Niels Erik Nielsen

Root traits as tools for creating phosphorus efficient crop varieties

This paper provides a brief assessment of the genetic variation in root properties (root morphology, including root hairs), mycorrhizal symbiosis, uptake kinetics parameters and root-induced changes (pH, organic acids and acid phosphatase) in the rhizosphere of various crop species and their genotypes and then briefly discusses the opportunities and challenges of using such knowledge for enhancing P efficiency of future crop genotypes by genetic means. Wide genotypic variation and heritability of root morphology, root hair length and density and thereby P acquisition provide opportunities for selection and breeding for root characteristics for increasing P acquisition. The progress is challenged by the concerns of high carbon cost of larger root systems and by the lack of cost effective methods to determine root length of a large number of genotypes under field conditions. The carbon cost of root hairs is low. Furthermore, low cost methods now exist to compare root hair formation of field grown genotypes. The development and application of sophisticated methods has advanced our knowledge on the role of mycorrhizal symbiosis in P acquisition and also on the molecular basis of fungi and plant interactions. However, extensive studies to explore genotypic variation in mycorrhizal responsiveness are rare, which makes it difficult to assess, how mycorrhizal symbiosis can be manipulated through breeding efforts. The promising variation found in P uptake kinetics parameters of crop genotypes in few studies indicates that more genotypes may be screened by relatively simple nutrient solution culture techniques. The genetic manipulation of the overall differences in cation-anion uptake, which is the main cause of rhizosphere pH change, may be difficult. For manipulation of rhizosphere pH, agronomic measures such as applications of ammonium or nitrate fertilisers may be more useful than breeding approaches. Also it seems difficult to assess what kind of genetic analysis should be performed to support the breeding efforts. Phosphorus mobilisation effect of pH depends on soil P compounds, therefore will differ with soil type. Both the enhanced release of organic acids and higher acid phosphatase activity in the rhizosphere may be useful for increasing P acquisition from inorganic and organic P pools, respectively. Modification of these traits by genetic means should be considered. For successful breeding programmes, the role of various root traits needs to be targeted in an integrated manner and then methods need to be developed for studying their importance under natural soil conditions, so that the genotypic variation can be explored and their ecological significance in P acquisition can be established.

Soil surface CO2 flux as an index of soil respiration in situ: A comparison of two chamber methods

Predictions of global climate change have recently focused attention on soils as major sources and sinks for atmospheric CO2, and various methodologies exist for measuring soil surface CO2 flux. A static (passive CO2 absorption in an alkali trap over 24 h) and a dynamic (portable infra-red CO2 gas analyzer over 1–2 min) chamber method were compared. Both methods were used for 100 different site × treatment × time combinations in temperate arable, forest and pasture ecosystems. Soil surface CO2 flux estimates covered a wide range from 0 to ca. 300 mg CO2ue5f8C m−2 h−1 by the static method and from 0 to ca. 2500 mg CO2ue5f8C m−2 h−1 by the dynamic method. The relationship between results from the two methods was highly non-linear, and was best explained by an exponential equation. When compared to the dynamic method, the static method gave on average 12% higher flux rates below 100 mg CO2ue5f8C m−2 h−1, but much lower flux rates above 100 mg CO2ue5f8C m−2 h−1. Spatial variability was large for both methods, necessitating a large number of replicates for reliable field data, with typical coefficients of variation being in the range 10–60%, usually higher with the dynamic than the static method. Diurnal variability in soil surface CO2 flux was partly correlated with soil temperature, whereas day-to-day variability was more unpredictable. However, use of a mechanistic simulation model of CO2 transport in soil, SOILCO2, showed that very large day-to-day changes in soil surface CO2 flux can result from rainfall events causing relatively small changes in soil water content above field capacity (ca. −10 kPa), even if CO2 production rates remained relatively unaffected.

