Factors affecting cation-anion uptake balance and iron acquisition in peanut plants grown on calcareous soils

Dicotyledonous plants subjected to Fe-deficiency stress can decrease pH in the rhizosphere by proton excretion and reduce ferric iron by an activated reduction system in the plasma membranes of the root or by reductants released from the roots. The efficiency by which these plants take up Fe may strongly depend on their cation-anion balance. This study presents results of two experiments conducted to evaluate the effect of K, growth stage and cultivar on ionic balance and Fe acquisition of peanut (Arachis hypogaea L.) plants.Potassium applications to the high calcareous soil (30.3% CaCO₃) favoured proton release, but did not ameliorate plant Fe acquisition. At the earliest stages of plant growth, anion uptake exceeded cation uptake due to intensive N uptake. With time, a shift in the ionic balance was observed as a result of predominant cation uptake. It appears that the relationship between H/OH-ion release and Fe nutrition of peanut plants is actually a complex phenomenon under soil conditions and depends on some soil parameters, such as CaCO₃ content. Even by enhanced H-ion release Fe nutrition of plants can be impaired if soil CaCO₃ is too high.     

Growth responses of tropical forage plant species to vesicular-arbuscular mycorrhizae

Summary Growth and mineral uptake of twenty-four tropical forage legumes and grasses were compared under glasshouse conditions in a sterile low P oxisol, one part inoculated and the other not inoculated with mycorrhizal fungi. Shoot and root dry weights and total uptake of P, N, K, Ca, and Mg of all the test plants were significantly increased by mycorrhizal inoculation. Mycorrhizal inoculation, with few exceptions, decreased the root/shoot ratio. Non-mycorrhizal plants contained always lower quantities of mineral elements than mycorrhizal plants. Plant species showed differences in percentage mycorrhizal root length and there was no correlation between percentage mycorrhizal infection and plant growth parameters. A great variation in dependence on mycorrhiza was observed among forage species. Total uptake of all elements by non-mycorrhizal legumes and uptake of P, N and K by non-mycorrhizal grasses correlated inversely with mycorrhizal dependency. Mycorrhizal plants of all species used significantly greater quantities of soil P than the nonmycorrhizal plants. Utilization of soil P by non-mycorrhizal plants was correlated inversely with mycorrhizal dependency.     

Modeling magnesium,phosphorus and potassium uptake by loblolly pine seedlings using a Barber-Cushman approach

The Barber-Cushman mechanistic nutrient uptake model, which has been utilized extensively to describe and predict nutrient uptake by crop plants, was evaluated for its ability to predict K, Mg, and P uptake by loblolly pine (Pinus taeda L.) seedlings. Sensitivity analyses were also used to investigate the impact of changes in soil nutrient supply, root morphological, and root uptake kinetics parameters on simulated nutrient uptake. Established experimental techniques were utilized to define the 11 parameters needed to model uptake by 1-0 seedlings of K, Mg, and P from a modified A horizon soil (Lilly series). Model predictions of K and P uptake over a 180-d growth period were underestimated by 6 and 11%, respectively. Estimates of Mg uptake were underestimated by 62%. While the level of agreement between predicted and observed K and P values was quite acceptable, analysis of parameter values and results of sensitivity analyses both indicated that the model underestimation of Mg uptake was the result of applying an I_max value developed under relatively low Mg concentration to a situation in which the functional I_max would be much higher due to the dominance of passive versus active uptake. Overall results of sensitivity analyses indicate that under the circumstances investigated, I_max, was the primary variable controlling plant uptake of K, Mg, and P. The dominance of this term over others was due to the relatively high C_li values for all three nutrients. Reducing (-50%) or increasing (+ 100%) other soil supply, root morphological, and remaining root uptake kinetics values did not substantially alter model estimates of nutrient uptake.     

