Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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Calcium-(organo)aluminum-proton competition for adsorption to tomato root cell walls: Experimental data and exchange model calculations
Riemsdijk, W.H. van; Keltjens, W.G. ; Postma, J.W.M. - \ 2005
Environmental Science and Technology 39 (2005)14. - ISSN 0013-936X - p. 5247 - 5254.
aluminum-toxicity - conformational transitions - calcium pectate - organic-acids - ion behavior - al-binding - growth - pectin - elongation - hypothesis
Aluminum interacts with negatively charged surfaces in plant roots, causing inhibition of growth and nutrient uptake in plants growing on acid soils. Pectins in the root cell wall form the major cation adsorption surface, with Ca2+ as the main adsorbing cation. Adsorption of Al3+ and Ca2+ to isolated cell wall material of tomato (Lycopersicon esculentum L.) roots was examined at pH 3.00-4.25 and in the presence of the aluminum chelators citrate and malate. Al3+ displaced Ca2+ from its pectic binding sites in the cell wall to a large extent but apparently also bound to non-Ca binding groups, displacing protons. Aluminum adsorption depended on the pH of the solution, with little Al adsorbing to the cell wall material at very low pH (
Effect of ectomycorrhizal fungi on the uptake of Ca, Mg and Al by Pinus sylvestris under aluminium toxicity
Schöll, L. van; Keltjens, W.G. ; Hoffland, E. ; Breemen, N. van - \ 2005
Forest Ecology and Management 215 (2005)1-3. - ISSN 0378-1127 - p. 352 - 360.
norway spruce seedlings - fungus suillus-variegatus - picea-abies seedlings - paxillus-involutus - mineral-nutrition - rigida seedlings - nutrient-uptake - critical loads - organic-acids - forest soils
Aluminium toxicity has been considered an important factor in forest decline. In earlier pot experiments, ectomycorrhizal tree seedlings were reported to have higher growth rates than non-mycorrhizal seedlings under aluminium toxicity. In this paper, we test that if this is caused by exclusion of Al and higher uptake of Ca and Mg by the ectomycorrhizal roots. Pinus sylvestris seedlings, grown for 3 months on a semi-hydroponic system, were continuously drip-irrigated with nutrient solution, containing 0 or 1.5 mM Al. The seedlings were non-mycorrhizal or colonized by ectomycorrhizal fungal species from a podzol soil. The presence of 1.5 mM Al in solution significantly decreased the dry weights of needles and roots compared to the control, and increased mycorrhizal colonization. Yet growth was not affected by mycorrhizal colonization. Concentrations of Al in the needles were significantly higher at 1.5 mM Al in solution than at 0 mM Al, and significantly higher in ectomycorrhizal seedlings than in non-mycorrhizal seedlings. Concentrations of Ca and Mg in the needles were significantly lower at 1.5 mM Al in solution than at 0 mM Al, but were not affected by ectomycorrhizal colonization. In conclusion, ectomycorrhizal colonization did not mitigate aluminium toxicity in our semi-hydroponic system. We suggest that better growth of soil-grown ectomycorrhizal tree seedlings compared to non-mycorrhizal tree seedlings should be explained by improved uptake of immobile nutrients such as P through a better soil exploration by the external mycelium or by detoxification of Al by organic anions excreted by the fungi
Metabolism and root exudation of organic acid anions under aluminium stress
Mariano, E.D. ; Jorge, R.A. ; Keltjens, W.G. ; Menossi, M. - \ 2005
Brazilian Journal of Plant Physiology 17 (2005)1. - ISSN 1677-0420 - p. 157 - 172.
Numerous plant species can release organic acid anions (OA) from their roots in response to toxic aluminium (Al) ions present in the rooting medium. Hypothetically OA complex Al in the root apoplast and/or rhizosphere and thus avoid its interaction with root cellular components and its entry in the root symplast. Two temporal patterns of root OA exudation are observed. In pattern I, OA release is rapidly activated after the contact of the root with Al ions while in pattern II there is a lag phase between the addition of Al and the beginning of OA release. Compounds other than OA have been detected in root exudates and are also correlated with Al resistance in plants. Plant species like buckwheat and tea show mechanisms of Al tolerance, which confer them the capacity to inactivate and store Al internally in the leaves. Disturbances in metabolic pathways induced by Al are still obscure and their relation to the altered OA concentration observed in roots under Al stress is not yet established. High concentrations of OA in roots do not always lead to high rates of OA release even when the spatial distribution of these two characteristics along the root axis is taken into account. Al induces high permeability to OA in young root cells and anion channels located in the cell membrane have been proposed to mediate the transport of OA to outside the cell. Genetically modified plants that overexpress genes involved in the biosynthesis and transport of OA as well as in Al toxicity events at the cell level have been generated. In most cases the transformations resulted in an improved ability of the plant to cope with Al stress. These promising findings reinforce the possibility of engineering plants with superior resistance to Al-toxic acid soils. The environmental impact of the large amounts of root exudates possibly conferred by these genetically modified plants is discussed, with special emphasis on soil microbiota
Long-term effects of aluminium exposure on nutrient uptake by maize genotypes differing in aluminium exposure
Mariano, E.D. ; Keltjens, W.G. - \ 2005
Journal of Plant Nutrition 28 (2005)2. - ISSN 0190-4167 - p. 323 - 333.
growth - sorghum - tolerance - plants