Root hairs and phosphorus acquisition of wheat and barley cultivars

Root-soil contact is an important factor for uptake of a less mobile soil nutrient such as phosphorus (P) by crop plants. Root hairs can substantially increase root-soil contact. Identification of crop cultivars with more and longer root hairs can, therefore, be useful for increasing P uptake in low input agriculture. We studied the root hairs of wheat (Triticum aestivum L. cvs. Kosack, Foreman, Kraka) and barley cultivars (Hordeum vulgare L. cvs. Angora, Hamu, Alexis, Canut) in relation to P depletion from the rhizosphere in three soils of different P levels (0.45, 1.1 and 1.6 mmoles P kg−1 soil; extracted with 0.5 M NaHCO3). Root hairs were measured in solution culture having nutrients and concentration similar to soil solution. Root hairs of Kraka were much longer (1.27 ± 0.26 mm) and denser (38 ± 3) hairs mm−1 root) than those of Kosack which had shorter (0.49 ± 0.2 mm) and fewer (24 ± 3) hairs mm−1 root) root hairs. Root hairs increased root surface area (RSA) of Kraka by 341%. The increase with Foreman was 142% and with Kosack it was 95%. For winter barley, the length (1.1 ± 0.3 mm) and density (30 ± 1 hairs mm−1 root) of root hairs of Hamu differed from root hair length (0.52 ± 0.18 mm) and density (27 ± 1 hairs mm−1 root) of Angora. Root hairs of spring barley cultivars differed in length (Canut 1.0 ± 0.24 mm; Alexis 0.64 ± 0.19 mm) but not in density (Canut 31 ± 1, Alexis 30 ± 2 hairs mm−1 root). Root hair diameter (12 ± 1µm) did not differ among the cultivars. Root hairs increased RSA of Canut by 245%, Hamu by 237%, Alexis by 143% and Angora 112%. The variation in root hair parameters of the cultivars was related to quantity of P depleted from rhizosphere. The correlation (R2) between the root hair lengths of wheat cultivars and the quantity of P depleted from the rhizosphere soil (Q) was (0.99***) in low-P, (0.85***) in medium-P and (0.78**) in high-P soil. The values of (R2) between the root hair surface areas of wheat cultivars and Q were (1.00***) in low-P, (0.74**) in medium-P and (0.66**) in high-P soil. Similar high values of R2 were found for barley. These results show that the variation in root hairs of cereal cultivars can be considerable and it can play a significant role in P acquisition, especially in low-P soils.

The effects of root-induced pH changes on the depletion of inorganic and organic phosphorus in the rhizosphere

A new method allowing control of rhizosphere pH and mineral nutrition was applied to study depletion of various organic and inorganic phosphorus fractions extractable sequentially with 0.5M KHCO3 (pH 8.5), 0.1M NaOH and residual P extractable with 6M H2SO4 from the rhizosphere soil.Soil pH was affected about 2 mm from the root mat. Depletion zones of inorganic P (KHCO3-Pi) extractable with 0.5M KHCO3 extended up to about 4 mm but the depletion zones of all other P fractions were about 1 mm only. The root-induced decrease of soil pH from 6.7 to 5.5 increased the depletion of total P from all fractions by 20% and depletion of KHCO3-Pi and residual P by 34% and 43%, respectively. Depletion of organic P (KHCO3-Po) extractable with 0.5M KHCO3 was not affected by a change in rhizosphere pH. With constant or increased pH, depletion of inorganic P (NaOH-Pi) was 17% and organic P (NaOH-Po) was 22% higher than with decreased pH. Only 54–60% of total P withdrawn from all fractions was from KHCO3-Pi. Substantial amounts of KHCO3-Po and NaOH-Po were mineralized and withdrawn from the rhizosphere within 1 mm from the root mat, as 11–15% of total P withdrawn originated from the organic P fractions. A remaining 11–16% was derived from NaOH-Pi, and 15–18% from residual P fractions likely to be rather immobile. Thus, 40–46% of the P withdrawn near the root mat of rape originated from non-mobile P fractions normally not included in 0.5M NaHCO3 extraction used to obtain an index of plant-available soil P.

Barley genotypes with long root hairs sustain high grain yields in low-P field

Superior root traits, like long root hairs, enhance phosphorus (P) uptake and hence the selection for root hair trait offers the possibility to sustain yields in low-P soils. It is yet unknown whether root hair promoted P uptake of barley genotypes is related to the grain yield in low -P field soil. To investigate this, a set of barley genotypes was pre-screened using hydroponics for long (about 1xa0mm, cvs. Pongo, Linus Barke, Tofta, Henni) and short root hairs (about 0.5xa0mm, cvs. AC91/5606/17, Meltan, Scarlett, Century, Otira, and Cecilia). The selected genotypes were cultivated in low-P field plots (no P in 35xa0years, 3xa0μM P in soil solution) and in plots amended by moderate (10xa0kg Pxa0ha−1, 6xa0μM P in soil solution) and high (20xa0kg Pxa0ha−1, 10xa0μM P in soil solution) P fertilisation. The ranking of the genotypes root hairs in laboratory remained consistence in the field, except for cv. Barke (1.05xa0mm). The genotypes varied in specific root length (SRL, mxa0g−1) and root hair length (RHL), but the estimated volume of soil explored by root system clearly depended on RHL. The correlations of RHL (R2=0.60***), volume of soil explored by root system (R2=0.57***) and SRL (R2=0.40**) with the P uptake in the field were highly significant. The correlation of root-shoot ratio with the P uptake was non-significant (R2=0.11). The genotypes with long root hairs preserved economical stable grain yield in low, moderate and high P plots. In contrast, the genotypes with short root hairs produced lower grain yield in low P soil, but they responded to moderate and high P fertilisation by significant increase in their grain yields. From the results of this field-based case study, it is concluded that barley genotypes with long root hairs are better adapted in low P soils and they express high yield potentials both in low and high P soils.

