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.     

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.     

The kinetics of ammonium and nitrate uptake by young rice plants

Summary An important process which affects the fate of fertilizer nitrogen (N) applied to a rice crop is crop N uptake. This uptake rate is controlled by many factors including the N-ion species and its concentration. In this study the relation between N concentration at the root surface and N uptake was characterized using Michaelis-Menten kinetics. The equation considers two parameters, Vmax and Km, which are measures of the maximum rate of uptake and the affinity of the uptake sites for the nutrient, respectively. Uptake rates of intact rice plants growing in a continuously flowing nutrient solution system were fitted to the Michaelis-Menten model using a weighted regression analysis. For NH₄−N the Km values for 4- and 9-week-old rice plants indicated a high affinity for the ammonium ions relative to concentrations reported for rice soils after fertilization. The Vmax values expressed on a unit-root-mass basis decreased with plant age, indicating a reduction in the average density of uptake sites on the root surface. The kinetics of NO₃−N uptake was similar to that of NH₄−N when NO₃−N was the only N source. However, if NH₄−N and NO₃−N were present simultaneously in the solution the Vmax for the uptake of NO₃−N was severely reduced, while the Km was affected very little. This inhibition appears to be noncompetitive. Fertilization of young rice plants leading to concentration of N at the root surface above approximately 900 μM will not increase crop uptake and may contribute to inefficient N recovery by the crop. The existence of NH₄−N and NO₃−N simultaneously at the root surface may also lead to inefficient N recovery because of reduced uptake of NO₃−N.     

Nutrient uptake by potato crops grown on two soils with contrasting physical properties

Potatoes were grown on two contrasting soils but in adjacent sites to investigate the effect of soil type on tuber production, nutrient uptake and nutrient inflow rates (uptake rate per unit length of root). The year of the study was wetter than normal. Tuber growth, root growth and nutrient uptake were all greater on the coarse rather than the fine-textured soil. However there was no difference in nutrient inflow rates between plants growing in the two soils. Therefore, it was concluded that the crop on the finer textured soil did not have an adequate nutrient supply, particularly of N, relative to the crop on the coarser-textured soil. The reasons for the low supply of nitrogen in the fine textured soil are not clear, but it might have been due to the smaller root system or to enhanced losses of nitrogen by denitrification caused by the combination of soil physical properties and poor drainage in a wet year.     

Root length dynamics in agroforestry with Gliricidia sepium as compared to sole cropping in the semi-deciduous rainforest zone of West Africa

Tree root systems may improve soil fertility through carbon inputs, uptake of leachable nutrients and maintenance of soil biomass, but can at the same time reduce crop yields by competition for water and nutrients. Quantitative information about the positive and negative effects of tree roots and their changes in space and time are necessary for the optimization of agroforestry associations. An alley cropping experiment was layed out as a randomized complete block design on a Plinthic Lixisol/Ferralic Cambisol with Gliricidia sepium hedgerows at 5 m distance, including a sole cropping control. The development of root systems was monitored by sequential soil coring (eight samplings) during one year, with maize and groundnut as crops. Additional information is presented from a single sampling for rice during the foregoing year. Pronounced fluctuations of live root length density indicated an important variability in the nutrient and water uptake capacity of the vegetation. At low total root length density, the hedgerows affected the root development in the agroforestry plots directly by the presence of their root systems. At high root length density, they affected root development mainly by improving crop root growth and influencing the composition of the spontaneous vegetation. The root length density of the hedgerows was too low to compete with the crops for soil resources. The hedgerows tended to increase root length densities in the subsoil when few roots were present, thus possibly reducing the risk of nutrient leaching. However, the length density of the perennial root systems decreased during the cropping season, presumably as an effect of repeated pruning, and attained minimum values almost at the same time as the crops. Trees with denser root systems which are less frequently pruned may be more efficient in achieving closer nutrient cycles, though at the cost of higher root competition with crops.     

Nitrogen use efficiency of rice reconsidered: What are the key constraints?