Genotypic differences in resistance to aluminum (Al) found in many plant species grown in conditions of Al stress seem to include differences in Al-induced inhibition of absorption and utilization of nutrients. Aiming to study the Al effects on nutrient uptake of maize genotypes differing in Al resistance and to check whether differences in mineral nutrition under Al stress correspond with resistance to Al in maize, an experiment involving 10 maize genotypes differing in Al resistance and 2 concentrations of Al (0 and 100 µM Al) was established. Total plant (shoot + root) uptake of phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), copper (Cu), iron (Fe), manganese (Mn), and zinc (Zn) was determined in maize plants after 14 days of growth in culture solution at the two concentrations of Al. The relative uptake [(uptake at 100 µM Al/uptake at 0 µM Al)×100] of the nutrients studied varied from 22% to 157%, indicating the existence of intraspecific variation for such feature in the presence of Al. Generally, Al had negative effects on the uptake of macro- and micronutrients. Al effects were most pronounced on the uptake of Ca and Mg, with respective reductions of 61% and 72%, when averaged across the 10 genotypes. Among the micronutrients, the most pronounced effects of Al were noted on Mn and Zn. Despite showing significant reductions in uptake of Ca and Mg, the maize genotypes showed a rather variable sensitivity to the Al stress imposed, which was related to their general resistance to Al previously assessed using root length as indicator. Under conditions of Al stress, genotypes more resistant to Al maintained a relatively higher absorption of both Ca and Mg than those more sensitive to Al, suggesting that the ability of a genotype to maintain a less disturbed nutrient uptake under Al stress can be an important component in resistance to Al
Role of ectomycorrhizal fungi in the uptake of Mg by Scots Pine (Pinus sylvestris)
Schöll, L. van; Hoffland, E. ; Keltjens, W.G. ; Breemen, N. van - \ 2005
In: Book of Abstracts of the International Congress Rhizosphere 2004 - Perspectives and challenges - A tribute to Lorenz Hiltner, Munich, Germany, 12 t/m 17 September 2004. - Munich : - p. 109 - 109.
Variation for aluminium resistance among maize genotypes evaluated with three screening methods
Mariano, E.D. ; Keltjens, W.G. - \ 2004
Communications in Soil Science and Plant Analysis 35 (2004)17-18. - ISSN 0010-3624 - p. 2617 - 2637.
organic-acids - nutrient solutions - root elongation - tropical maize - tolerance - cultivars - soil - toxicity - sorghum - growth
Genetically determined differences in aluminum (Al) resistance exist among plant species and genotypes, and efforts to select and breed maize germplasm with higher resistance to Al have been made worldwide. This work aimed to study genotypic variation for Al resistance in maize genotypes by using three different screening techniques, to compare the results of the screening techniques, and to select genotypes with differential sensitivity to Al for further research on the mechanisms of Al resistance in maize. The effects of Al on various plant characteristics were studied with 10 maize genotypes in a series of experiments that comprised short-term (4 and 14 d) exposures to Al in culture solution (up to 100 µM Al), as well as longer-term (40 d) growth in an acid soil (soil solution pH range 3.4-4.1). Al resistance varied widely among the maize genotypes, as revealed by the different screening methods used. A screening method based on root elongation rate of seedlings growing in culture solution was effective in discriminating resistance to Al. A concentration of 40 µM Al gave the best differential responses among the 10 genotypes studied, causing reductions in root elongation rate of 10 to 68%. The best indicators of differential Al resistance were root characteristics, especially root length. Internal root concentrations of citrate and malate, however, did not reflect plant resistance to Al. The Al resistance rankings established with the screening techniques were consistent and indicated genotypes with contrasting sensitivity to Al to be used in further studies of mechanisms of Al resistance in maize.
Contribution of ectomycorrhizal fungi to plant uptake of Ca, Mg and K and to mineral weathering
Tunneling, L. ; Schöll, L. van; Smits, M.M. ; Hoffland, E. ; Keltjens, W.G. ; Jongmans, A.G. ; Breemen, N. van - \ 2004
In: Book of Abstracts, Fourth International Conference on Mycorrhizas, Montreal, 10-15 August 2003. - Montreal : - p. 338 - 338.
Aluminium concentration versus the base cation to aluminium ratio as predictors for aluminium toxicity in Pinus sylvestris and Picea abies seedlings
Schöll, L. van; Keltjens, W.G. ; Hoffland, E. ; Breemen, N. van - \ 2004
Forest Ecology and Management 195 (2004)3. - ISSN 0378-1127 - p. 301 - 309.
norway spruce seedlings - gleditsia-triacanthos l - critical loads - root-growth - forest soils - calcium - magnesium - plants - inhibition - resistance
Aluminium (Al) toxicity is considered an important factor in forest deterioration caused by soil acidification. A ratio of base cations (BC) to Al in the soil solution lower than 1 is widely used as an indicator for potentially adverse effects on tree health. In our view, the validity of the assumptions underlying the use of the BC:AI ratio as an indicator for Al toxicity in trees has never been evaluated properly. Here, we evaluate the importance of the base cations Ca and Mg in counteracting Al toxicity. Pinus sylvestris and Picea abies seedlings were grown on nutrient solution with a range of Al (0-0.25-0.5-1-2 mM) and base cation (0.25-0.5-2 mM) concentrations, giving BC:Al ratios of 1 at different levels of Al. Increasing concentrations of Al in solution caused growth reductions, which could not be counteracted by increasing concentrations of BC in solution with P. sylvestris and only partly counteracted with P. abies. Increased concentrations of Al in solution decreased the concentrations in shoot and root of both Ca and Mg, while increased concentrations of BC in solution increased tissue concentrations of BC. Growth reductions were, however, not a result of BC deficiencies, as growth reduction already occurred in tree seedlings that maintained adequate concentrations of Ca and Mg. All growth and uptake variables measured showed a higher or equal correlation with the absolute concentrations of Al or Al + BC in solution than with the BC:Al ratio. We conclude that Al toxicity is determined solely by the concentration of Al in solution. Shoot growth decreased significantly as dissolved Al increased at a constant BC:Al ratio of 1. In P. abies, but not in P. sylvestris, dissolved BC can positively affect uptake of BC and growth, which might partly alleviate the toxic effects of Al. Our results show that the mechanistic explanation for the effect of the BC:Al ratio is insufficient to describe Al toxicity. Care should be taken when using models based on the BC:AI ratio to predict the effect of Al on tree growth. (C) 2004 Elsevier B.V. All rights reserved.
Status en perspectieven van biologische H2 en CH4 productie : inventarisatie van R&D en expertise bij Nederlandse kennisinstellingen, betekenis voor duurzame ontwikkeling, R&D issues en ontwikkelingsrichtingen, mogelijkheden voor stimulering van ontwikkeling en implementatie
Reith, J.H. ; Albracht, S.P.J. ; Bleijlevens, B. ; Hagen, W.R. ; Stams, A.J.M. ; Keltjens, J.T. ; Claassen, P.A.M. ; Akkerman, I. ; Wijffels, R.H. ; Zeeman, G. ; Lier, J.B. van; Groenestijn, J.W. van; Jansen, D. - \ 2003