Evaluation in field conditions of a three-dimensional architectural model of the maize root system: Comparison of simulated and observed horizontal root maps

Most existing water and nutrient uptake models are based on the assumption that roots are evenly distributed in the soil volume. This assumption is not realistic for field conditions, and significantly alters water or nutrient uptake calculations. Therefore, development of models of root system growth that account for the spatial distribution of roots is necessary.The objective of this work was to test a three dimensional architectural model of the maize root system by comparing simulated horizontal root maps with observed root maps obtained from the field. The model was built using the current knowledge on maize root system morphogenesis and parameters obtained under field conditions. Simulated root maps (0.45 × 0.75 m) of horizontal cross sections at 3 depths and 3 dates were obtained by using the model for a plant population. Actual root maps were obtained in a deep, barrier-free clay-loamy soil by digging pits, preparing selected horizontal planes and recording root contacts on plastic sheets.Results showed that both the number of cross-sections of axile roots, and their spatial distribution characterized with the R-index value of Clark and Evans (1954), were correctly accounted for by the model at all dates and depths. The number of cross-sections of laterals was also correctly predicted. However, laterals were more clustered around axile roots on simulated root maps than on observed root maps. Although slight discrepancies appeared between simulated and observed root maps in this respect, it was concluded that the model correctly accounted for the general colonization pattern of the soil volume by roots under a maize crop.     

Modelling zinc uptake by rice crop using a Barber-Cushman approach

The Barber-Cushman mechanistic nutrient uptake model which has been utilized extensively to describe and predict nutrient uptake by crop plants at different stages of crop growth was evaluated for its ability to predict the Zn uptake by rice seedlings. Uptake of the nutrient is, therefore, determined by the rate of nutrient supply to the root surface by mass flow and diffusion. Inter root competition and time dependent root density are accounted for by soil volume that delivers nutrients. The radii of these cylinders decline with increasing density. Since mass flow and diffusion each supply zinc to the root, the process can be described mathematically using the model of Barber-Cushman (1984). The 11 parameters of the model for the uptake by rice cultivars were measured by established experimental techniques. Zinc uptake at different growth stages predicted by the model was compared to measured zinc uptake by rice cultivars grown on sandy loam soil in a green house. Predicted zinc uptake was significantly correlated with observed uptake r ²=0.99^**. Sensitivity analysis was also used to investigate the impact of changes in soil nutrient supply, root morphological and root uptake kinetic parameters on simulated nutrient uptake. Overall results of sensitivity analysis indicate that the half distance between root axes, rate of root growth and water flux affect the uptake of zinc particularly at their higher values rather than at lower values and DaZn is the most sensitive parameter for zinc uptake at its lower values.     

Nutrient uptake relationship to root characteristics of rice

Data on root parameters and distribution are important for an improved understanding of the factors influencing nutrient uptake by a crop. Therefore, a study was conducted on a Crowley silt loam at the Rice Research and Extension Center near Stuttgart, Arkansas to measure root growth and N, P and K uptake by three rice (Oryza sativa L.) cultivars at active tillering (36 days after emergence (DAE)), maximum tillering (41 DAE), 1.25 cm internode elongation (55 DAE), booting (77 DAE) and heading (88 DAE). Soil-root core samples were taken to a depth of 40 cm after plant samples were removed, sectioned into 5 cm intervals, roots were washed from soil and root lengths, dry weights and radii were measured. Root parameters were significantly affected by the soil depth × growth stage interaction. In addition, only root radius was affected by cultivar. At the 0- to 5-cm soil depth, root length density ranged from 38 to 93 cm cm^-3 throughout the growing season and decreased with depth to about 2 cm cm^-3 in the 35- to 40-cm depth increment. The increase in root length measured with each succeeding growth stage in each soil horizon also resulted in increased root surface area, hence providing more exposed area for nutrient uptake. About 90% of the total root length was found in the 0- to 20-cm soil depth throughout the season. Average root radius measured in the 0- to 5-cm and 35- to 40-cm depth increments ranged from 0.012 to 0.013 cm and 0.004 to 0.005 cm, respectively throughout the season. Total nutrient uptake by rice differed among cultivars only during vegetative growth. Differences in total nutrient uptake among the cultivars in the field appear to be related to absorption kinetics of the cultivars measured in a growth chamber study. Published with permission of the Arkansas Agricultural Experiment Station.     