Recent field studies on irrigated rice at the IRRI research farm indicate efficient use of fertilizer-N based on plant uptake of applied N, (estimated by N difference), and utilization of acquired N for increased grain yield. These findings contrast with ¹⁵N uptake in microplot studies which underestimate the actual increase in plant N from added fertiliser. Constraints other than uptake efficiency, however, may govern fertiliser-N efficiency in farmers fields. In a study of 44 farmers' fields in Central Luzon, rice yields ranged from 2.5 to 6.2 t ha^-1 and N uptake from 35 to 95 kg N ha^-1 in plots without fertiliser-N addition. Farmers applied from 35 to 240 kg N ha^-1, but there was no relationship between the N rate used by each farmer and the effective soil N supply. Mean N uptake efficiency from fertiliser by N difference was only 36%. We conclude that improved fertiliser-N efficiency by farmers will require a more information-intensive management strategy that makes N fertiliser inputs better fitted to the seasonal pattern of crop N demand and soil N supply.     

Responses of various crop species and cultivars to fertilizer application

Summary Crop response to fertilizer application depends not only on the level of available plant nutrients in the soil but is also related to crop physiology and morphology. For a well balanced nutrition the rate of nutrient supply to the roots must correspond with the rate of nutrient required for growth. Species or cultivars with a high growth rate generally respond more favourably to fertilizer application than those with low growth rates. An analogous relationship holds for the biomass produced per unit soil surface. Thus modern rice and wheat cultivars tolerate a more dense spacing than older ones. Due to the dense stand the yield and particularly the grain yield of the modern varieties may be several times higher than those of older cultivars, and therefore also the nutrient requirement, especially the demand for N and P, is higher for the modern cultivars.Modern cereal cultivars are characterized by a high crop index which means that after flowering a high proportion of grain filling material must be produced by photosynthesis. Assimilation and translocation of photosynthates are favoured by K⁺. Thus in particular modern cultivars require a high K⁺ content for optimum grain filling.Nutrient exploitation of soils by plant roots depends on root morphology and root physiology. Grasses generally have much longer roots than dicots. Thus the rate of K⁺ and phosphate uptake per unit root length is lower for grasses than for dicots. It is for this reason that dicots respond earlier to a K⁺ and phosphate dressing than grasses.Species living symbiotically with Rhizobium may depress the rhizosphere pH considerably and thus promote the dissolution of phosphate rock.     

Interactive effects of potassium and sodium on root growth and expression of K/Na transporter genes in rice

Sufficient supply of potassium (K) can alleviate the adverse effects of excess sodium (Na) on plant growth. However, it remains unclear if such a beneficial function is related to regulation of root growth and/or expression of K/Na transporters. Herein we report the responses of a rice cultivar, which was pretreated with normal nutrient solution for 1 month, to three levels of Na (0, 25, and 100 mM) without or with supply of K for 9 days. High Na (100 mM) significantly decreased plant growth, root activity, and total K uptake, and increased biomass ratio of roots to shoots. Short-term removal of K supply (9 days) did not affect root morphology and biomass ratio of roots to shoots, but decreased root activity of seedlings grown in high Na solution. K deficiency increased uptake of Na and transport of K from roots to shoots. Moreover, expression of OsHAK1, a putative K transporter gene, was upregulated by low Na (25 mM) and downregulated by high Na (100 mM) in roots. In leaves, its expression was suppressed by the Na treatments when K supply was maintained. Expression of OsHKT2;1, which encodes a protein that acts mainly as a Na transporter, was downregulated by high Na, but was enhanced by K deficiency both in roots and leaves. Expression of five other putative K/Na transporter or Na⁺/H⁺ genes, OsHKT1;1, OsHKT1;2, OsHKT2;3, OsNHX1, and OsSOS1, was not affected by the treatments. The results suggest that OsHAK1 and OsHKT2;1 were involved in the interactive effects of K and Na on their uptake and distribution in rice. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Ammonia-oxidizing bacteria on root biofilms and their possible contribution to N use efficiency of different rice cultivars