Petten : ECN (Rapport ECN C-03-021) - 66
waterstof - methaan - methaanproductie - biogas - bio-energie - vervangbare hulpbronnen - biologische productie - biomassa - energiebronnen - anaërobe afbraak - biobased economy - biobrandstoffen - hydrogen - methane - methane production - biogas - bioenergy - renewable resources - biological production - biomass - energy sources - anaerobic digestion - biobased economy - biofuels
Deze rapportage beschrijft de uitkomsten van een inventarisatie onder onderzoekers verbonden aan Nederlandse kennisinstellingen. Het doel van de inventarisatie is: 1. Het in kaart brengen van lopend onderzoek, expertise en infrastructuur bij Nederlandse kennisinstellingen op het gebied van biologische waterstof- en methaan productie; 2. Identificatie van de visie van de deelnemende onderzoekers ten aanzien van de potentiële betekenis en toepassingen van biologische H2 en CH4 productieprocessen voor duurzame ontwikkeling; R&D issues en kansrijk geachte onderzoeksrichtingen, en mogelijkheden voor bevordering van de verdere ontwikkeling en praktijk implementatie. Onderzoek en expertise op het gebied van biologische waterstof en methaan productie bij Nederlandse kennisinstellingen staan op een hoog peil. Gezien het vóórkomen van zowel gedeelde als complementaire expertise biedt samenwerking tussen de twee werkgebieden een duidelijke toegevoegde waarde. De meerwaarde voor duurzame ontwikkeling betreft met name: hernieuwbare energieproductie (H2, CH4) uit biomassa- en reststromen, het sluiten van nutriënt en waterkringlopen, en de ontwikkeling van goedkope waterstofkatalysatoren voor brandstofcellen en voor de vervanging van chemische reacties (hydrogenering, ¿water-gas-shift¿) door biologische alternatieven. Belangrijke onderzoeksrichtingen op het gebied van zowel biologische waterstof als methaan productie worden geïdentificeerd. Tevens worden aanbevelingen gedaan voor de bevordering van de verdere ontwikkeling.
Citrate exudation by maize roots; A possible mechanism of resistance to aluminium
Mariano, E.D. - \ 2003
Wageningen University. Promotor(en): Willem van Riemsdijk, co-promotor(en): W.G. Keltjens. - [S.l.] : S.n. - ISBN 9058088928 - 128
zea mays - maïs - citraten - aluminium - wortels - exudatie - bodemgiftigheid - plantenontwikkeling - genetische variatie - voedingsstoffenopname (planten) - zea mays - maize - citrates - aluminium - roots - exudation - soil toxicity - plant development - genetic variation - nutrient uptake
Al toxicity and plant nutrient uptake: a role for root cell walls, pH and organic chelators
Postma, J. - \ 2003
Wageningen University. Promotor(en): Willem van Riemsdijk, co-promotor(en): W.G. Keltjens. - Wageningen : S.n. - ISBN 9058089290 - 169
planten - aluminium - voedingsstoffenopname (planten) - plantenvoeding - fytotoxiciteit - wortels - celwanden - chelaatvormers - plants - aluminium - nutrient uptake - plant nutrition - phytotoxicity - roots - cell walls - chelating agents
Evaluating the role of root citrate exudation as a mechanism of aluminium resistance in maize genotypes
Mariano, E.D. ; Keltjens, W.G. - \ 2003
Plant and Soil 256 (2003). - ISSN 0032-079X - p. 469 - 479.
zea-mays l - triticum-aestivum l - organic-acids - tolerance mechanism - tropical maize - citric-acid - toxicity - plants - wheat - translocation
Organic anion exudation by roots as a mechanism of aluminium (Al) resistance has been intensively studied lately. In the present study, we evaluated qualitative and quantitative aspects of root exudation of organic anions in maize genotypes of distinct sensitivity to Al in response to Al exposure. Roots of maize seedlings were grown axenically in nutrient solution and root exudates were collected along the whole seminal root axis for a short period ( 4 h) using a divided-root-chamber technique. In root exudates collected from 10-mm long root apices, citrate accounted for 67% of the total organic anions found, followed by malate (29%), trans-aconitate (3%), fumarate ( <1%), and cis-aconitate ( 1%). Rates of citrate exudation from root apices of two genotypes with differential resistance to Al were consistently higher in the Al resistant one, differing by a factor of 1.7-3.0 across a range of external Al concentrations. Furthermore, relative Al resistance of eight maize genotypes correlated significantly well with their citrate exudation rate measured at 40 μM Al. Higher exudation rates were accompanied by a less inhibited root elongation. The exudation of citrate along the longitudinal axis of fully developed seminal roots showed a particular pattern: citrate was exuded mainly in the regions of root apices, either belonging to the main root or to the lateral roots in the most basal part of the main root. The involvement of citrate in a mechanism of Al resistance is evaluated in terms of protection of the root from the effects of excess Al on root elongation and on nutrient uptake along a root axis showing distinct sites of citrate exudation.
Phosphorus levels in shoots of bambara groundnut in Botswana soils
Ramolemana, G.M. ; Keltjens, W.G. ; Wessel, M. ; Maphanyane, G.S. - \ 2002
Journal of Plant Nutrition 25 (2002)9. - ISSN 0190-4167 - p. 2035 - 2049.
The critical phosphorus (P) concentration and range of bambara groundnut (Vigna subterranea) plants were determined from a sand culture experiment at Botswana College of Agriculture. Twelve P levels (0.207-159 mg P pot(-1)) were used for growing the plants for 78 days (early podding stage) and the shoot P concentrations determined. The critical P concentration was used for assessing the P nutritional status of bambara groundnut plants grown in farmers fields in Botswana. Plants from 10 farmers' fields were sampled at 78 days after sowing and analyzed for P. The critical shoot P concentration and range for bambara groundnut determined visually from graphs were 0.15% and 0.15-0.20%, respectively. In 80% of the farms, plants showed shoot P concentrations between 0.10% and 0.13% which correspond with moderate to severe P deficiency. Therefore, most of bambara groundnut farmers in Botswana grow the crop under limiting P conditions.
The phosphorus and nitrogen nutrition of bambara groundnut (Vigna subterranea (L.) Verdc.) in Botswana soils : an exploratory study
Ramolemana, G.M. - \ 1999
Agricultural University. Promotor(en): M. Wessel; W.G. Keltjens. - S.l. : Ramolemana - ISBN 9789058080202 - 89
vigna subterranea - fosfor - stikstof - plantenvoeding - bevruchting - botswana - vigna subterranea - phosphorus - nitrogen - plant nutrition - fertilization - botswana

