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Aim: Assessing water consumption in intercropping systems relative to sole crop systems is important for planting system planning. Methods: This study focused on wheat/maize (Triticum aestivum L./Zea mays L.) relay intercropping to evaluate water use advantage of relay intercropping system based on the same period. Field experiments were conducted from 2014 to 2016, and three cropping patterns were designed: sole wheat, sole maize, wheat/maize intercrops. Method for evaluating water use advantage in intercropping was revised by considering water consumption of sole crops during the fallow period (periods where no plants are grown). Results: Intercropping grain yield was higher than sole cropping yield. Based on the existing method, the intercropping system had 20.4% greater water use than the sole cropping system and the water equivalent ratio (WER) was lower than 1; after revision, intercropping had 7.7% less water use than sole cropping, and the WER was greater than 1. Before revision, intercropped wheat used 3.1% more water than sole wheat, and the intercropped maize used 38.8% more water than sole maize; after revision, intercropped wheat used 24.4% less water than sole wheat, and intercropped maize used 11.0% more water than sole maize. Conclusion: These results indicated that the revised method can be better used to evaluate water use advantage of relay intercropping system. This study implied that wheat/maize intercropping increased both yield and water use based on revised method, and relay intercropping can be a suitable production practice employed in fallow policy, especially in areas with limited water. [ABSTRACT FROM AUTHOR]

Purpose: Legume crops are advocated for integration into crop rotation systems, and cereal-based rotations with the presence of legumes have a substantial effect on improving soil fertility and health. It is not yet clear whether the frequency of legume inclusion in crop rotation systems influences soil biochemical properties and soil organic matter (SOM) mineralization. Methods: An incubation experiment was conducted with 13C-glucose addition to evaluate the influences of soybean (Glycine max L.) cropping frequency on SOM mineralization under long-term wheat (Triticum aestivum L.)- and maize (Zea mays L.)-based rotation systems. Phospholipid fatty acids (PLFAs) and 13C-PLFAs were measured to explore microbial biomass, community structure and microbial utilization of glucose in wheat and maize systems. Results: Glucose addition increased native SOM mineralization, i.e. positive priming effect. Compared with less soybean cropping frequency under long-term wheat- and maize-based rotation systems, wheat-soybean-soybean-soybean rotation and maize-soybean-soybean rotation increased the total biomass (PLFAs), fungal biomass and decreased the ratio of bacteria to fungi. Furthermore, the ratio of bacteria to fungi was negatively correlated with PE intensity, indicating that greater fungal biomass played a key role in stimulating SOM priming. That the proportion of 13C-glucose in G- and fungi had a positive relationship with PE intensity also supported this conclusion. The presence of soybean in wheat- and maize-based rotations increased SOM priming, while the soybean cropping frequency had no significant influence on SOM priming. However, in contrast to a maize-based rotation system, the same frequency of soybean in a wheat-based rotation system had lower soil C/N ratio and higher B/F ratio, and resulted in lower PE intensity. Conclusions: Our findings indicated that the presence of soybean in wheat- and maize-based rotation systems increased PE intensity because of higher soil C/N ratio and lower B/F ratio, while the soybean cropping frequency had no significant influence on SOM priming. Furthermore, the presence of soybean in maize system induced more SOM priming than that in wheat system with glucose addition. [ABSTRACT FROM AUTHOR]

Background and Aim: Success in agronomic biofortification of maize and wheat is highly variable. This study aimed to elucidate the differences in uptake and translocation of foliar-applied zinc (Zn) in maize (Zea mays L.) and wheat (Triticum aestivum L.) using different experimental approaches. Methods: Old leaves of plants cultured with low or adequate Zn supply in soil were treated with natural Zn (as ZnSO4) or enriched stable isotope 70Zn as SO4 salt. Treated leaves, and the remaining untreated shoot and roots were analyzed for Zn using quantitative (ICP-OES and ICP-MS after acid digestion) and qualitative (fluorescence microscopy after Zinpyr-1 staining) techniques. Results: Application of Zn to a single old leaf resulted in significantly higher increments in Zn concentration of the treated leaf, remaining untreated shoot and roots in wheat than in maize, particularly under low Zn supply. The relative Zn uptake (i.e., the portion of 70Zn taken up by the treated leaf) was also estimated to be significantly higher in wheat than maize, irrespective of the rate of Zn supply to plants. Higher uptake and translocation of Zn in wheat compared to maize was also visualized and demonstrated by using Zinpyr-1 fluorescent dye. Conclusions: Our results conclude that wheat is superior to maize in terms of leaf penetration and uptake, as well as subsequent translocation of foliar-applied Zn, therefore, the agronomic biofortification potential is better for wheat than for maize. [ABSTRACT FROM AUTHOR]