Ammonia-oxidizing bacteria (AOB) populations were studied on the root surface of different rice cultivars by PCR coupled with denaturing gradient gel electrophoresis (DGGE) and fluorescence in situ hybridization (FISH). PCR-DGGE of the ammonium monooxygenase gene (amoA) showed a generally greater diversity on root samples compared to rhizosphere and unplanted soil. Sequences affiliated with Nitrosomonas spp. tended to be associated with modern rice hybrid lines. Root-associated AOB observed by FISH were found within a discrete biofilm coating the root surface. Although the total abundance of AOB on root biofilms of different rice cultivars did not differ significantly, there were marked contrasts in their population structure, indicating selection of Nitrosomonas spp. on roots of a hybrid cultivar. Observations by FISH on the total bacterial community also suggested that different rice cultivars support different bacterial populations even under identical environmental conditions. The presence of active AOB in the root environment predicts that a significant proportion of the N taken up by certain rice cultivars is in the form of NO₃ ^–-N produced by the AOB. Measurement of plant growth of hydroponically grown plants showed a stronger response of hybrid cultivars to the co-provision of NH₄ ⁺ and NO₃ ^–. In soil-grown plants, N use efficiency in the hybrid was improved during ammonium fertilization compared to nitrate fertilization. Since ammonium-fertilized plants actually receive a mixture of NH₄ ⁺ and NO₃ ^– with ratios depending on root-associated nitrification activity, these results support the advantage of co-provision of ammonium and nitrate for the hybrid cultivar.     

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.     

Enhancing phosphorus and zinc acquisition efficiency in rice: a critical review of root traits and their potential utility in rice breeding

Background

Rice is the world''s most important cereal crop and phosphorus (P) and zinc (Zn) deficiency are major constraints to its production. Where fertilizer is applied to overcome these nutritional constraints it comes at substantial cost to farmers and the efficiency of fertilizer use is low. Breeding crops that are efficient at acquiring P and Zn from native soil reserves or fertilizer sources has been advocated as a cost-effective solution, but would benefit from knowledge of genes and mechanisms that confer enhanced uptake of these nutrients by roots.

Scope

This review discusses root traits that have been linked to P and Zn uptake in rice, including traits that increase mobilization of P/Zn from soils, increase the volume of soil explored by roots or root surface area to recapture solubilized nutrients, enhance the rate of P/Zn uptake across the root membrane, and whole-plant traits that affect root growth and nutrient capture. In particular, this review focuses on the potential for these traits to be exploited through breeding programmes to produce nutrient-efficient crop cultivars.

Conclusions

Few root traits have so far been used successfully in plant breeding for enhanced P and Zn uptake in rice or any other crop. Insufficient genotypic variation for traits or the failure to enhance nutrient uptake under realistic field conditions are likely reasons for the limited success. More emphasis is needed on field studies in mapping populations or association panels to identify those traits and underlying genes that are able to enhance nutrient acquisition beyond the level already present in most cultivars.     

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.     

Seed vigour contributes to yield improvement in dry direct‐seeded rainfed lowland rice

Dry direct‐seeded rice (DSR) cultivation is widely spreading in tropical Asia, but drought and nutrient deficiency stresses often cause crop failure in rainfed lowlands. The objective of this study was to dissect the physio‐morphological characteristics associated with crop establishment and early vigour of DSR under drought and P deficiency conditions in the Philippines. It was found that new drought‐resistant cultivars bred for DSR (Rc348 and Rc192) had faster germination and sprout growth than popular irrigated rice cultivars (Rc222 and Rc10) under soil water deficit due to rapid moisture acquisition by the germinating seeds from drying soils. There was a significant correlation between seed moisture content and the reduction in seed dry weight, and between reduction in seed dry weight and shoot elongation under both control and drought stress treatments at the germination stage. At the seedling stage, the root growth of Rc348 under drought tended to be more vigorous with its higher root‐to‐shoot ratio compared to Rc222 and Rc10. The seedling vigour of Rc348 under P deficiency was also greater than that of Rc222 due to its greater root growth and P uptake. The yields of Rc348 and Rc192 grown under rainfed condition at the target drought‐prone site where a dry spell of 13 days occurred during crop establishment were higher (4.0–4.1 t ha^?1) than the yield of Rc10 (3.0 t ha^?1). These results suggest that quick germination and seedling vigour with quick root anchorage and great nutrient uptake capacity, even with limitations of soil moisture and nutrients, would be important traits for DSR in rainfed lowlands.     