Bambara groundnut ( Vigna subterranea (L.) Verdec.) is a legume crop grown especially by small farmers mainly in semi-arid parts of Africa both in mixed cultivation and pure stands. It is considered as a hardy crop because of its drought tolerance, resistance to pests and diseases and ability to yield on chemically poor soils. The crop produces edible seeds which can be eaten unripe or stored as dried pulse for later consumption. In Botswana it is grown under semi-arid conditions by small farmers as a minor crop. Attempts have been made to increase the bambara groundnut yield through application of nutrients and improvement of the nitrogen-fixing capacity.

This research has not been undertaken systematically and especially the responses to P and N fertilizers did not show a clear pattern. Given the low available P levels and the limited N supply in Botswana soils, a research programme was undertaken to investigate the P and N nutrition of bambara groundnut. The main objectives of the study were:(i) to investigate the effects of applied P and N on growth and development of bambara groundnut in Botswana soils, and (ii) to investigate the P and N uptake, shoot concentrations at different growth and development stages of bambara groundnut and (iii) how uptake, internal concentration and growth are related. The experimental programme consisted of five pot experiments and a large field experiment with P and irrigation as treatments in which bambara groundnut was grown on a representative low P soil.

In addition a sand culture experiment was conducted to establish the critical P concentration in bambara groundnut shoots. The critical P levels were subsequently used to determine the P status of bambara groundnut plants in farmers' fields.