Soil solution phosphate,root uptake kinetics and nutrient acquisition: implications for a patchy soil environment

Summary The importance of increased root phosphate (P) uptake kinetics, root proliferation and local increases of soil solution P (P₁) for P acquisition from fertile soil microsites was explored with a simulation model and calculated uptake was compared with experimental data. Based on the partitioning of added P in microsites to P₁ and P adsorbed on soil particles and the results of a dual-isotope-labeling experiment (Caldwell et al. 1991a), acquisition of P from the fertile microsites was some 20 X that of uptake from an equal volume of soil which received only water. Simulations were in general agreement and also showed that elevation of root P uptake kinetics could contribute more to the increased acquisition than did root proliferation under these circumstances. Although increased physiological uptake capacity for P has generally been considered to be of little benefit because of diffusion limitation, in patchy soil environments selective elevation of P uptake kinetics in fertile microsites may be of considerable benefit. These tests were conducted in calcareous soil which releases much less P into the soil solution than do many other soils. In many noncalcareous soils the benefits of selective elevation of root uptake kinetics would likely be greater.     

Influence of Nitrogen Supply and Water Stress on Growth and Nitrogen, Phosphorus, Potassium and Calcium Contents in Pearl Millet

Influence of supra-optimal concentrations of N on growth and accumulation of N, K, P and Ca in the shoots and roots in Pennisetum glaucum (L.) R.Br. under water stress was assessed in a pot experiment under glasshouse conditions. Thirty four-day-old plants of two lines, ICMV94133 and WCA-78, were subjected to 224, 336, or 448 mg(N) kg^–1(soil) and soil moisture 100 or 30 % of field capacity for 30 d. Increasing soil N supply decreased growth of both lines under water deficit. Nitrogen content in the shoots of both lines was not affected by supra-optimal levels of N or different watering regimes, but in contrast, the root N content was increased consistently in WCA-78 with increase in soil N content. Shoot P content increased considerably in WCA-78 at the two higher N contents, but it was significantly lower at drought stress than at well-watered treatment. In contrast, shoot or root P content in ICMV94133 did not differ under both watering regimes. Potassium content in the shoots of WCA-78 was considerably increased at the two higher N contents under drought conditions. Root K content was increased in WCA-78 at the highest N content under well-watered conditions, whereas the reverse was true in ICMV94133. Calcium content in the shoots of ICMV94133 was higher under drought stress compared with that at well-watered conditions, but such pattern was not observed in WCA-78. However, root Ca content increased in both lines with increase in N supply.     

Effects of belowground grazing by collembola on growth,mycorrhizal infection,and P uptake of Geranium robertianum

We hypothesized that the grazing of vesicular-arbuscular mycorrhizal (VAM) hyphae by soil animals could be responsible for the lack of a direct relationship between mycorrhizal infection intensity and nutrient uptake under field conditions. To test this hypothesis, we determined the effect of a range of densities of the collembola, Folsomia candida, on growth, VAM infection, and P uptake in Geranium robertianum, a common forest herb, under greenhouse conditions. Total and aboveground growth were greater at low collembola density than either at higher collembola density or without collembola. These differences were greater when the plants were grown in a high organic content soil mix than when grown in sand. Root mass was not affected by collembola density. In the soil mix, root length decreased with increasing collembola density, but not in the sand. The percent of root length infected with VAM was lower at any collembola density than when collembola were absent. Total infected root length decreased linearly with increasing collembola density. Few significant differences in P uptake or tissue concentration were found. Thus, plant growth (but not P uptake) may be stimulated at low collembola density and inhibited at high. We discuss mechanisms which may be responsible for this non-linear response, and the implications of the pattern of response to studies of plant competition, nutrient turnover, and revegetation.     

