Modelling potassium uptake by wheat (Triticum aestivum) crops

Spring wheat was grown in the field under deficient and sufficient levels of soil K and with high and low supplies of fertiliser nitrogen. Measurements were made of K uptake, soil nutrient supply parameters, root growth and, in solution culture, root influx parameters. Mechanistic models predicted uptake reasonably well under K-deficient conditions, but over-predicted uptake, by as much as 4 times, under K-sufficient conditions. The over-prediction was apparently due to poor characterisation of plant demand.     

Agarose as a suitable substrate for use in the study of Al dynamics in the rhizosphere

Because of experimental difficulties, few authors have studied the dynamics of aluminium in the rhizosphere. The aim of this paper is to present a suitable method for studying rhizosphere Al dynamics. It is based on the use of agarose as a substrate for plant growth. Agar and agarose gels are often used in rhizosphere studies, but most are poorly characterized and occasionally give rise to experimental artefacts, especially with low mobility elements like Al. The results reported here show that agarose is a relatively pure substrate, nearly devoid of phosphorus and other Al-complexing substances. Aqueous extracts of agarose also exhibit Al phytotoxicity equivalent to that of a nutrient solution. Since this substrate has the properties of a variable charge exchange complex, it can be considered as a physico-chemical model for organic matter. Finally, its Al adsorption capacity is high enough for the Al reserve in the substrate not to exert a limiting effect on plants and low enough to allow accurate measurement of Al depletion in the rhizophere.     

Relationships between dynamics of nitrogen uptake and dry matter accumulation in maize crops. Determination of critical N concentration

The concept of critical nitrogen concentration(%N _c) has been proposed as the minimum%N in shoots required to produce the maximum aerial biomassat a given time. Several authors have shown that%N _c declines as a function of aerial biomassaccumulation (W) and the %N _c –W relationship has been proposed as a diagnostic tool of N statusin different crops, excluding maize. From data obtained in five nitrogenfertilisation experiments in irrigated maize crops, 26 critical data-pointswere selected with a precise statistical procedure. An allometric relationwas fitted and a critical %N−W relationshipmodel is proposed in maize as: If W < 1 t ha^-1%N _c = 3.40 If 1 t ha^-1≤ W ≤ 22 t ha^-1%N _c = 3.40(W)^−0.37 The model is applicable to maize crop development between emergenceand silking + 25 days. The model was tested and validated with dataobtained in a network of 17 N fertilisation experiments conducted inFrance under contrasting pedoclimatic conditions. In only nineout of 280 data-points (3.2%), the plant N status was mispredictedwhen ±5% error around %N _c wasallowed. A critical N uptake model (Nu_c, kg Nha^-1) is proposed as Nu_c = 34 (W)^0.63 A comparison between Nu_c and N uptake observedin N treatments giving the maximal grain yields has shown that maizecrops assimilate at least 30 kg N ha^-1 in a storage N poolat the silking stage. The significance of the critical%N−W and Nu−W relationships is discussed in relation to theoretical models proposed inwhole plant ecophysiology. Different relationships calculated betweenleaf area index and aerial biomass accumulation, and between N uptakeand leaf area were consistent with previous results for other crops.This strengthens the interest of the critical%N−W relationship for use as diagnostictool of nitrogen status in maize crops. This revised version was published online in June 2006 with corrections to the Cover Date.     

Manganese reduction in the rhizosphere of mycorrhizal and nonmycorrhizal maize

The influence of rhizosphere microorganisms and vesicular-arbuscular (VA) mycorrhiza on manganese (Mn) uptake in maize (Zea mays L. cv. Tau) plants was studied in pot experiments under controlled environmental conditions. The plants were grown for 7 weeks in sterilized calcareous soil in pots having separate compartments for growth of roots and of VA mycorrhizal fungal hyphae. The soil was left either uninoculated (control) or prior to planting was inoculated with rhizosphere microorganisms only (MO-VA) or with rhizosphere microorganisms together with a VA mycorrhizal fungus [Glomus mosseae (Nicol and Gerd.) Gerdemann and Trappe] (MO+VA). Mycorrhiza treatment did not affect shoot dry weight, but root dry weight was slightly inhibited in the MO+VA and MO-VA treatments compared with the uninoculated control. Concentrations of Mn in shoots decreased in the order MO-VA > MO+VA > control. In the rhizosphere soil, the total microbial population was higher in mycorrhizal (MO+VA) than nonmycorrhizal (MO-VA) treatments, but the proportion of Mn-reducing microbial populations was fivefold higher in the nonmycorrhizal treatment, suggesting substantial qualitative changes in rhizosphere microbial populations upon root infection with the mycorrhizal fungi. The most important microbial group taking part in the reduction of Mn was fluorescent Pseudomonas. Mycorrhizal treatment decreased not only the number of Mn reducers but also the release of Mn-solubilizing root exudates, which were collected by percolation from maize plants cultivated in plastic tubes filled with gravel quartz sand. Compared with mycorrhizal plants, the root exudates of nonmycorrhizal plants had two fold higher capacity for reduction of Mn. Therefore, changes in both rhizosphere microbial population and root exudation are probably responsible for the lower acquisition of Mn in mycorrhizal plants.     