In Chapter 2 the question was whether bambara groundnut will respond to P fertilization in a P-poor soil in the absence and presence of added Rhizobium inoculum, whether bambara groundnut selections are different with respect to P nutrition, and whether soil moisture level under field conditions limits P uptake. In a preliminary pot experiment, which was terminated at flowering (51 days after sowing; DAS), the response of bambara groundnut shoot dry matter (DM) to P fertilization (fertilizer ground and thoroughly mixed with the soil) was positive and linear. There was no effect of P fertilization on nodulation both in the absence and presence of added inoculum. Leaf blade P and N concentrations were not affected by P fertilization.

The two selections showed basically the same response to P fertilization. The indigenous rhizobia seemed to be sufficiently effective in nodulating bambara groundnut. In the field, P fertilization (fertilizer broadcast after ploughing and worked into the soil with a digging fork) had no effect on bambara groundnut plants, while irrigation increased all plant growth parameters except root DM. Total seed yield was 2.8 t and 4.2 t ha -1for rainfed and irrigated treatments, respectively. Leaf blade P and N concentrations were not affected neither by P fertilization nor by soil moisture content.

However, irrigation increased the uptake of P. Evidently, part of the P pool present in the soil becomes available under improved soil moisture conditions, probably due to improved P diffusion to the roots. The lack of response to applied P in the field experiment with a low P soil might imply that when root growth is not restricted by soil volume as is the case in pot experiments, available soil P was enough to meet the P requirements of bambara groundnut. This could mean that compared to other crops like cereals bambara groundnut has a low P requirement possibly combined with a high P efficiency.

This hypothesis was tested in a pot experiment (reported in Chapter 3.1) in which the P uptake and the internal P use efficiency of bambara groundnut were compared to those of maize and pigeon pea. Maize is a crop with a high P requirement and pigeon pea has the capacity to use soil P normally not available to other crops. The response of shoot dry matter to added P was highest for maize, intermediate for bambara groundnut and lowest for pigeon pea. Compared to the other two crops bambara groundnut had a very low root-shoot ratio. The P uptake of maize was two and five times higher than that of bambara groundnut and pigeon pea, respectively.

Contrary to maize, most of the P requirement of the other two crops could be met with P originating from soil. Maize had the highest internal P use efficiency, on average producing 1.89 g DM per mg P taken up, against values of 0.79 for bambara groundnut and 0.56 for pigeon pea. Therefore, lack of response by bambara groundnut to P fertilization in the field cannot be attributed to an exceptionally high P efficiency, but probably to its low P requirement.

