The root length density (RLD) distribution in the soil is a key factor for water and nutrient uptake but its direct assessment is not easy. A field method is presented here to assess RLD of sorghum (Sorghum bicolor L. Moench) from the root intersection density (RID) and taking root orientations into account. The method was developed with four varieties cropped in 2006 in Bambey (Senegal) on a sandy soil, and validated on the basis of independent data obtained on two other varieties cropped in 2008 in Thiès in Senegal on a sandy loam soil. Sets of 1 × 10^-3 m³ cubic undisturbed soil samples were collected at different positions and at three different dates for the four varieties during the first experiment. RIDs of thick and fine roots were assessed on the sides of the soil cubes and RLDs were measured in the volumes. Fine roots appeared to be isotropic when thick roots were horizontal near the surface and gradually became vertical in deeper horizons. RLD was tenfold lower for thick roots than for fine ones. This led to the development of a model to determine RLD from RID. The relationship for all roots was modeled as RLD = RID × CO, where the root orientation coefficient (CO) is equal to 1.97, indicating that the global root distribution is almost isotropic (CO = 2 for isotropic conditions). The model was validated on the basis of independent data from the second experiment. This model enables RLD mapping and description of RLD variability on sorghum from RID observations on trench profiles (with 10 × 10 cm or 5 × 5 cm surface area used for RID measurement).

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Recent findings on the functions of roots from the chemical and biological standpoints have strengthened the need for research on the mechanisms of root growth by physical and mathematical approaches. This study was aimed at developing a root growth model, based on dynamic interactions between the root and its ambient environment. As a tool for modelling, we employed the discrete element method (DEM) with the assumption that root growth could be expressed in terms of dynamic interactions between growing root elements and soil elements. Using a corresponding time scale, the results of a DEM simulation indicated a sufficiently accurate shape of root elongation, which could be verified by a root growth experiment. Moreover, a comparison of fractal dimensions from DEM results and experimental results showed almost similar values of 1.0, which is the dimension of a line.

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Kurume azaleas (Rhododendron Kurume group) are susceptible to environmental stresses such as drought, high temperatures, and soil physical and chemical conditions. We inferred that this susceptibility depends largely on their rooting potential. The rooting rate, the number of new roots, and root regenerating potential (RRP) obtained by incubating lignified root cuttings deteriorated remarkably or reduced during June and August. During summer, rooting did not respond to 1-naphtyleacetic acid (NAA) application. Reserve carbohydrates in the xylem tissue almost disappeared during June and August. These facts suggest that the rooting potential of root systems was distinctively low during summer. New root formation was inferred to be dependent on physiological conditions of parental roots, not only on external stresses, such as drought and high temperatures. In Kurume azalea, seasonal variation in rooting potential might be a major factor determining damage by environmental stresses.

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In leguminous plants, infection with rhizobia leads to the formation of root nodules that fix atmospheric nitrogen and supply it as ammonium to the host plant cells. The formation of nitrogenfixing nodules is nutritionally beneficial, but excessive nodule production appears to be detrimental to the host legumes because of the resultant over-consumption of photosynthetic products. Therefore, the number of root nodules in leguminous plants is tightly regulated by shoot-root signaling known as a feedback or autoregulation of nodulation. Recently, we found that foliar application of polyamines and an effective inhibitor of polyamine biosynthesis regulates the number of nodules in soybean plants. In this review, we discuss the possible function of polyamines as a systemic regulator of nodule formation.

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A lengthy cavity usually forms in the vascular cylinders of pea (Pisum sativum) primary roots in response to sudden flooding at 25^oC. This is thought to be a form of aerenchyma. Ethylene has been shown to mediate inducible aerenchyma in maize, therefore the role of ethylene in the formation of cavities in pea roots was examined. Pea seedlings grown for 4 d in 2 L beakers in vermiculite moistened below field capacity - conditions that do not favor cavity formation - were flooded with solutions containing ethylene inhibitors (AOA, EGTA, and STS). Pea seeds were germinated and grown in suitable containers (0.8-1.0 L) for 4 d at 25^oC in the dark in similar vermiculite. These were then exposed to various concentrations of ethylene for 1 d, or they were flooded and endogenous ethylene was measured periodically by gas chromatography. Observations of roots exposed to exogenous ethylene were made by light microscope. All three inhibitors of ethylene suppressed cavity formation in flooded roots. Exogenous ethylene exposure caused cavities to frequently form in a dose-dependent manner in unflooded roots and caused an increase in mean cortical cell size and number. Flooding increased the rate of ethylene release into the air space above the medium surface. These results indicate ethylene mediates vascular cavity formation and add to the evidence that vascular cavities are the result of programmed cell death and may function as a type of aerenchyma.