Effect of soil moisture and phosphate level on root hair growth of corn roots

Summary Root hairs have been shown to enhance P uptake by plants growing in low P soil. Little is known of the factors controlling root hair growth. The objective of this study was to investigate the influence of soil moisture and P level on root hair growth of corn (Zea mays L.). The effect of volumetric soil moistures of 22% (M₀), 27% (M₁), and 32% (M₂) and soil (Raub silt loam, Aquic Argiudoll) P levels of, 0.81 (P₀), 12.1 (P₁), 21.6 (P₂), 48.7 (P₃), and 203.3 (P₄) mol P L^–1 initially in the soil solution, on shoot and root growth, P uptake, and root hair growth of corn was studied in a series of pot experiments in a controlled climate chamber. Root hair growth was affected more by soil moisture than soil P. The percentage of total root length with root hairs and the density and length of root hairs on the root sections having root hairs all increased as soil moisture was reduced from M₂ to M₀. No relationship was found between root hair length and soil P. Density of root hairs, however, was found to decrease with an increase in soil P. No correlation was found between root hair growth parameters and plant P content, further suggesting P plays a secondary role to moisture in regulating root hair growth in soils. The increase in root hair growth appears to be a response by the plant to stress as yield and P uptake by corn grown at M₀ were only 0.47 to 0.82, and 0.34 to 0.74, respectively, of that measured at M₁ across the five soil P levels. The increase in root hair growth at M₀, which represents an increase of 2.76 to 4.03 in root surface area, could offset, in part, the reduced rate of root growth, which was the primary reason for reduced P uptake under limited soil moisture conditions.Journal Paper No. 10,066 Purdue Univ. Agric. Exp. Stn., W. Lafayette, IN 47907. Contribution from the Dep. of Agron. This paper was supported in part by a grant from the Tennessee Valley Authority.     

Der Einfluss der K-Konzentration der Nährlösung auf die Ertragsbildung,die Qualität und den K-Aufnahmeverlauf bei Hafer

Summary 1. In solution culture with oats (Avena sativa) the effect of the K concentration of the nutrient solution on yield and K uptake rates has been investigated. The K concentration of the different treatments were: 0.1, 0.3, 0.9 and 2.7 me K/l. These concentrations were kept constant during the whole growing period. 2. The growth of the plants in all treatments was normal. Even with the lowest K concentration no visible symptoms of K deficiency occurred. The highest grain yield was obtained with a K concentration of 0.9 me K/l. This yield was about 40% higher than the yield with the lowest K concentration and about 5% higher than the grain yield with the highest K concentration. 3. The increasing K supply had a favorable effect on the fat and crude protein content of the grains. The highest K concentration (2.7 me K/l) resulted in the highest content of crude protein (20%) and of fat (6.2%). 4. The rates of K uptake throughout the growing period differed considerably. The highest rates were measured during the shooting stage and the blossom stage. The K concentration of the nutrient solution influenced the uptake rates significantly. High yields were obtained if the K uptake at the beginning of the blossom stage was higher than 50% of the total K uptake. 5. The optimal K concentration for the yield performance, 0.9 me K/l, found in this experiment cannot be compared with K concentrations of the soil solution under field conditions without reservations. Under field conditions root growth and the contact surface between soil solution and root surface are different. Assuming that under field condition this contact surface nutrient solution/root surface is not larger than in solution culture, also a K concentration of 1 me K/l of the soil solution should meet the K demand of the plants, necessary for the production of high grain yields.      

Simulation of nutrient uptake by a growing root system considering increasing root density and inter-root competition

A simulation model is presented which describes uptake of a growth limiting nutrient from soil by a growing root system. The root surface is supposed to behave like a zero-sink. Uptake of the nutrient is therefore determined by the rate of nutrient supply to the root surface by mass flow and diffusion. Inter-root competition and time dependent root density are accounted for by assigning to each root a finite cylindrical soil volume that delivers nutrients. The radius of these cylinders declines with increasing root density. Experiments with rape plants grown on quartz sand were used to evaluate the model. Simulated nitrogen uptake agreed well with observed uptake under nitrogen limiting conditions. In case no nitrogen limitation occurred nitrogen uptake was overestimated by the model, probably because the roots did not behave like a zero-sink any more.     

