began to develop shortly thereafter. Potato roots extended to depths of 60 cm by 4 to 6 weeks after planting, and maximum root density in the hill and furrow was observed by tuber initiation to early tuber bulking. Temporal and spatial trends were similar to previous results using destructive sampling. The method has promise for studying the root growth and development of field-grown plants.     

A rapid quantitative measurement of root length and root branching by microcomputer image analysis

A computer program was made for fast and reliable measurement of root length and for estimating the number of root tips and branching points. Image-processing procedures available in a program package for image analysis by means of a personal computer were used. The method is described in this paper and some results of tests on variance and systematic errors (bias) are discussed.Time required for analysis of an evenly spread root (sub-)sample with a total length of max. 300 cm was reduced to less than 20 seconds. Random deviations from the real length, determined by measuring known lengths of wire, did not exceed 5%, after correction for length density dependent bias. Counts of root tips appeared to be unreliable, but branching ratios could be determined fairly accurately, after correction for the length density dependent number of pseudo-branches (e.g. crossings). Rhizotron root photographs were also analysed satisfactorily, after modification of a few steps in the program.     

Effect of lime-pelleting on the nodulation of lucerne (Medicago sativa L.) in an acid soil: A comparative study carried out in the field,in pots and in rhizotrons

The nodulation of lucerne was studied in soil (pH-H₂O 5.2) with seeds either inoculated with Rhizobium meliloti (R), or inoculated and pelleted with lime (RP). For comparison, experiments were done in the field and in two types of micro-cosmos: pots and rhizotrons. In the field experiments, lime-pelleting improved the establishment of seedlings and augmented the nitrogen yield of the first harvest. These positive responses in plant growth were the consequence of a better nodulation on the upper 10 mm of the seedling tap root. The number of seedlings carrying crown nodules increased from 18% (R) to 56% (RP) at 26 days after sowing.In both, pots and rhizotrons, lime-pelleting also increased crown nodulation: in pots from 32% (R) to 60% (RP), and in rhizotrons from 5% (R) to 90% (RP). Rhizotrons, made of plastic petri dishes, allowed for continuously following of early root developments and nodule formation. Crown nodulation could already be measured after 14 days. Based on these experiments, it was concluded (i) that crown nodulation is an adequate parameter to quantify the benefit of lime-pelleting, and (ii) that rhizotrons, because of the more pronounced effects and shorter incubation time, are more suitable to study the nodulation responses in the soil caused by the addition of rhizobia and lime.     

A study of a population of birches in Glen Gairn

Summary

A small population of birches was studied with a view to measuring the rangeof morphological variation in Betula pendula Roth, and B. pubescens Ehrh. The general morphological features of crown and bark of trees in the area indicated that both species were present. A more detailed study was made of leaf and fruit characters, accepted by other investigators as being useful in discriminating between the species. It was shown that such characters varied continuously throughout the population and no tree was wholly pubescens- like or pendula-like in all characters.

The implications of these results are discussed in relation to the situation in Glen Gairn and to the general taxonomic situation of the British birches.     

A growth chamber for idealized studies of seedling root growth dynamics and structure

A root growth chamber is described which allows seedling root growth dynamics and structure to be monitored continuously under a variety of conditions for several weeks. The chamber consists of two cells with inner dimensions 18×20×0.12 cm. To simulate the soil matrix, each cell was filled with spherical glass beads of 0.1 cm diameter. Given the 0.12 cm width of each cell, the glass bead matrix was approximately one bead layer thick. Roots were therefore grown in a quasi -two-dimensional and transparent environment. This enabled root images of high spatial and temporal resolution to be collected and analysed quantitatively using standard image analysis techniques. The chamber was constructed such that the root environment could be manipulated with regard to nutrient distribution, `soil' matrix structure and other perturbations to the system. Preliminary experiments of the growth dynamics of lentil roots (Lens culinaris L. cv. Verte du Puy) in the chamber were conducted. The majority of the primary and lateral roots followed a similar growth pattern with high growth rates between days 5 and 9 and days 14 and 18 separated by a period of low growth rate between days 10 and 12 after seeding in the chamber. Thus, primary and lateral root growth was to a certain extent synchronized. Lateral roots developed after 3 to 8 days on the outer curve (convex side) of the primary root. The roots shared many of the characteristics of roots developed in three-dimensional systems indicating that the chamber did not induce artificial root behaviour. Thus, the idealized and quantitative studies that can be conducted in the chamber may enable many aspects of the complex interactions between the root system and environment to be studied.