Background and aims: Previous studies showed that the competition-recovery principle is one of the mechanisms underpinning overyielding in wheat/maize intercropping. However, few studies have focused on the effects of root morphological and physiological changes during the recovery growth of late-maturing species. The present study aimed to determine the mechanism underlying the recovery growth in terms of root distribution and nitrogen (N) uptake in response to different N supplies. Methods: The roots of maize were sampled three times by auger after wheat harvest in the intercropping system with six levels of N application and sole crops at one N-application rate under field condition. Results: Intercropped maize adjusted its root length density (RLD) and root distribution and enhanced its N absorption per unit root length with increasing soil N concentration. Soil inorganic N concentration had a direct influence on RLD of intercropped maize which was related to shoot N concentration. In addition, maize took up 93% more N per unit root length when intercropped with maize compared with sole cropping. Conclusions: Our findings show that the recovery growth of late-maturing species involves phenotypic plasticity of maize root architecture, and the enhanced N uptake resulted from extra soil N acquired from the area where wheat was growing before its harvest. [ABSTRACT FROM AUTHOR]

Background and Aims: Understanding the characteristics of root system is important for optimization of intercropping systems and increased utilization efficiency of limited resources. However, few studies have examined root development and water uptake in wheat/maize intercropping system under different water conditions. Methods: A two-year field experiment of sole wheat, sole maize, and intercropped wheat/maize was carried out under rainfed conditions and supplementary irrigation. Roots of wheat and maize were sampled seven times over the two growing seasons (2014–2015 and 2015–2016) using auger and monolith methods. Results: Root growth and distribution were promoted in intercropped wheat, while in intercropped maize roots both were suppressed. The maximum lateral extension distance of intercropped wheat under rainfed conditions varied from 40 to 50 cm during the co-growth period, which was significantly (p < 0.05) greater than that under supplementary irrigation conditions (25–30 cm), allowing intercropped wheat to access soil water in the area between the wheat and maize strips. Meanwhile, after wheat harvest, intercropped maize roots grown under supplementary irrigation showed growth recovery mainly in a horizontal direction with maximum lateral distance of 50 cm. Conclusions: Intercropping boosted the capacity of wheat roots but suppressed root growth in maize, and rainfed conditions encouraged the lateral distribution of intercropped wheat roots. [ABSTRACT FROM AUTHOR]

Aims: Traits of the plant root system architecture (RSA) play a key role in crop performance. Therefore, architectural root traits are becoming increasingly important in plant phenotyping. In this study, we use a mathematical model to investigate the sensitivity of characteristic root system measures, obtained from different classical field root sampling schemes, to RSA parameters. Methods: Root systems of wheat and maize were simulated and sampled virtually to mimic real field experiments using the root system architecture (RSA) model CRootBox. By means of a sensitivity analysis, we found RSA parameters that significantly influenced the virtual field sampling results. To identify correlations between sensitivities, we carried out a principal component analysis. Results: We found that the parameters of zero order roots are the most sensitive, and parameters of higher order roots are less sensitive. Moreover, different characteristic root system measures showed different sensitivity to RSA parameters. RSA parameters that could be derived independently from different types of field observations were identified. Conclusions: Selection of characteristic root system measures and parameters is essential to reduce the problem of parameter equifinality in inverse modeling with multi-parameter models and is an important step in the characterization of root traits from field observations. [ABSTRACT FROM AUTHOR]