Studies on in vitro selection of Fe-efficient lines in sugarcane

The objective of this research was to show how a mechanistic uptake model that accurately predicts phosphorus (P) uptake by maize (Zea mays L.) in a pot experiment may be used to evaluate the reasons for the differences in P availability observed when soil pH is varied. The model predicts P uptake by integrating soil P supply by mass flow and diffusion; size, shape and growth rate of roots; and P uptake kinetics of the root. The P supply parameters of the model that may be affected by soil pH are P_li, initial P concentration in the soil solution; b, the buffer power of P in the soil, P_si, for P_li, and D_e, effective diffusion coefficient. The effect of these changes on P uptake was predicted with the model by using measured values of the three soil supply parameters and of size, shape, and growth rate of roots and keeping the other parameters at values characteristic of maize. Values for three soil supply parameters can be calculated from measurements of P_li, P_si, and , volumetric water content. The predictions of the model closely agreed with observed uptake when form of P present at the higher pH's was accounted for. There was a significant positive correlation (r=0.94) between P_li and observed P uptake and a significant negative correlation (-0.93) between P_si and observed P uptake. The use of the model demonstrated the significance of P form and the importance of P_li in P uptake. It also showed importance of root growth rate.     

Effect of iron plaque outside roots on nutrient uptake by rice (Oryza sativa L.). Zinc uptake by Fe-deficient rice

This solution culture study examined the effect of the deposition of iron plaque on zinc uptake by Fe-deficient rice plants. Different amounts of iron plaque were induced by adding Fe(OH)₃ at 0, 10, 20, 30, and 50 mg Fe/L in the nutrient solution. After 24 h of growth, the amount of iron plaque was correlated positively with the Fe(OH)₃ addition to the nutrient solution. Increasing iron plaque up to 12.1 g/kg root dry weight increased zinc concentration in shoots by 42% compared to that at 0.16 g/kg root dry weight. Increasing the amount of iron plaque further decreased zinc concentration. When the amounts of iron plaque reached 24.9 g/kg root dry weight, zinc concentration in shoots was lower than that in shoots without iron plaque, implying that the plaque became a barrier for zinc uptake. While rice plants were pre-cultured in –Fe and +Fe nutrient solution in order to produce the Fe-deficient and Fe-sufficient plants and then Fe(OH)₃ was added at 20, 30, and 50 mg Fe/L in nutrient solution, zinc concentrations in shoots of Fe-deficient plants were 54, 48, and 43 mg/kg, respectively, in contrast to 32, 35, and 40 mg/kg zinc in shoots of Fe-sufficient rice plants. Furthermore, Fe(OH)₃ addition at 20 mg Fe/L and increasing zinc concentration from 0.065 to 0.65 mg Zn/L in nutrient solution increased zinc uptake more in Fe-deficient plants than in Fe-sufficient plant. The results suggested that root exudates of Fe-deficient plants, especially phytosiderophores, could enhance zinc uptake by rice plants with iron plaque up to a particular amount of Fe.     