Root growth and water uptake in winter wheat under deficit irrigation

Root growth is critical for crops to use soil water under water-limited conditions. A field study was conducted to investigate the effect of available soil water on root and shoot growth, and root water uptake in winter wheat (Triticum aestivum L.) under deficit irrigation in a semi-arid environment. Treatments consisted of rainfed, deficit irrigation at different developmental stages, and adequate irrigation. The rainfed plots had the lowest shoot dry weight because available soil water decreased rapidly from booting to late grain filling. For the deficit-irrigation treatments, crops that received irrigation at jointing and booting had higher shoot dry weight than those that received irrigation at anthesis and middle grain filling. Rapid root growth occurred in both rainfed and irrigated crops from floral initiation to anthesis, and maximum rooting depth occurred by booting. Root length density and dry weight decreased after anthesis. From floral initiation to booting, root length density and growth rate were higher in rainfed than in irrigated crops. However, root length density and growth rate were lower in rainfed than in irrigated crops from booting to anthesis. As a result, the difference in root length density between rainfed and irrigated treatments was small during grain filling. The root growth and water use below 1.4 m were limited by a caliche (45% CaCO₃) layer at about 1.4 m profile. The mean water uptake rate decreased as available soil water decreased. During grain filling, root water uptake was higher from the irrigated crops than from the rainfed. Irrigation from jointing to anthesis increased seasonal evapotranspiration, grain yield, harvest index and water-use efficiency based on yield (WUE), but did not affect water-use efficiency based on aboveground biomass. There was no significant difference in WUE among irrigation treatments except one-irrigation at middle grain filling. Due to a relatively deep root system in rainfed crops, the higher grain yield and WUE in irrigated crops compared to rainfed crops was not a result of rooting depth or root length density, but increased harvest index, and higher water uptake rate during grain filling.     

Potassium uptake by plants,as characterized by root density,species and K/Rb ratio

Summary A small fraction of the plant K requirement is attained by root interception. The bulk of K has to be transported to the growing roots by mass-flow and diffusion in which diffusion mechanism plays the major role. Studies were undertaken to evaluate soil and plant parameters that might have influence on K supply mechanisms in soil and on plant uptake of K. Increasing wheat plant density led to competition for K absorption and resulted in lower K uptake by plant. In high plant density treatment, about 60% of the K requirement was met by diffusion process whereas in low plant density treatment mass-flow contributed most of the K demand. Solution diffusion and mass-flow were the major mechanisms of K supply to wheat roots. The mechanism of K supply to wheat root was compared with corn and onion. The major mechanism of K supply to corn and onion roots was exchange and solution diffusion. The mechanism of K supply to different crop species is attributable to differences in the K requirements, water flux rates and to the differences in root parameters.     

Mineralization of nitrogen from15N labelled fungi,soil microbial biomass and roots and its uptake by barley plants

The effect of vesicular-arbuscular mycorrhizal fungi (VAM) on field bean and spring wheat dry matter production and on phosphorus, zinc, copper, iron and manganese uptake was determined under greenhouse conditions. Nutrient availability was varied by using different sizes of pots and by diluting the soil with sand. VAM increased plant dry matter production under all sets of growth conditions. VAM were found to directly increase the uptake of P, Zn, Cu and Fe by field beans and of P and Zn for wheat in both experiments. Increased uptake of the other nutrients measured was attributed to increased dry matter production or other factors. The effect of VAM decreased as the pot size holding the host plants decreased, but was not affected by the ratio of soil to sand if the pot size was kept constant. Nutrient uptake by beans as a proportion of total amount of nutrient present increased as the amount of nutrient decreased. Increase in root-soil contact and altered chemical equilibria are probable reasons for increased efficiency of nutrient uptake by beans as the level of available nutrient decreased. For wheat, which has a relatively fibrous root system, decreasing the nutrient availability had minimal effects on nutrient uptake in these experiments. Increases in total uptake of a particular nutrient resulting from inoculation with VAM are not necesarily indicative of a direct uptake of that nutrient by the VAM.     