Background and aims: Cereals are the main dietary source of cadmium (Cd). Rice grains often contain higher levels of Cd than other cereals, but the reasons are unknown. The aims of this study were to compare Cd uptake, translocation and influx kinetics between rice, wheat and maize and to investigate whether the expression and functionality of Nramp5 genes differ between the three crop species.Methods: Two cultivars each of rice, wheat and maize were grown hydroponically and exposed to a range of Cd concentrations. Nramp5 genes were cloned from the three plants and their expression levels determined. The Cd transport activities of Nramp5 proteins were tested in yeast.Results: Under hydroponic conditions, Cd uptake in rice was 2.5-8.1 and 1.1-3.6 times that of wheat and maize, respectively. The maximum Cd influx velocity of rice was 6.5 and 2.2 times that in wheat and maize, respectively. Wheat showed the lowest Cd uptake but the highest Cd root-to-shoot translocation. The absolute expression level of OsNramp5 in rice roots was 4-5 times that of TaNramp5A and TaNramp5D in wheat or ZmNramp5 in maize. All Nramp5 proteins were localized to the plasma membrane. When expressed in yeast, OsNramp5 showed a greater Cd transport activity than wheat or maize Nramp5.Conclusions: Rice has a greater Cd uptake ability than wheat or maize, likely because OsNramp5 is more highly expressed and the protein has a higher Cd transport activity than wheat or maize Nramp5. [ABSTRACT FROM AUTHOR]

Background and aims: Boron (B) deficiency depresses grain set and grain yield of wheat and maize while having little effect on their vegetative growth. This paper describes effects of B deficiency in rice and how these vary with planting season and variety. Methods: Three rice varieties (KDML105, CNT1, SPR1) were grown in sand culture without (B0) and with 10 μM (B10) B added to the nutrient solution, in the cool season of 2007/08 and 2008/09 and the hot season of 2011 in Chiang Mai, Thailand (18°47′N, 98°59′E). Boron responses were measured in growth and yield parameters, pollen viability and B concentration of the flag leaf and anthers at anthesis. Results: Grain weight was strongly depressed by B deficiency ranging from 28 % in SPR1 to 79 % in CNT1, and the yield was much lower in the cool season than in the hot season plantings. The variation in grain weight was closely associated with grain set and number of spikelets but not with shoot dry weight or tillering. Grain set was closely related to pollen viability, and both were increased with increasing anther B concentration at >20 mg B kg. In addition to its adverse effect on grain set, B deficiency also depressed grain filling and weight of individual grains in rice. Conclusions: Boron deficiency depressed rice grain yield through adverse effects on reproductive growth, panicle and spikelet formation and grain filling, in addition to grain set as in wheat and maize. [ABSTRACT FROM AUTHOR]

Aims: A field experiment was conducted where maintenance of indigenous arbuscular mycorrhizal (AM) fungal populations was attempted using AM host cover crops arranged temporally or spatially during growth of nonmycorrhizal crops. Methods: To arrange AM hosts temporally, sunflower or oat was grown as a cover crop after non-host cropping (cabbage) or fallowing. In order to arrange AM hosts spatially, red clover, white clover or vetch was intercropped during growth of non-host cabbage. Results: The AM colonization and growth of maize with previously introduced sunflower or oat were much greater than those without introduction of cover crops or those with introduction of non-host cover crops. The AM colonization and yield of winter wheat grown after cabbage with AM host intercropping were greater than those after cabbage only cropping, suggesting that arrangement of AM hosts between cabbage rows is effective for maintaining the AM fungal population in soil during non-host cropping. Conclusions: Mycorrhizal hosts cropped after or during non-host cropping is an effective means to increase indigenous AM fungal populations. The results show that AM colonization, P uptake and productivity of crops after cultivation of nonmycorrhizal crops can be improved by arranging AM hosts temporally or spatially as cover crops. [ABSTRACT FROM AUTHOR]

Croplands mainly act as net sources of the greenhouse gases carbon dioxide (CO) and nitrous oxide (NO), as well as nitrogen oxide (NO), a precursor of troposheric ozone. We determined the carbon (C) and nitrogen (N) balance of a four-year crop rotation, including maize, wheat, barley and mustard, to provide a base for exploring mitigation options of net emissions. The crop rotation had a positive net ecosystem production (NEP) of 4.4 ± 0.7 Mg C ha y but represented a net source of carbon with a net biome production (NBP) of -1.3 ± 1.1 Mg C ha y. The nitrogen balance of the rotation was correlated with the carbon balance and resulted in net loss (−24 ± 28 kg N ha y). The main nitrogen losses were nitrate leaching (−11.7 ±1.0 kg N ha y) and ammonia volatilization (−9 kg N ha y). Dry and wet depositions were 6.7 ± 3.0 and 5.9 ±0.1 kg N ha y, respectively. Fluxes of nitrous (NO) and nitric (NO) oxides did not contribute significantly to the N budget (NO: -1.8 ± 0.04; NO: -0.7 ± 0.04 kg N ha y) but NO fluxes equaled 16% of the total greenhouse gas balance. The link between the carbon and nitrogen balances are discussed. Longer term experiments would be necessary to capture the trends in the carbon and nitrogen budgets within the variability of agricultural ecosystems. [ABSTRACT FROM AUTHOR]