Temporal variation of C and N mineralization, microbial biomass and extractable organic pools in soil after oilseed rape straw incorporation in the field

Abstract The temporal variation of soil microbial biomass C and N, extractable organic C and N, mineral N and soil-surface CO2 flux in situ in two arable soils (a sandy loam and a coarse sandy soil) was examined periodically for a full year after field incorporation of 0, 4 or 8 t dry mass ha−1 of oilseed rape straw in late summer. Both unlabelled and 15N-labelled straw were applied. Soil-surface CO2 flux, used as an index of soil respiration, was up to 2-fold higher in the straw-amended treatments than in the unamended treatment at both sites during the first 6–8 wk, but the general temporal pattern was mainly controlled by soil temperature and soil water content. Microbial biomass C and N increased very rapidly after the straw amendments and the 31–49% difference from the unamended treatment persisted throughout the winter. Temporal variations in soil microbial biomass C and N were only within ±13–22% of the mean at both sites and in all straw treatments over the 1 y period. Microbial biomass C-to-N ratios were not significantly different between straw treatments and were relatively constant over time. Extractable organic C and N were slightly higher in the straw-amended treatments and were higher in spring and summer than in autumn and winter. More than 90% of the added straw N could be accounted for initially and there was no loss of straw N over the winter period, in spite of a winter rainfall that was twice the 25 y average. Between 52 and 80% of the initial increase in microbial biomass N was derived from the straw N, with up to 27% of the straw N being incorporated into the microbial biomass. Rapid immobilization of soil mineral N occurred simultaneously and the sum of this and the straw-derived microbial biomass N on day 7 exceeded the total increase in microbial biomass N, indicating a very rapid turnover of microbial biomass in the first few days. Significant differences in microbial biomass C and N between the straw treatments were still found after nearly 1 y and the decay constant of straw-derived microbial biomass N was estimated to be ca. 0.26 y−1.

Variation in root hairs of barley cultivars doubled soil phosphorus uptake

Length and density (number mm-1 root) of root hairs of two barley (Hordeum vulgare L.) cultivars Salka and Zita and their capability to absorb phosphorus (P) from nutrient solution as well as from rhizosphere soil were studied. The cultivars were chosen because they differed most among 30 cultivars in ability to absorb P from low P soil in two field conditions. In nutrient solution culture, Salka had 32±4 root hairs mm-1 root, 1.02±0.22 mm long. Zita had 21±3 hairs mm-1 root, 0.54±0.14 mm long. In soil, the root hairs of both the cultivars were slightly longer (Salka 1.10 ±0.16 mm; Zita 0.63±0.18 mm) than in solution culture but the difference was non-significant (p<0.05). The root hairs increased the effective root surface area of Salka by 206% and that of Zita by 81%. In solution culture, Salka produced 163±9 m g-1 and Zita 153±11 m g-1 dry roots in 21 days. Salka produced 1.65±0.22 g and Zita 1.51±0.31 g of green dry matter (DM). The cultivars did not differ in uptake of P from nutrient solution culture. The P content of DM was 0.42±0.1% in Salka and 0.41±0.08% in Zita. In soil, Salka depleted two times more P from rhizosphere than Zita. The longer root hairs of Salka increased the extension of the depletion zone for NaHCO3-Pi (inorganic P extracted with 0.5 M NaHCO3) in the rhizosphere. The cultivars also depleted NaOH-Pi (inorganic P extracted with 0.1 M NaOH) from the rhizosphere soil, but the difference between the cultivars was non-significant (p<0.05). The results suggested that the ability of Salka to absorb more inorganic soil P was due to its longer and denser root hairs.

Direct evidence on participation of root hairs in phosphorus (32P) uptake from soil