The difference of potassium dynamics between yellowish red soil and yellow cinnamon soil under rapeseed (Brassica napus L.)–rice (Oryza sativa L.) rotation

To increase the use efficiency of potassium (K) fertilizer, special attention was paid to the dynamics of soil K in the root zone and non-root zone. Difference in K dynamics between yellowish red soil and yellow cinnamon soil under rapeseed (Brassica napus L.)rice (Oryza sativa L.) rotation was studied using a rhizobox system. Results showed that soil water soluble K (Sol-K) and exchangeable K (Ex-K) in the root zone of both soils were reduced in the early stage of rapeseed growth. Along with plant growth and K uptake, soil Sol-K in the inner (0–20 mm to root zone), middle (20–40 mm) and outer (40–60 mm) compartments of the non-root zone of yellowish red soil migrated towards the root zone. As a result, soil Ex-K was transformed into Sol-K. The changes in soil Sol-K and Ex-K in the non-root zone of yellow cinnamon soil were similar to yellowish red soil, and soil non-exchangeable K (Nonex-K) in the root zone also decreased significantly. In the early stage of rice growth, waterlogging promoted diffusion of soil Sol-K from non-root zone to root zone and transformation of Ex-K into Sol-K. Along with the growth of rice and K uptake, soil Ex-K in each compartment of yellowish red soil decreased significantly. Soil Sol-K and Ex-K in the yellow cinnamon soil declined to a certain extent, and then remained unchanged, while soil Nonex-K kept on decreasing. It revealed that the plants first absorbed K in the root zone, of which K reserve was replenished by a gradual diffusion of K from the non-root zone. The closer to the root zone, the greater the contribution to K uptake by plants. Within one rotation cycle, Ex-K and Sol-K in yellowish red soil were the main forms of K available to the plants, and little Nonex-K could be absorbed. However, in the yellow cinnamon soil, Nonex-K was the main form of K available to the plants, followed by Ex-K and Sol-K.     

Rice cultivar evaluation for phosphorus use efficiency

Phosphorus deficiency is one of the most growth-limiting factors in acid soils in various parts of the world. The objective of this study was to screen 25 rice cultivars (Oryza sativa L.) at low, medium, and high levels of soil P. Number of tillers, root length, plant height, root dry weight and shoot dry weight were related to tissue P concentrations, P uptake and P-use efficiency. Shoot weight was found to be the plant parameter most sensitive to P deficiency. Significant cultivar differences in P use efficiency were found. Phosphorus use efficiency was higher in roots than shoots and decreased with increasing levels of soil P. Positive correlations were found among growth parameters such as plant height, tillers, root and shoot weight, and P content of roots and shoots. These results indicate selection of rice cultivars for satisfactory performance under low P availability can be carried out using shoot and root dry weight as criteria.     

Root dynamics of Melaleuca halmaturorum in response to fluctuating saline groundwater

Melaleuca halmaturorum is a salt and waterlogging tolerant tree and thus often occurs in saline areas fringing permanent wetlands and in ephemeral swamps. The dominance of this tree in natural groundwater discharge areas may result in M. halmaturorum transpiration making a major contribution to groundwater discharge. To quantify this the seasonal changes in tree water sources in response to fluctuating soil salinity and waterlogging were examined. This study was conducted in a natural system where seasonally fluctuating saline groundwater (64 dS m^–1; 0.3–1.2 m deep) allowed the patterns of M. halmaturorum root water uptake to be followed over a 15 month period. Tree water sources were examined using the naturally occurring stable isotopes of water, while new root growth was examined using a field root observation window and from soil cores. The presence of isotopic fractionation of ²H under conditions of soil salinity and waterlogging was tested in a glasshouse experiment. Measurements of soil and leaf water potential were also made to examine the possible water sources and limits to water uptake. No isotopic fractionation was found by tree roots under conditions of salinity and waterlogging. M. halmaturorum trees were active in taking up groundwater at most times and combined this with a shallower soil water source replenished by rainfall in winter. Water uptake was concentrated in the deeper parts of the soil profile when the groundwater was at its deepest and salt had accumulated in the surface soils, at the end of summer. When groundwater rose, at the end of winter, roots responded by extracting water from near the soil surface (0–0.1 m), at the new watertable. This pattern of water uptake in response to groundwater fluctuations and salt accumulation in the surface soil was also reflected in new root tip appearance at the root observation window. Fluctuations in leaf water potential fallowed fluctuations in surface soil (0.1 m depth) water potential at all times. In winter leaf water potential reflected the absolute values of the surface soil water potential but in summer it was between surface soil and groundwater water potentials. We conclude that M. halmaturorum used groundwater in summer and a combination of rainfall and groundwater from the surface soils in winter. The ability to take up water from saline substrates through the maintenance of low leaf water potential, combined with this ability to rapidly alter root water uptake in response to changes in soil water availability contributed to the survival of M. halmaturorum in this saline swamp.     