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A unique integrative analysis is proposed to monitor changes in the developmental processes of a particular cell type in the root, i.e. the rates of cell differentiation, production, and elongation. As a model case, effects of exogenous ethylene on differentiation, division and elongation of endodermal cells were analyzed in maize primary roots. The distance from the lowest position of the Casparian strip, a morphological marker for endodermal cell differentiation, to the root tip decreased in response to ethylene in a dose-dependent manner. The endodermal cell flux in a single cell file, an indicator for cell division, decreased in response to ethylene, although the estimated time required for an individual cell to complete the formation of the Casparian strip, as an indicator for the cell differentiation rate, did not. As indicators for cell elongation, the mature cell length did not change in the presence of ethylene, although the period for cell elongation increased. The Casparian strip formed after the endodermal cell had ceased elongation, irrespective of the presence of ethylene.

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Adventitious (crown) roots account for the majority of the root system of monocots. It is reported that auxin plays an important role in the formation of crown roots, but the underlying molecular mechanisms are still unknown. We characterized a rice (Oryza sativa L.) mutant crown rootless4 (crl4) that was found to have defective crown root formation. Besides reduced crown root number, the crl4 mutant showed auxin-related abnormal phenotypical character-istics such as reduced lateral root number and impaired root gravitropism. CRL4 encodes a protein highly homologous with Arabidopsis GNOM, which mediates auxin-dependent plant growth by coordinating the polar localization of auxin efflux carrier PIN1. In the crl4 mutant, auxin transport was impaired in shoots and roots. Besides, the GUS staining controlled by DR5 promoter in the node of crl4 mutant was fainter and spread wider than that of wild-type. These results indicate that maintaining an appropriate auxin accumulation and gradient through CRL4 in the basal portion of shoots is essential for crown root formation in rice.

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In order to achieve continuous monitoring of the rhizosphere, we developed a simple method that uses a wide-view optical scanner inserted into the ground. The scanner system facilitates the analysis of image data and allows continuous monitoring by automating the capture and by fixing the position of the optical scanner. The system was tested in the laboratory and installed under CO₂ measurement system in the field. Clear images were obtained, suggesting the possibility of analyzing live-dead-decomposition root dynamics continuously.

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Effects of sucrose and cytokinin (trans-zeatin riboside [t-ZR]) on the formation of tuberous root of sweetpotato (Ipomoea batatas (L.) Lam.) were studied by directly applying these substances to young roots. In the roots that originally contained approximately 9 μg of endogenous sucrose per milligram of fresh weight (FW), further increase in the sucrose level did not necessarily result in tuberous root formation, whereas t-ZR application invariably induced tuberous root formation. However, t-ZR did not induce morphogenesis when the sucrose level in the root was lowered to approximately 2 μg mg^-1 of FW. Our results imply that high levels of endogenous sucrose are necessary for tuberous root formation and that in the presence of high sucrose concentrations, cytokinins can trigger tuberous root formation.

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We present a simple procedure for measuring the pH in the apoplast of the rice root, using the fluorescent ratio method and Oregon Green 488 (OG). Staining with 5 μM OG for 3 min followed by 5-min washing with calcium solution was an effective method for measuring the pH of the apoplast at the root surface. In order to prevent the diffusion of OG from the apoplast, the circulation of a low concentration of OG (0.02 μM) in a root vessel was found to be a useful modality. However, aluminum ion disturbed pH measure-ment. In the presence of 50 μM AlCl₃, the pH value was inaccurate at the pH rage higher than pH 5.0.

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