mospheric nitrogen (N) deposition is a serious problem on the North China Plain (NCP) because it imposes a considerable nutrient burden on the local environment. However, it also makes a substantial contribution to agricultural crop N requirements. The integrated total N input (ITNI) system is a method to quantify total atmospheric N deposition by using N-labeled monitor plants grown in pots. The effect of pot surface area and variety of indicator plant on the amount of airborne N input quantified by the ITNI system was investigated in this study. Total N deposition to the soil-maize/soil-wheat plant system at key growth stages was also quantified to improve N-fertilizer recommendations. When indicator plants having a high space requirement were used a correction factor was needed and this could be obtained only by simulating commercial field conditions, especially plant density, because the factor depends largely on pot area or the difference in plant density between pot conditions and field conditions. The total airborne N input measured by the ITNI system was not influenced by the variety of monitoring plant. N deposition was 20-25 kg N ha during growth from three expanded leaf to ten expanded leaf and also from ten expanded leaf to maturity of maize. N deposition was 29.1 kg N ha between planting and the jointing stage and 10.1 kg N ha from jointing to maturity of wheat. This high measured N deposition indicates that N deposition should be taken into account when calculating the N fertilizer requirements of maize and wheat in this region. [ABSTRACT FROM AUTHOR]

Interspecific interactions and soil nitrogen supply levels affect intercropping productivity. We hypothesized that interspecific competition can be alleviated by increasing N application rate and yield advantage can be obtained in competitive systems. A field experiment was conducted in Wuwei, Gansu province in 2007 and 2008 to study intercropping of faba bean/maize, wheat/maize, barley/maize and the corresponding monocultures of faba bean ( Vicia faba L.), wheat ( Triticum aestivum L.), barley ( Hordeum vulgare L.) and maize ( Zea mays L.) with N application rates of 0, 75, 150, 225 and 300 kg N ha. Total land equivalent ratios (TLER) were 1.22 for faba bean/maize, 1.16 for wheat/maize, and 1.13 for barley/maize intercropping over the 2-year study period. Maize was overyielding when intercropped with faba bean, but underyielding when intercropped with wheat or barley according to partial land equivalent ratios (PLER) based on grain yields of individual crops in intercropping and sole cropping. There was an interspecific facilitation between intercropped faba bean and maize, and interspecific competition between maize and either wheat or barley. The underyielding of maize was higher when intercropped with barley than with wheat. Fertilizer N alleviated competitive interactions in intercrops with adequate fertilizer N at 225 kg ha. Yield advantage of intercropping can be acquired with adequate nitrogen supply, even in an intensive competitive system such as barley/maize intercropping. This is important when using intercropping to develop intensive farming systems with high inputs and high outputs. [ABSTRACT FROM AUTHOR]

Permanent raised bed planting with crop residue retention is a form of conservation agriculture that has been proposed as an alternative to conventional tillage for wheat production systems in the Central Highlands of Mexico. A field experiment comparing permanent and tilled raised beds with different residue management under rainfed conditions was started at El Batán (State of Mexico, Mexico) in 1999. The percentage of small and large macroaggregates and mean weight diameter (MWD) was significantly larger in permanent raised beds compared to conventionally tilled raised beds both with full crop residue retention (average for maize and wheat), while the percentages free microaggregates was lower. The percentages of small and large macroaggregates and mean weight diameter (MWD) was significantly larger in permanent raised beds with residue retention compared to permanent raised beds with removal of the residue (average for maize and wheat), while the percentages free microaggregates and silt and clay fraction was lower. Cultivation of maize significantly reduced the large macroaggregates, while wheat reduced the silt and clay fraction (average over all systems). Cultivation of maize reduced the C and N content of the free microaggregates compared to soil cultivated with wheat, while removal of plant residue reduced the C and N content of the silt and clay fraction compared to soil where residue was retained. The C and N content of the coarse particulate organic matter (cPOM) and microaggregates within the macroaggregates was significantly larger in permanent raised beds compared to conventionally tilled raised beds both with full residue retention, while C and N content of the cPOM was significantly lower when residue was removed or partially removed compared to the soil where the residue was retained. The δ 13C ‰ signatures of the macroaggregates, microaggregates, the silt and clay fraction, cPOM and microaggregates within the macroaggregates were not affected by tillage or residue management when wheat was the last crop, but removal of residue reduced the δ 13C ‰ signatures of the macro-, microaggregates and microaggregates within the macroaggregates significantly compared to soil where the residue was retained. Retaining only 30–50% of the organic residue still improved the soil structure considerably compared to plots where it was removed completely. Permanent raised beds without residue retention, however, is a practice leading to soil degradation. [ABSTRACT FROM AUTHOR]