The contribution of different regions of the seminal roots of wheat to uptake of nitrate from soil

A technique was developed to determine the physiological activity of defined sections of seminal roots of wheat grown in sand. Wheat plants were grown for 2 weeks in narrow columns of N-deficient sand to which all other nutrients had been added. The columns were split longitudinally and ¹⁵N-labelled nitrate, in an agar medium, supplied to 2 cm sections of root. Shoots and roots were analysed after 24 h to determine the uptake of ¹⁵N. Three sections were examined on either the secondary or tertiary seminal root: 1 cm from the seed (basal segment), 35 cm from the seed (middle segment) and 4 cm from the root apex (apical segment). Total uptake was greatest from the basal and middle segments, declining by 50% from the apical segment. However, uptake per unit root length, including exposed sections of lateral roots, was not significantly different along the root.     

A coupled model of photosynthesis-transpiration based on the stomatal behavior for maize (Zea mays L.) grown in the field

The study presents a theoretical basis of a stomatal behavior-based coupled model for estimating photosynthesis, A, and transpiration, E. Outputs of the model were tested against data observed in a maize (Zea mays L.) field. The model was developed by introducing the internal conductance, g _ic, to CO₂ assimilation, and the general equation of stomatal conductance, g _sw, to H₂O diffusion, into models of CO₂ and H₂O diffusion through the stomata of plant leaves. The coupled model is easier for practical use since the model only includes environmental variables, such as ambient CO₂ concentration, leaf temperature, humidity and photosynthetic photon flux received at the leaves within the canopy. Moreover, concept of g _ic, and factors controlling A and E were discussed, and applicability of the model was examined with the data collected in the maize field.     

The Wageningen Rhizolab — a facility to study soil-root-shoot-atmosphere interactions in crops

Roots in the Wageningen Rhizolab are observed using two methods: (i) non-destructively, using horizontal, glass minirhizotrons at intervals of 14 days between observations; (ii) with destructive sampling using augers on three dates in the season. This paper reports changes with depth and time in root numbers per unit interface area of the minirhizotron tube (number of intersections) of four crop species (wheat, Brussels sprouts, leek and potato). The number of root intersections of Brussels sprouts, wheat and potato declined with depth at any time, whereas leek showed a different pattern because maximum root growth was observed at a depth of 10–20 cm. Root density generally decreased in the following order: Brussels sprouts, wheat, potato and leek. Plots of root length densities, L_rv(cm. cm^-3), obtained by auger sampling, versus the number of intersections showed considerable variation in slope with species, time in the season and year, implying that a single, universal equation to convert minirhizotron observations into volumetric root densities does not exist. Causes of variation in the slopes are discussed. It is concluded that limited auger sampling combined with minirhizotron observations yield adequate quantitative estimates of relevant root properties.     

Plant and bacterial mucilages of the maize rhizosphere: Comparison of their soil binding properties and histochemistry in a model system