Manganese dynamics in the rhizosphere and Mn uptake by different crops evaluated by a mechanistic model

Understanding of the mechanisms of Mn supply from the soil and uptake by the plants can be improved by using simulation models that are based on basic principles. For this, a pot culture experiment was conducted with a sandy clay loam soil to measure Mn uptake by summer wheat (Triticum aestivum L. cv. Planet), maize (Zea mays L. cv. Pirat) and sugar beet (Beta vulgaris L. cv. Orbis) and to simulate Mn dynamics in the rhizosphere by means of a mechanistic model. Seeds of three crops were sown in pots containing 2.9 kg soil in a controlled growth chamber. Root and shoot weight, Mn content of plants, root length and root radius were determined 8 (13 days in case of sugar beet) and 20 days after germination. Soil and plant parameters were determined to run nutrient uptake model calculations. Manganese content of the shoot varied from 25 mg kg^-1 for sugar beet to 34 mg kg^-1 for maize. Sugar beet had the lowest root length/shoot weight ratio but the highest relative shoot growth rate, resulting in the highest shoot demand on the root. This is reflected by the Mn influx which was 0.9 × 10^-7, 1.7 × 10^-7 and 2.5 × 10^-7 nmol cm^-1 s^-1 for wheat, maize and sugar beet, respectively. Nutrient uptake model calculations predicted similar influx values. Initial Mn concentration of 0.2 μM in the soil solution decreased to only 0.16 μM for wheat, 0.13 μM for maize and 0.11 μM for sugar beet at the root surface. This shows that manganese transport to the root was not a limiting step. This was confirmed by the fact that an assumed 20 times increase in maximum influx (I_max) increased the calculated Mn influx by 3.7 times. Sensitivity analysis demonstrated that for controlling Mn uptake the initial soil solution concentration (C _Li), the root radius (r₀), I_max and the Michaelis constant (K _m) were the most sensitive factors in the listed order. This revised version was published online in August 2006 with corrections to the Cover Date.     

Root growth,macro-nutrient uptake dynamics and soil fertility requirements of a high-yielding winter oilseed rape crop

Shoot growth, root growth and macro-nutrient uptake by a high-yielding (5t/ha grain) winter oilseed rape crop have been measured. Maximum rooting density in the top 20cm of soil was 9.4 cm cm⁻³ and roots reached a depth of at least 1.8 m. Maximum nutrient uptakes were 364 kg ha⁻¹ for N, 43 kg ha⁻¹ for P, 308 kg ha⁻¹ for K, 287 kg ha⁻¹ for Ca and 16 kg ha⁻¹ for Mg. A 30-day drought coincided with the flowering period and root and shoot growth, as well as nutrient uptake rates, were reduced. Nutrient concentrations in the soil solution necessary to sustain the nutrient fluxes into the root system by diffusive supply have been calculated. Peak values were in the range 10 μM for P to 87 μM for N, lower than the observed concentrations, and it was concluded that nutrient transport to roots was not a limitation to uptake by this rape crop.     

Ontogenetic variations on absorption and utilization of phosphorus in common bean cultivars under biological nitrogen fixation

Salinity is a major yield-reducing factor in coastal and arid, irrigated rice production systems. Salt tolerance is a major breeding objective. Three rice cultivars with different levels of salt tolerance were studied in the field for growth, sodium uptake, leaf chlorophyll content, specific leaf area (SLA), sodium concentration and leaf CO₂ exchange rates (CER) at photosynthetic active radiation (PAR)-saturation. Plants were grown in Ndiaye, Senegal, at a research station of the West Africa Rice Development Association (WARDA), during the hot dry season (HDS) and the wet season (WS) 1994 under irrigation with fresh or saline water (flood water electrical conductivity = 3.5 mS cm^-1). Relative leaf chlorophyll content (SPAD method) and root, stem, leaf blade and panicle dry weight were measured at weekly intervals throughout both seasons. Specific leaf area was measured on eight dates, and CER and leaf sodium content were measured at mid-season on the first (topmost) and second leaf. Salinity reduced yields to nearly zero and dry-matter accumulation by 90% for the susceptible cultivar in the HDS, but increased leaf chlorophyll content and CER at PAR- saturation. The increase in CER, which was also observed in the other cultivars and seasons, was explained by a combination of two hypotheses: leaf chlorophyll content was limited by the available N resources in controls, but not in salt-stressed plants; and the sodium concentrations were not high enough to cause early leaf senescence and chlorophyll degradation. The growth reductions were attributed to loss of assimilates (mechanisms unknown) that must have occurred after export from the sites of assimilation. The apparent, recurrent losses of assimilates, which were between 8% and 49% according to simulation with the crop model for potential yields in irrigated rice, ORYZA S, might be partly due to root decomposition and exudation. Possibly more importantly, energy-consuming processes, such as osmoregulation, interception of sodium and potassium from the transpiration stream in leaf sheaths and their subsequent storage, drained the assimilate supply. This revised version was published online in August 2006 with corrections to the Cover Date.     