Root hairs substantially extend root surface for ion uptake. Although many reports suggest a relationship between root hairs and phosphorus (P) uptake of plants, the role of root hairs in phosphorus uptake from soils is still debated. We measured uptake of phosphorus from soil directly via root hairs. Root hairs only were allowed to penetrate through a tightly stretched nylon screen (53 µm) glued to the bottom of a PVC tube. The penetrating root hairs grew for 2 and 4 days in soil labelled with radioisotope phosphorus (P) tracer 32P (185 kBq g-1 dry soil) filled in another PVC tube. Transparent plastic rings of thickness ranging from 0.25 mm to 2.0 mm were inserted between the two PVC tubes. This provided slit width for microscopic observations in situ, which confirmed that only root hairs were growing into the 32P labelled soil. In some cases no rings were inserted (slit width = 0) where both root hairs and root surface were in contact with the labelled soil (total 32P uptake). The uptake of32 P from soil via the root hairs only was quantified by measuring activity of 32P in the plant shoot (32P uptake only via root hairs).The results showed that when 70 percent of the root hairs grew into the labelled soil, they contributed to 63 percent of the total P uptake. With decreasing number of root hairs growing into the 32P labelled soil, the quantity of 32P in the plant shoot decreased. In this study, P uptake via root hairs was measured in a soil-based system, where root hairs were the only pathway of 32P from soil to the plant shoot. Therefore, this study provides a strong evidence on the substantial participation of root hairs in uptake of phosphorus from soil.

A root hairless barley mutant for elucidating genetic of root hairs and phosphorus uptake

This paper reports a new barley mutant missing root hairs. The mutant was spontaneously discovered among the population of wild type (Pallas, a spring barley cultivar), producing normal, 0.8 mm long root hairs. We have called the mutant bald root barley (brb). Root anatomical studies confirmed the lack of root hairs on mutant roots. Amplified Fragment Length Polymorphism (AFLP) analyses of the genomes of the mutant and Pallas supported that the brb mutant has its genetic background in Pallas. The segregation ratio of selfed F2 plants, resulting from mutant and Pallas outcross, was 1:3 (−root hairs:+root hairs), suggesting a monogenic recessive mode of inheritance.In rhizosphere studies, Pallas absorbed nearly two times more phosphorus (P) than the mutant. Most of available inorganic P in the root hair zone (0.8 mm) of Pallas was depleted, as indicated by the uniform P depletion profile near its roots. The acid phosphatase (Apase) activity near the roots of Pallas was higher and Pallas mobilised more organic P in the rhizosphere than the mutant. The higher Apase activity near Pallas roots also suggests a link between root hair formation and rhizosphere Apase activity. Hence, root hairs are important for increasing plant P uptake of inorganic as well as mobilisation of organic P in soils.Laboratory, pot and field studies showed that barley cultivars with longer root hairs (1.10 mm), extracted more P from rhizosphere soil, absorbed more P in low-P field (Olsen P=14 mg P kg−1 soil), and produced more shoot biomass than shorter root hair cultivars (0.63 mm). Especially in low-P soil, the differences in root hair length and P uptake among the cultivars were significantly larger. Based on the results, the perspectives of genetic analysis of root hairs and their importance in P uptake and field performance of cereals are discussed.

Effect of heavy metals on peppermint and cornmint

Heavy metal pollution of agricultural soils and air is one of the most severe ecological problems on a world scale and in Bulgaria in particular. The biggest sources of pollution in Bulgaria are some non-ferrous metals smelters, such as the Non-Ferrous Metals Combine (NFMC) near Plovdiv, situated on very fertile soils. Vegetable, arable and animal production in this area results in contaminated produce with excessive amounts of Cd, Pb, Cu, Mn and Zn.In order to discover some crops which could be grown on these areas without contamination of the end product, we conducted (in 1991–1993) field experiments in the vicinities of NFMC near Plovdiv. As experimental material we used Mentha piperita L. (cv Tundza and Clone No 1) and Mentha arvensis var piperascens Malinv. (cv Mentolna-14). Plants have been grown on three Plots: Plot No 1-at a distance of 400 m from the source of pollution; Plot No 2-at 3 km from the source of pollution and on a control Plot-in the experimental gardens of University of Agriculture in Plovdiv, at 10 km from the source of pollution. It was established that heavy metal pollution of soil and air at a distance of 400 m from the source of pollution decreased the yields of fresh herbage by 9–16% and the yield of essential oil by up to 14% compared to the control, but did not negatively affect the essential oil content and its quality.Oils obtained from Plot 1 at a distance of 400 m from the source of pollution have not been contaminated with heavy metals.Cultivar response to heavy metal pollution was established. A positive correlation between Pb concentration in leaves and in essential oil was found.Heavy metal concentration in the plant parts was found to be in order: for Cd roots > leaves > rhizomes > stems; Pb roots = leaves > rhizomes = stems; Cu roots > rhizomes = stems = leaves; Mn roots > leaves > stems = rhizomes; Zn leaves > roots > stems = rhizomes.The tested cultivars of peppermint and cornmint could be successfully grown in highly heavy metal polluted areas, as in the area around NFMC near Plovdiv, without contamination of the end product-the essential oils.Despite of the yield reduction (up to 14%), due to heavy metal contamination, mint still remained a very profitable crop and it could be used as substitute for the other highly contaminated crops.