Mucilages from the root tips of axenically-grown maize and from a bacterium (Cytophaga sp.) isolated from the rhizosheaths of field-grown roots, were immobilized by drying onto nylon blotting membrane. The mucilage plaques remained in place through repeated rewettings and histochemical treatments. Staining of the plaques showed that both mucilages included acidic groups, and 1,2 diols (the latter notably fewer in bacterial mucilage). Bacterial mucilage plaques stained strongly for protein, plant mucilage was unstained. Plaques of both mucilages bound soil particles strongly if soil was applied to wet mucilage and then dried. Bound soil was not lost with rewetting. Dry weight and densitometer measurements showed that bacterial mucilage bound about 10% more soil than the same surface area of root-cap mucilage. Pretreatment of plaques with periodate oxidation eliminated most soil binding by root-cap mucilage but this was completely reversible by reduction with borohydride. Soil binding to bacterial mucilage was unaffected by periodate but much diminished by borohydride pretreatment (partially restored by subsequent oxidation). Neither pretreatment with cationic dyes nor preincubation in pectinase, pectin methylesterase or protease affected subsequent soil binding by the mucilage plaques. Pretreatment of root-cap mucilage plaques with lectins specific for component sugars also did not alter soil binding. It is concluded that mucilages of both plant and bacterial origin can contribute to the adhesion and cohesion of maize rhizosheaths, but each by a different mechanism. Binding by root-cap mucilage depends on 1,2 diol groups of component sugars, that of bacterial mucilage does not, and is likely to be protein mediated. ei]Section editor: R O D Dixon     

Ethylene and carbon dioxide concentrations of soils as influenced by rhizosphere of crops under field and pot conditions

A method for collecting low volumes of soil gas from a small region, and a technique for determining small concentrations of ethylene using an enrichment process are described. Using these methods, it was found that ethylene and carbon dioxide (CO₂) concentrations of soils varied considerably depending on the presence or absence of a rhizosphere. Ethylene was much higher (31–375 nL L^–1; mean: 207) in non-cropped areas (i.e., soils without rhizosphere) than in the rhizosphere region (8–136 nL L^–1; mean: 38) of a field in which maize or soybean were grown. On the other hand, CO₂ concentrations were higher in rhizosphere than in non-rhizosphere soil, especially in pot experiments. The rate of ethylene decomposition was, however, much greater in rhizosphere soil (55 nL g^–1 day^–1) than in non-rhizosphere soil (34 nL g^–1 day^–1). Higher microbial activity was presumed to result in the decrease of ethylene concentration and the increase in CO₂ in rhizosphere regions. The implications of these results in relation to the influence of ethylene in rhizosphere on plant growth, and the role of soil microbes on decomposition of ethylene is discussed.     

Modeling the water use efficiency of soybean and maize plants under environmental stresses: application of a synthetic model of photosynthesis-transpiration based on stomatal behavior

Understanding the variability of plant WUE and its control mechanism can promote the comprehension to the coupling relationship of water and carbon cycle in terrestrial ecosystem, which is the foundation for developing water-carbon coupling cycle model. In this paper, we made clear the differences of net assimilation rate, transpiration rate, and WUE between the two species by comparing the experiment data of soybean (Glycine max Merr.) and maize (Zea mays L.) plants under water and soil nutrient stresses. WUE of maize was about two and a half times more than that of soybean in the same weather conditions. Enhancement of water stresses led to the marked decrease of Am and Em of two species, but water stresses of some degree could improve WUE, and this effect was more obvious for soybean. WUE of the two species changed with psiL in a second-order curve relation, and the WUE at high fertilization was higher than that at low fertilization, this effect was especially obvious for maize. Moreover, according to the synthetic model of photosynthesis-transpiration based on stomatal behavior (SMPTSB) presented by Yu et al. (2001), the WUE model and its applicability were discussed with the data measured in this experiment. The WUE estimated by means of the model accorded well with the measured values. However, this model underestimated the WUE for maize slightly, thus further improvement on the original model was made in this study. Finally, by discussing some physiological factors controlling Am and WUE, we made clear the physiological explanation for differences of the relative contributions of stomata- and mesophyll processes to control of Am and WUE, and the applicability of WUE model between the two species. Because the requirement to stomatal conductance by unit change of net assimilation rate is different, the responses of opening-closing activity of stomata to environmental stresses are different between the two species. To obtain the same level of net assimilation rate, soybean has to open its stomata more widely to keep small stomatal resistance, as compared with maize.     

