Mehmet Senbayram

Potassium in agriculture--status and perspectives.

In this review we summarize factors determining the plant availability of soil potassium (K), the role of K in crop yield formation and product quality, and the dependence of crop stress resistance on K nutrition. Average soil reserves of K are generally large, but most of it is not plant-available. Therefore, crops need to be supplied with soluble K fertilizers, the demand of which is expected to increase significantly, particularly in developing regions of the world. Recent investigations have shown that organic exudates of some bacteria and plant roots play a key role in releasing otherwise unavailable K from K-bearing minerals. Thus, breeding for genotypes that have improved mechanisms to gain access to this fixed K will contribute toward more sustainable agriculture, particularly in cropping systems that do not have access to fertilizer K. In K-deficient crops, the supply of sink organs with photosynthates is impaired, and sugars accumulate in source leaves. This not only affects yield formation, but also quality parameters, for example in wheat, potato and grape. As K has beneficial effects on human health, its concentration in the harvest product is a quality parameter in itself. Owing to its fundamental roles in turgor generation, primary metabolism, and long-distance transport, K plays a prominent role in crop resistance to drought, salinity, high light, or cold as well as resistance to pests and pathogens. Despite the abundance of vital roles of K in crop production, an improvement of K uptake and use efficiency has not been a major focus of conventional or transgenic breeding in the past. In addition, current soil analysis methods for K are insufficient for some common soils, posing the risk of imbalanced fertilization. A stronger prioritization of these areas of research is needed to counter declines in soil fertility and to improve food security.

Contribution of nitrification and denitrification to nitrous oxide emissions from soils after application of biogas waste and other fertilizers

The attribution of nitrous oxide (N(2)O) emission to organic and inorganic N fertilizers requires understanding of how these inputs affect the two biological processes, i.e. denitrification and nitrification. Contradictory findings have been reported when the effects of organic and inorganic fertilizers on nitrous oxide emission were compared. Here we aimed to contribute to the understanding of such variation using (15)N-labelling techniques. We determined the processes producing N(2)O, and tested the effects of soil moisture, N rates, and the availability of organic matter. In a pot experiment, we compared soil treated with biogas waste (BGW) and mineral ammonium sulphate (Min-N) applied at four rates under two soil moisture regimes. We also tested biogas waste, conventional cattle slurry and mineral N fertilizer in a grassland field experiment. During the first 37 days after application we observed N(2)O emissions of 5.6 kg N(2)O-N ha(-1) from soils supplied with biogas waste at a rate of 360 kg N ha(-1). Fluxes were ca. 5-fold higher at 85% than at 65% water holding capacity (WHC). The effects of fertilizer types and N rates on N(2)O emission were significant only when the soil moisture was high. Organic fertilizer treated soils showed much higher N(2)O emissions than those receiving mineral fertilizer in both, pot and field experiment. Over all the treatments the percentage of the applied N emitted as N(2)O was 2.56% in BGW but only 0.68% in Min-N. In the pot experiment isotope labelling indicated that 65-95% of the N(2)O was derived from denitrification for all fertilizer types. However, the ratio of denitrification/nitrification derived N(2)O was lower at 65% than at 85% WHC. We speculate that the application of organic matter in conjunction with ammonium nitrogen first leads to a decrease in denitrification-derived N(2)O emission compared with soil receiving mineral fertilizer. However, at later stages when denitrification becomes C-limited, higher N(2)O emissions are induced when the soil moisture is high.

Long-term influence of manure and mineral nitrogen applications on plant and soil 15N and 13C values from the Broadbalk Wheat Experiment

The Broadbalk Wheat Experiment at Rothamsted Research in the UK provides a unique opportunity to investigate the long-term impacts of environmental change and agronomic practices on plants and soils. We examined the influence of manure and mineral fertiliser applications on temporal trends in the stable N ((15)N) and C ((13)C) isotopes of wheat collected during 1968-1979 and 1996-2005, and of soil collected in 1966 and 2000. The soil delta(15)N values in 1966 and 2000 were higher in manure than the mineral N supplied soil; the latter had similar or higher delta(15)N values than non-fertilised soil. The straw delta(15)N values significantly decreased in all N treatments during 1968 to 1979, but not for 1996-2005. The straw delta(15)N values decreased under the highest mineral N supply (192 kg N ha(-1) year(-1)) by 3 per thousand from 1968 to 1979. Mineral N supply significantly increased to straw delta(13)C values in dry years, but not in wet years. Significant correlations existed between wheat straw delta(13)C values with cumulative rainfall (March to June). The cultivar Hereward (grown 1996-2005) was less affected by changes in environmental conditions (i.e. water stress and fertiliser regime) than Cappelle Desprez (1968-1979). We conclude that, in addition to fertiliser type and application rates, water stress and, importantly, plant variety influenced plant delta(13)C and delta(15)N values. Hence, water stress and differential variety response should be considered in plant studies using plant delta(13)C and delta(15)N trends to delineate past or recent environmental or agronomic changes.