Genotypic differences in root penetration ability of wheat through thin wax layers in contrasting water regimes and in the field

A significant proportion of arable land in south-western Australia is highly susceptible to subsoil compaction, which limits access of roots of wheat to water and nutrients at depth. Genotypic variation in the ability of roots to penetrate a hardpan has been reported for other cereals, using a pot technique, where a thin wax-layer of paraffin wax and petroleum jelly is placed in a soil column to simulate a hardpan. Previously we have modified and validated this technique for measuring root penetration ability of wheat seedlings under contrasting water regimes. Here we report on a series of five experiments (runs), two in well-watered and three in drought stress conditions, which evaluated seminal and nodal root penetration ability through thin wax layers among 24 Australian wheat cultivars and breeding lines (entries). These results were compared with observations on their rooting depths in two contrasting soil types in field trials, including a sandy duplex that contained a hardpan and a red clay that increased in soil strength with depth. Nodal roots ceased growth early under soil water deficit, and water uptake was instead dependant on seminal roots under conditions imposed in the pots. Plants were then reliant on the ability of seminal roots to penetrate the wax layer. Eight entries had superior root penetration ability in both well-watered and drought stressed conditions. Roots of three other entries, which failed to penetrate the wax layers, died under drought stress conditions. In field trials, there was a significant interaction between site and entry for maximum root depth. Our results from the pot studies and field trials indicate that there exists genotypic variation in root traits that are required to penetrate uniformly hard soil, dry soil or soil containing a hardpan. As four of the eight superior entries also showed superior root penetration ability at both sites in the field, there was an overall consistency, but there were exceptions at individual field sites. Factors likely to result in such exceptions were discussed, and topics for further research identified.     

Soil potassium mobility and uptake by corn under differential soil moisture regimes

This study examined the effects of soil moisture on soil K mobility, dynamics of soil K, soil K fixation, plant growth and K uptake. A pot experiment, with and without corn (Zea maysL.), was conducted over a 16-d duration using a Yolo silt loam treated with two soil moisture regimes, i.e. constant moisture vs. wetting–drying (W–D) cycles. Soil K dynamics were determined using both ion exchange resin and direct extraction of soil solution. Soil K mobility increased significantly with soil moisture content (θ_v) and there was a positive curvilinear relationship between θ_v and effective diffusion coefficient (D_e), suggesting that more K⁺ can diffuse to the plant roots at sufficient soil moistures. Increase in D_e could be attributed to the decrease of impedance factor. During W–D cycles, soil solution K concentration increased as soil solution volume decreased, but soil solution K and NH₄ ⁺-extractable K pools decreased. In the constant moisture regime, available K pools decreased over the 16-d duration, but to a lesser extent than in W–D regime. The W–D cycles significantly enhanced K fixation and reduced available K pools in the soil in contrast to the constant moisture regime. Potassium fixation by the soil showed a biphasic pattern under the W–D regime, with a rapid fixation within the first 2 d after re-wetting, followed by a slower fixation. In the soil with constant moisture, K fixation was rapid during the first 8 h after wetting the soil, and then proceeded so slowly that no significant K fixation was observed after 4 d. The W–D cycles decreased root and shoot growth and K uptake by corn compared to constant moisture condition. Our results support the hypothesis that W–D cycles enhance soil K fixation, reduce soil K mobility and plant growth, and therefore reduce plant K⁺ uptake. This revised version was published online in June 2006 with corrections to the Cover Date.