Possible Involvement of Cytokinin in Nitrate-mediated Root Growth in Maize

Response of root system architecture to nutrient availability in soils is an essential way for plants to adapt to soil environments. Nitrate can affect root development either as a result of changes in the external concentration, or through changes in the internal nutrient status of the plant. Nevertheless, less is known about the physiological mechanisms. In the present study, two maize (Zea mays L.) inbred lines (478 and Wu312) were used to study a possible role of cytokinin in nitrate-mediated root growth in nutrient solutions. Root elongation of 478 was more sensitive to high nitrate supply than that of Wu312. Medium high nitrate (5 mM) inhibited root elongation in 478, while, root elongation in Wu312 was only inhibited at high NO ₃ ⁻ supply (20 mM). Under high nitrate supply, the root elongation zone in 478 became swollen and the site of lateral root elongation was close towards the root tip. Both of the phenomena are typical of root growth induced by exogenous cytokinin treatments. Correspondingly, zeatin and zeatin nucleotide (Z + ZR) concentrations were increased at higher nitrate supply in 478, whereas they were constant in Wu312. Furthermore, exogenous cytokinin 6-benzylaminopurine (6-BA) completely reversed the stimulatory effect of low nitrate on root elongation. Therefore, it is supposed that the inhibitory effect of high concentration of nitrate on root elongation is, at least in part, mediated by increased cytokinin level in roots. High nitrate supply may have negative influences on root apex activity by affecting cytokinin metabolism so that root apical dominance is weakened and, therefore, root elongation is suppressed and lateral roots grow closer to the root apex. Nitrate suppressed lateral root elongation in Wu312 at concentration higher than 5 mM. In 478, however, this phenomenon was not significant even at 20 mM nitrate. Although exogenous 6-BA (20 nM) could suppress lateral root elongation as well, the inhibitory effect of high NO ₃ ⁻ concentration of nitrate on lateral root growth cannot be explained by changes in endogenous cytokinin alone.     

The role of VAM fungi in nitrogen dynamics in maize-bean intercrops

Nitrogen (N) transfer from N-fixing legumes via vesicular-arbuscular mycorrhizal (VAM) fungi to associated non-fixing plants has been demonstrated in greenhouse experiments. To date, this transfer has been shown only where mineral N is applied shortly before harvest, and hence is readily available. We have yet to demonstrate VAM-mediated N transfer where soil-N is limiting, a condition under which most traditional legume-nonlegume intercrops are grown.In this study, ¹⁵N-enriched soil (with 0.28%N) was used to distinguish between the uptake of soil- and atmospherically-derived N in maize grown with beans in the presence or absence of VAM fungi. VAM infection did not result in transfer of fixed N or soil N from bean to maize, despite a VAM-stimulated increase in N fixation in bean. In fact, beans were more competitive for soil N when mycorrhizal. N content in beans increased by 75% with a concomitant 22% decrease in mg N per maize plant. The competitive effect may have resulted from a VAM-mediated shift in carbon allocation in beans (but not maize) from shoots to roots.     

Tillage effects on nitrogen uptake by maize from fine textured soils in the northwestern Corn Belt,USA

Summary Nitrogen (N) accumulation data from a replicated field study were fitted to a tanh (time) function and the derivate obtained to determine relative maximum rates of accumulation by maize. Both positive and negative effects of tillage on N accumulation rates were observed. Most of the N accumulation occurred over a 30-day period and time of N accumulation was not affected by tillage. Tilled profiles tend to contain greater NO₃–N, greater aeration, and lower moisture contents than untilled profiles, and these characteristics interact to affect plant N accumulation.     

Carbon translocation to the rhizosphere of maize and wheat and influence on the turnover of native soil organic matter at different soil nitrogen levels

Wheat and maize were grown in a growth chamber with the atmospheric CO₂ continuously labelled with ¹⁴C to study the translocation of assimilated carbon to the rhizosphere. Two different N levels in soil were applied. In maize 26–34% of the net assimilated ¹⁴C was translocated below ground, while in wheat higher values (40–58%) were found. However, due to the much higher shoot production in maize the total amount of carbon translocated below ground was similar to that of wheat. At high N relatively more of the C that was translocated to the root, was released into the soil due to increased root respiration and/or root exudation and subsequent microbial utilization and respiration. The evolution rate of unlabelled CO₂ from the native soil organic matter decreased after about 25 days when wheat was grown at high N as compared to low N. This negative effect of high N in soil was not observed with maize.     