Another reason for the difference in response of bambara groundnut to applied P as found in pot- and field experiment could be the mode of fertilizer application. In the pot experiment, P fertilizer was thoroughly mixed with the soil before planting while in the field experiment P was broadcast and worked into the top-soil with a digging fork. This means that in pots the P fertilizer is directly after germination available to the plant and in the field at a later stage or not at all during the growing period.

This idea led to a pot experiment described in Chapter 3.2, in which different timing of P application was studied. The effects of application at sowing, two weeks and four weeks after sowing were studied. It was found that large significant DM responses are only found when P is applied within two weeks after sowing and that this early application is also needed for a high seed yield. This indicates that under field conditions broadcasting of fertilizer at sowing is not an effective way to supply bambara groundnut plants in time (e.g. directly after germination) with additional P.

In the field and pot experiments (Chapters 2 and 3), P fertilization had no effect on the shoot P concentration of bambara groundnut plants irrespective of the treatment (P rate, time of P application and irrigation). This led to the question whether internal shoot P concentrations found in bambara groundnut plants in previous experiments were adequate or not. To answer this question, the critical shoot P concentration of bambara groundnut was determined in a sand culture experiment using 12 P rates varying between 0.21 and 159 mg P plant -1for the whole experimental period. The critical P value, determined graphically and associated with 10% reduction in maximum shoot dry weight, was about 0.15% with a critical range of 0.15 to 0.20%.

The critical P value showed that bambara groundnut plants in our previous experiments (pots and field) were growing under suboptimal P levels. Subsequently a farm survey was conducted on ten farms to assess the soil P status of bambara groundnut fields in Botswana under farmers' conditions, and to compare the P nutritional status of those field-grown plants with the critical shoot P concentration determined from our sand culture experiment (Chapter 4.2). Soil and plant samples from each of the ten farms selected for the survey were collected at about 78 DAS and analysed for P. The soil P levels from 80% of the farms were below five milligrams P kg -1(P-Bray). On two farms, shoot P concentrations were at the critical level and on the other farms below the critical level.

In all previous experiments, bambara groundnut plants were growing under marginal P conditions with shoot P concentrations around or below the critical level, irrespective of the P fertilization and irrigation treatments, while the N nutrition was adequate. This raises the question whether the plant N supply will still be adequate if bambara groundnut plants are grown under conditions where P will be no longer marginal or deficient. Under such conditions, higher DM production will not only increase the plant N requirement, but also the N 2 fixation might behave differently.

To answer these questions, a pot experiment described in Chapter 5 was conducted to find out whether (i) the capacity of the N 2 fixation process, possibly supplemented with some native soil mineral N is high enough to meet the plant N requirement also under non-limiting P conditions, and (ii) at internal P concentrations exceeding the critical level, the amount of fixed N increases with increasing internal P.

In the experiment, bambara groundnut plants were fertilized with different rates of P with or without additional mineral N. Phosphorus fertilization increased shoot, root, and nodule dry weights, seed yield, shoot P and shoot N concentrations and shoot P and shoot N contents at all sampling periods. Nitrogen fertilization on the other hand had no effect on the parameters measured but influenced the time to plant maturity. Under non-limiting P conditions bambara groundnut seems to be able to meet its N requirement from N 2 fixation and soil mineral N. The results indicate that no mineral fertilizer N should be supplied at sowing, but this does not rule out the possibility that at a later stage additional N may be needed. One of those might be the onset of podding, when a major shift in assimilate partitioning from vegetative growth to pod filling takes place.

To investigate this, the effect of N fertilization at different development stages of bambara groundnut: at sowing (as in the previous experiment), at the early vegetative growth, at (50%) flowering and (50%) podding, was investigated in a pot experiment (Chapter 6). To separate the contributions of soil mineral N and N 2 fixation to the N nutrition of bambara groundnut plants, treatments with sterilized and unsterilized soil (control) were incorporated in the experiment. Nitrogen fertilization did not affect shoot dry weight and seed yield. Only N fertilization at 50% flowering decreased nodule number and nodule dry weight, shoot N concentration and shoot N content. This can mean that the flowering stage is critical for nodule formation in bambara groundnut.

Sterilization of soil decreased nodule number and nodule dry weight. Shoot N concentration of plants grown in sterilized soil was decreased only at the 50% flowering stage, and poor N nutrition at early stages of growth delayed the flowering and podding stages by two weeks. Shoot N concentration of the sterilized soil treatment recovered despite the low nodule number and nodule dry weight, implying a high N 2 fixation efficiency of nodules present. It was not possible to quantify the contribution of the soil mineral N to the N nutrition of bambara groundnut because nodules were present in the sterilized soil treatment throughout the experimental period. Nitrogen fertilization, irrespective of time of application or growth stage was not beneficial to plant growth and reproduction.