It is essential to have an indication of the sustainability of an agricultural system in addition to the potential, immediate benefits well before the catastrophic consequences of non-sustainability become apparent. Long-term experiments are best suited to test sustainability of a given system. This paper has a dual objective: (1) evaluate the Greenseeker handheld NDVI sensor as a tool for measuring within plot spatial variability, (2) address the question whether different management practices affect spatial within-plot crop growth variability and what this spatial variability tells us about the cropping system performance. Therefore, spatial and time variability of crop performance were measured during the 2004 and 2005 crop cycle for all plots of the different management treatments of a long-term (started 1991) tillage and residue management trial. The NDVI readings measured with the handheld sensor correlated well with the visual scoring in the field. The hand-held sensor is time-efficient and gave reproducible results. The potential for using this tool to detect spatial crop variability, both within and between plots/treatments, is promising. The coefficient of variation (CV) for the NDVI measurement sequence in each plot was determined. The CV’s throughout the crop season reflected the canopy expansion and senescence curve of maize and wheat. The CV was high at the beginning of the crop season, however, once the canopy began to close, leaves from larger plants covered the leaves and whorl of smaller plants, extending further into the linear row. Measurements to investigate spatial variability related to crop performance should thus be done after this initial stage at the end of the vegetative period when the vegetative biomass of the crops is fully established. Zero tillage systems without surface residue retention produced high CVs of the NDVI sequence and high spatial crop variability throughout the season, even after the vegetative period. As the only factors differing between the different plots are the tillage/residue/rotation practices and as similar patterns were found for all plots representing repetitions of the same management practice (zero tillage without residue retention), increased variability is an indicator of agronomic mismanagement or, conversely, of sound agriculturally production practices. [ABSTRACT FROM AUTHOR]

A field experiment was carried out to quantify biological nitrogen fixation (BNF) using the 15N isotope natural abundance method in maize ( Zea mays L.)/faba bean ( Vicia faba L.) and wheat ( Triticum aestivum L.)/faba bean intercropping systems. Faba bean was yielding more in the maize/faba bean intercropping, but not in the wheat/faba bean intercropping. Biomass, grain yield and N acquisition of faba bean were significantly increased when intercropped with maize, and decreased significantly with wheat, irrespective of N-fertilizer application, indicating that the legume could gain or lose productivity in an intercropping situation. There was yield advantage of maize/faba bean intercropping, but no in wheat/faba bean intercropping. The grain yield of the faba bean intercropped with maize was greater than that of faba bean monoculture due to increases of the stems per plant and the pods per stem of faba bean. N fertilization inhibited N fixation of faba bean in maize/faba bean and wheat/faba bean intercropping and faba bean monoculture. The responses of different cropping systems to N-fertilizer application, however, were not identical, with competitive intercropping (wheat/faba bean) being more sensitive than facilitative intercropping (maize/faba bean). Intercropping increased the percentage of N derived from air (%Ndfa) of the wheat/faba bean system, but not that of the maize/faba bean system when no N fertilizer was applied. When receiving 120 kg N/ha, however, intercropping did not significantly increase %Ndfa either in the wheat/faba bean system or in the maize/faba bean system in comparison with faba bean in monoculture. The amount of shoot N derived from air (Ndfa), however, increased significantly when intercropped with maize, irrespective of N-fertilizer application. Ndfa decreased when intercropped with wheat, albeit not significantly at 120 kg N/ha. Ndfa was correlated more closely with dry matter yield, grain yield and competitive ratio, than with %Ndfa. This indicates that that total dry matter yield (sink strength), not %Ndfa, was more critical for the legume to increase Ndfa. The results suggested that N fixation could be improved by yield maximization in an intercropping system. [ABSTRACT FROM AUTHOR]