Hydrotropism and its interaction with gravitropism in roots

We have studied hydrotropism and its interaction with gravitropism in agravitropic roots of a pea mutant and normal roots of peas (Pisum sativum L.) and maize (Zea mays L.). The interaction between hydrotropism and gravitropism in normal roots of peas or maize were also examined by nullifying the gravitropic response on a clinostat and by changing the stimulus-angle for gravistimulation. Depending on the intensity of both hydrostimulation and gravistimulation, hydrotropism and gravitropism of seedling roots strongly interact with one another. When the gravitropic response was reduced, either genetically or physiologically, the hydrotropic response of roots became more unequivocal. Also, roots more sensitive to gravity appear to require a greater moisture gradient for the induction of hydrotropism. Positive hydrotropism of roots occurred due to a differential growth in the elongation zone; the elongation was much more inhibited on the moistened side than on the dry side of the roots. It was suggested that the site of sensory perception for hydrotropism resides in the root cap, as does the sensory site for gravitropism. Furthermore, an auxin inhibitor, 2,3,5-triiodobenzoic acid (TIBA), and a calcium chelator, ethyleneglycol-bis-(-aminoethylether)-N,N,N,N- tetraacetic acid (EGTA), inhibited both hydrotropism and gravitropism in roots. These results suggest that the two tropisms share a common mechanism in the signal transduction step.     

Wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.) somatic embryogenesis from isolated scutellum: Days post anthesis,days of spike storage,and sucrose concentration affect efficiency

Summary To improve the efficiency of somatic embryogenesis of isolated scutella from commercial wheat (Triticum aestivum L.) cultivars, two factorial experiments were conducted to examine effects of days post anthesis (DPA), days of spike storage (DSS) at 4°C, and sucrose concentrations (SC) on the percentage of scutella producing mature embryos and the number of mature embryos produced per responsive scutellum. In the first experiment, scutella isolated from spikes collected at 10, 11, 12, 13, 14, 15, and 16 DPA and stored at 4°C for 7, 10, 13, and 16d were placed on embryo induction medium [Murashige and Skoog plus 9.96 μM 2,4-dichlorophenoxyacetic acid (2,4-D) and 110 mg l⁻¹ casamino acids], incubated in darkness for 12–14 d and then under light for 2 wk. The interaction of DPA × DSS significantly affected the percentage of scutella producing mature embryos, while only DPA affected the number of mature embryos per responsive scutellum. In the second experiment, scutella isolated from spikes collected at 12 DPA and stored for 15, 16, 17, 18, and 19d were placed on embryo induction medium containing 2, 3, 4, and 5% sucrose. The interaction of DSS × SC significantly affected both the percentage of scutella producing mature embryos and the number of mature embryos per responsive scutellum. In general, DPA/DSS/SC combinations, 12/17/3, 12/18/3, and 12/19/2, yielded the numerically highest embryogenesis efficiencies.     

Tolerance of wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.) to high soil and solution selenium levels

The fertilisation of wheat crops with Se is a cost-effective method of enhancing the concentration of organic Se in grain, in order to increase the Se intake of animals and humans. It is important to avoid phytotoxicity due to over-application of Se. Studies of phytotoxicity of Se in wheat grown in Australia, where rainfall and grain yield are usually relatively low, have not been reported previously, and overseas studies have had varied results. This study used trials conducted in the field, glasshouse and laboratory to assess Se phytotoxicity in wheat. In field trials that used rates of up to 120 g ha^–1Se as selenate, and in pilot trials that used up to 500 g ha^–1 Se soil-applied or up to 330 g ha^–1 Se foliar-applied, with soils of low S concentrations (2–5 mg kg^–1), no Se toxicity symptoms were observed. In pot trials of four weeks duration, the critical tissue level for Se toxicity was around 325 mg kg^–1 DW, a level attained by addition to the growth medium of 2.6 mg kg^–1 Se as selenate. Solution concentrations above 10 mg L^–1 Se inhibited early root growth of wheat in laboratory studies, with greater inhibition by selenite than selenate. For selenite, Se concentrations around 70 mg L^–1 were required to inhibit germination, while for selenate germination % was unaffected by a solution concentration of 150 mg L^–1 Se. Leaf S concentration and content of wheat increased three-fold with the addition of 1 mg kg^–1 Se as selenate to the growth medium. This effect is probably due to the induction of the S deficiency response of the main sulphate transporter. This study found wheat to be more Se-tolerant than did earlier studies of tobacco, soybeans and rice. We conclude that Se phytotoxicity in wheat will not be observed at the range of Se application rates that would be used to increase grain Se for human consumption (4–200 g ha^–1 Se as selenate, which would result in soil and tissue levels well below those seen in the above studies), even when – as is common in Australia – soil S concentration and grain yield are low.