Finally in Chapter 7 a general discussion of the main results on the responses of bambara groundnut to P and N fertilization in Botswana soils and the implications to bambara groundnut farmers is presented. It is concluded that P is important for growth and seed yield of bambara groundnut, and the low P requirement may be responsible for its ability to thrive in chemically poor soils. But for a positive response to P fertilization to occur, the soil moisture content must be adequate and the fertilizer should be available to the seedling within two weeks after sowing. Bambara groundnut can meet its nitrogen requirement from N 2 fixation and soil mineral N, and there is no need for supplementary mineral N fertilizer. The shoot P concentrations at the early podding stage seem to be a suitable guide for monitoring the P status and P requirement of bambara groundnut.

Verhoogde fosfaatvoeding van planten op P-arme gronden als gevolg van symbiose met Mycorrhiza
Keltjens, W.G. - \ 1999
Meststoffen : Dutch/English annual on fertilizers and fertilization (1999). - ISSN 0169-2267 - p. 49 - 56.
Understanding leaf area expansion in sunflower and wheat grown under low-phosphorus conditions
Rodríguez, D. ; Goudriaan, J. ; Keltjens, W.G. - \ 1998
In: Phosphorus in plant biology, regulatory roles in molecular, cellular, organismic and ecosystem processes / Lynch, J.P., Deikman, J., Rockville, Maryland, USA : (Current Topics in Plant Physiology, An American Society of Plant Physiologists Series 19) - p. 354 - 356.
Phytochelatins as biomarkers for heavy metal stress in maize (Zea mays L.) and wheat (Triticum aestivum L.): combined effects of copper and cadmium.
Keltjens, W.G. ; Beusichem, M.L. van - \ 1998
Plant and Soil 203 (1998)1. - ISSN 0032-079X - p. 119 - 126.

Heavy metal contaminated soils often show increased levels of more than one metal, e.g. copper (Cu), cadmium (Cd), zinc (Zn), lead (Pb) or nickel (Ni). In case such soils are used for crop production, prediction of yield reduction or quality decline due to heavy metals in the soil is inadequate when based only on chemical soil analysis. The use of biomarkers such as phytochelatins (PC), non-protein thiols specifically induced in plants upon exposure to heavy metals, may be an additional tool or diagnostic criterion in heavy metal research and in practice. In the present work, Cu and Cd uptake and induction of PC synthesis are studied with hydroponically grown maize and wheat plants exposed to mixtures of the two metals. We observed a close positive relationship between the concentrations of Cd and PC in the plant shoot material. A decreased shoot concentration of Cd after addition of Cu, due to metal competition at common root absorption sites, coincided with lower shoot PC levels. Also differences in metal uptake and xylary metal transport among the two plant species were reflected in corresponding differences in PC concentration. The observed direct relationship between shoot PC concentration and the degree of metal-induced growth inhibition makes the use of PC promising for the purpose tested for
Phytochelatins as biomarkers for heavy metal toxicity in maize: single metal effects of copper and cadmium.
Keltjens, W.G. ; Beusichem, M.L. van - \ 1998
Journal of Plant Nutrition 21 (1998). - ISSN 0190-4167 - p. 635 - 648.
Soil acidification effects on fine root growth of Douglas-fir on sandy soils
Olsthoorn, A.F.M. - \ 1998
Agricultural University. Promotor(en): R. Rabbinge; W.G. Keltjens; G.M.J. Mohren. - Wageningen : IBN-DLO - ISBN 9789076095059 - 153
bosbouw - brand - verontreiniging - bomen - zandgronden - verzuring - bodem ph - bodemaciditeit - plantenfysiologie - plantenontwikkeling - wortels - nederland - pseudotsuga menziesii - forestry - fire - pollution - trees - sandy soils - acidification - soil ph - soil acidity - plant physiology - plant development - roots - netherlands - pseudotsuga menziesii

The ammonium sulphate deposited in forest ecosystems in the Netherlands as a result of air pollution currently exceeds 80 kg N ha -1yr -1locally. To study the influence of this air pollution on fine root density and its dynamics, fine root growth was monitored for three years in two young mature stands of Douglas-fir on sandy soils in the central part of the Netherlands in the ACIFORN project (ACIdification of FORests in the Netherlands). In the drier site the fine root density in early summer was higher and more strongly reduced after a dry spring than at the more favourable site. The fine root density appeared to depend on the proximity of the tree stems in the stand. In normal years, the main peak of fine root growth occurred in spring, with less growth during summer. After a dry spring, fine root growth on the drier site reacted strongly to periods with rain in summer. In a greenhouse trial with one-year-old seedlings of Douglas-fir in sand, ammonium sulphate was added to simulate the present acidic deposition. Part of the ammonium was nitrified into nitrate. This resulted in different ammonium, nitrate, aluminium levels and pH in the different treatments. In the treatments with the highest applications, fine root length was reduced by 50 % compared to the control, and the specific root length was reduced by 40 %.