Using an alkaline calcareous soil, pot experiments were conducted to elucidate the effects of NHvs. NOnutrition (50 or 100 mg kg−1 soil) of wheat and maize on microbial activity in the rhizosphere and bulk soils. Dicyandiamide was used as nitrification inhibitor to maintain NHas the predominant N source for plants grown in NH-treated soil. While maize grew equally well on both N sources, root and shoot growth of wheat was higher under NHthan under NOnutrition. Bacterial population density on roots, but not in the rhizosphere soil, was higher under NHthan under NO supplied at 150 mg N kg−1 soil; whereas at both N levels applied, NHcompared to NOnutrition of wheat and maize significantly increased microbial biomass in the rhizosphere soil. Under both plant species, NH vs. NOnutrition also increased aerobic and anaerobic respiration, and dehydrogenase activity in the rhizosphere. As microbial activity in the planted bulk and unplanted soils was hardly affected by the N-source, we hypothesize that the stimulation by NHof the rhizosphere microbial activity was probably due to higher availability of root exudates under NH than under NOnutrition. [ABSTRACT FROM AUTHOR]

Critical levels of selenium in raya ( Brassica juncea Czern L.), maize ( Zea mays L.), wheat ( Triticum aestivum L.) and rice ( Oryza sativa L.) were worked out by growing these crops in an alkaline silty loam soil treated with different levels of selenite-Se ranging from 1 to 25 μg g−1 soil. Significant decrease in dry matter yield was observed above a level of 5 μg Se g−1 soil in raya and maize; 4 μg Se g−1 soil in wheat and 10 μg Se g−1 soil in rice shoots. The critical level of Se in plants above which significant decrease in yield would occur was found to be 104.8 μg g−1 in raya, 76.9 μg g−1 in maize, 41.5 μg g−1 in rice and 18.9 μg g−1 in wheat shoots. Significant coefficients of correlation were observed between Se content above the critical level and dry matter yield of raya as well as rice ( r = −0.99, P ≤ 0.01), wheat ( r = −0.97, P ≤ 0.01) and maize (( r = −0.96, P ≤ 0.01). A synergistic relationship was observed between S and Se content of raya ( r = 0.96, P ≤ 0.01), wheat ( r = 0.89, P ≤ 0.01), rice ( r = 0.85, P ≤ 0.01) and maize ( r = 0.84, P ≤ 0.01). Raya, maize and rice absorbed Se in levels toxic for animal consumption (i.e. > 5 mg Se kg−1) when the soil was treated with more than 1.5 μg Se g−1. In case of wheat, application of Se more than 3 μg g−1 soil resulted in production of toxic plants. [ABSTRACT FROM AUTHOR]

Precision agriculture was initiated in the mid 1980s, using newly available technologies, to improve the application of fertilizers by varying rates and blends as needed within fields. Presently, the concept has been adapted to a variety of practices, crops, and countries. Its adoption varies significantly by cropping system, regions, and countries but it is progressively introduced or evaluated around the world. Several types of challenges limit a broader adoption: socio-economical, agronomical, and technological. Socio-economical barriers are principally costs and lack of skills. Agronomical challenges are lack of basic information, inadequate sampling and scouting procedures, absence of site-specific fertilizer recommendations, misuse of information, and lack of qualified agronomic services. There are multiple technological barriers that relate to machinery, sensor, GPS, software, and remote sensing. However, these barriers will be progressively lifted and precision agriculture will be a significant component of the agricultural system of the future. It offers a variety of potential benefits in profitability, productivity, sustainability, crop quality, food safety, environmental protection, on-farm quality of life, and rural economic development. [ABSTRACT FROM AUTHOR]

Analyzes the use of phytochelatins as biomarkers for the diagnosis of heavy metals in maize and wheat. Relationship between the concentration of cadmium and phytochelatins in plant's shoot material; Decrease in shoot concentration of cadmium after the addition of copper; Inferences drawn from differences in metal uptake and xylary metal transport among the two plant species.