Does increased zinc uptake enhance grain zinc mass concentration in rice?

Rice (Oryza sativa) is the worlds’ most important cereal and potentially an important source of zinc (Zn) for people who eat mainly rice. To improve Zn delivery by rice, plant Zn uptake and internal allocation need to be better understood. This study reports on within‐plant allocation and potential Zn accumulation in the rice grain in four so‐called aerobic rice cultivars (Handao297, K150, Handao502 and Baxiludao). Two controlled‐condition experiments were carried out, one with a wide range of constant Zn concentrations in the medium and one with a range of plant growth rate‐related supply rates. In both experiments, increased Zn supply induced increased plant Zn uptake rate throughout crop development, when expressed as daily Zn uptake (μg day^?1) or as daily Zn uptake per gram of plant dry matter (μg g^?1). Zinc mass concentration (ZnMC) in all plant organs increased with an increase in Zn supply but to various degrees. At higher uptake levels, the ZnMC in stems increased most, while the ZnMC in hulled grains (brown rice) increased least. The increase in leaf ZnMC was generally small, but at toxic levels in the medium, leaf ZnMC increased significantly. It appears that regulation of grain Zn loading differs from regulation of Zn loading to other organs. A milling test on seeds of Baxiludao and Handao502 showed that when ZnMC in brown rice increased from 13 to 45 mg kg^?1, ZnMC in polished rice grains (endosperm) also increased from 9 to 37 mg kg^?1 but remained three to five times lower than that in the bran. Irrespective of the ZnMC in the brown rice, around 75% of total grain Zn was present in the endosperm. In both cultivars, there was a major difference in ZnMC between bran and endosperm (120 and 37 mg kg^?1, respectively), suggesting a barrier for Zn transport between the two tissues. There seems to be a second barrier between stem and rachis, as their ZnMCs also differed greatly (300 and 100 mg kg^?1, respectively) in both cultivars at higher plant ZnMC. It is concluded that there is too little scope from a human nutrition perspective to enhance ZnMC in rice endosperm by simply increasing the Zn supply to rice plants because Zn allocation to the endosperm is limited, while observed genotypic differences indicate scope for improvement through breeding.     

Morphology and response of roots of pasture species to phosphorus and nitrogen nutrition

The root morphology of ten temperate pasture species (four annual grasses, four perennial grasses and two annual dicots) was compared and their responses to P and N deficiency were characterised. Root morphologies differed markedly; some species had relatively fine and extensive root systems (Vulpia spp., Holcus lanatus L. and Lolium rigidum Gaudin), whilst others had relatively thick and small root systems (Trifolium subterraneum L. and Phalaris aquatica L.). Most species increased the proportion of dry matter allocated to the root system at low P and N, compared with that at optimal nutrient supply. Most species also decreased root diameter and increased specific root length in response to P deficiency. Only some of the species responded to N deficiency in this way. Root morphology was important for the acquisition of P, a nutrient for which supply to the plant depends on root exploration of soil and on diffusion to the root surface. Species with fine, extensive root systems had low external P requirements for maximum growth and those with thick, small root systems generally had high external P requirements. These intrinsic root characteristics were more important determinants of P requirement than changes in root morphology in response to P deficiency. Species with different N requirements could not be distinguished clearly by their root morphological attributes or their response to N deficiency, presumably because mass flow is relatively more important for N supply to roots in soil.Section editor: H. Lambers