A critical review of the results reported in the literature, to ascertain the total effect of the soil chemical changes since the start of the industrial revolution on the fine root density, revealed that aluminium is the main factor reducing fine root growth, even at sub-lethal concentrations in the soil solution. The fine root length is probably reduced by 50 % in the topsoil, and by 75 % in the subsoil. Excessive nitrogen availability reduces the total fine root biomass by 30 %. The net result is that the fine root system is less dense and more shallow than it would be without acidic deposition. This reduces the potential for water uptake on coarse sandy soils, and increases fine root mortality in long dry periods, especially on poor sites. An elementary model to describe fluctuations in the fine root density during and after a drought is used to illustrate the effects of soil acidification and nitrogen enrichment on drought susceptibility. Accumulated deposition has enhanced the risks of tree mortality or forest dieback induced by severe drought.

Leaf area expansion and assimilate production in sunflower (Helianthus annuus L.) growing under low phosphorus conditions.
Rodriguez, D. ; Zubillaga, M.M. ; Ploschuk, E.L. ; Keltjens, W.G. ; Goudriaan, J. ; Lavado, R.S. - \ 1998
Plant and Soil 202 (1998)1. - ISSN 0032-079X - p. 133 - 147.
Reductions in leaf area and plant growth as a consequence of phosphorus (P) limitations have been attributed both to direct effects of P shortage on leaf expansion rate and to a reduced production of assimilates required for growth. Canopy assimilation and leaf area expansion are closely interrelated processes. In this work we used experimental and simulation techniques to identify and study their importance in determining leaf area on sunflower (Helianthus annuus L.) growing under P-deficient conditions. Experiment 1 was done outdoors, in Buenos Aires, Argentina, and Experiment 2 in a glasshouse in Wageningen, The Netherlands. In both experiments we studied the effects of soil P addition on leaf appearance, leaf expansion, dry matter accumulation, and leaf photosynthesis of non-water stressed plants grown in pots containing a P-deficient soil. Before sowing the equivalent amounts of 0–600 kg of super phosphate ha-1 were added to the pots. Phosphorus deficiency delayed leaf appearance increasing the value of the phyllochron (PHY) up to 76%, the rate of leaf area expansion during the quasi-linear phase of leaf expansion (LER) was reduced by up to 74%, with respect to high P plants. Phosphorus deficiency reduced by up to 50% the rate of light saturated photosynthesis per unit of leaf area (AMAX) in recently expanded leaves, while at low levels of leaf insertion in the canopy, AMAX was reduced by up to 85%, when compared to that in high P plants. Phosphorus deficiency also reduced the duration of the quasi-linear phase of leaf expansion by up to eight days. The values of LER were related (r = 0.56, P < 0.05) to the mean concentration of P in all the leaves (Leaves P%) and not to the concentration of P in the individual leaf where LER was determined (r = 0.22, P < 0.4) suggesting that under P deficiency individual leaf expansion was not likely to be regulated by the total P concentration at leaf level. The values of AMAX of individual leaves were related (r = 0.79, P < 0.01) to the concentration of total P in the corresponding leaf (Leaf P%). LER showed a hyperbolic relationship with Leaves P% (R2 = 0.94, P < 0.01, n = 13) that saturate at 0.14%. AMAX showed a hyperbolic relationship with Leaf P% (R2 = 0.73, P < 0.01, n = 53) that saturated with values of Leaf P% higher than 0.22. A morphogenetic model of leaf area development and growth was developed to quantify the effect of assimilate supply at canopy level on total leaf area expansion, and to study the effects of model parameters on the growth of sunflower plants under P-deficient conditions. With this model we identified the existence of direct effects of P deficiency on individual leaf area expansion. However, we calculated that under mild P stress conditions up to 83% of the reduction in the observed leaf area was explained by the particular effects of P% on the rate of leaf appearance, on the duration of the linear period of leaf expansion, and on the value of AMAX. We also calculated that the effects of P deficiency on the value of AMAX alone, explained up to 41% of the observed reductions in total leaf area between the highest and the intermediate P level in Experiment 2. Possible mechanisms of action of the direct effects of P on individual leaf expansion are discussed in this paper.
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