Soil denitrification potential and its influence on N2O reduction and N2O isotopomer ratios

RATIONALE N2O isotopomer ratios may provide a useful tool for studying N2O source processes in soils and may also help estimating N2O reduction to N2. However, remaining uncertainties about different processes and their characteristic isotope effects still hamper its application. We conducted two laboratory incubation experiments (i) to compare the denitrification potential and N2O/(N2O+N2) product ratio of denitrification of various soil types from Northern Germany, and (ii) to investigate the effect of N2O reduction on the intramolecular (15)N distribution of emitted N2O. METHODS Three contrasting soils (clay, loamy, and sandy soil) were amended with nitrate solution and incubated under N2 -free He atmosphere in a fully automated incubation system over 9 or 28 days in two experiments. N2O, N2, and CO2 release was quantified by online gas chromatography. In addition, the N2O isotopomer ratios were determined by isotope-ratio mass spectrometry (IRMS) and the net enrichment factors of the (15)N site preference (SP) of the N2O-to-N2 reduction step (η(SP)) were estimated using a Rayleigh model. RESULTS The total denitrification rate was highest in clay soil and lowest in sandy soil. Surprisingly, the N2O/(N2O+N2) product ratio in clay and loam soil was identical; however, it was significantly lower in sandy soil. The IRMS measurements revealed highest N2O SP values in clay soil and lowest SP values in sandy soil. The η(SP) values of N2O reduction were between -8.2 and -6.1‰, and a significant relationship between δ(18)O and SP values was found. CONCLUSIONS Both experiments showed that the N2O/(N2O+N2) product ratio of denitrification is not solely controlled by the available carbon content of the soil or by the denitrification rate. Differences in N2O SP values could not be explained by variations in N2O reduction between soils, but rather originate from other processes involved in denitrification. The linear δ(18)O vs SP relationship may be indicative for N2O reduction; however, it deviates significantly from the findings of previous studies.

Role of magnesium fertilisers in agriculture: plant–soil continuum

Abstract. In this review, we summarise factors contributing to plant availability of magnesium (Mg) in soils, the role of Mg in plant physiological processes related to yield formation and abiotic stress tolerance, and soil and fertiliser parameters related to Mg leaching in fertilised soils. Mg is a common constituent in many minerals, comprising 2% of Earth’s crust; however, most soil Mg (90–98%) is incorporated in the crystal lattice structure of minerals and thus not directly available for plant uptake. Plants absorb Mg from the soil solution, which is slowly replenished by soil reserves. Duration and intensity of weathering, soil moisture, soil pH, and root–microbial activity in soil are key factors that determine plant-available Mg release from soils. On the other hand, the amount of Mg released from soil minerals is generally small compared with the amounts needed to sustain high crop yield and quality. Thus, in many agro-ecosystems, application of Mg fertilisers is crucial. Magnesium is involved in many physiological and biochemical processes; it is an essential element for plant growth and development and plays a key role in plant defence mechanisms in abiotic stress situations. An early effect of Mg deficiency in plants is the disturbed partitioning of assimilates between roots and shoots because the supply of sink organs with photosynthetic products is impaired, and sugars accumulate in source leaves. Thus, optimal supply of Mg is required to improve crop tolerance to various stresses and to increase yield and quality parameters of harvested products. Unlike other cations, Mg is very mobile in soils because it is less bound to the soil charges. Therefore, Mg losses by leaching might occur in sandy soils with high water conductivity. Leaching of Mg in soils when applied with various water-soluble fertilisers may also vary depending on the fertiliser’s chemical composition, granule size, and effect on soil pH and cation balance, as we discuss in detail.

Quantitative limitations to photosynthesis in K deficient sunflower and their implications on water-use efficiency

Potassium (K) is crucial for crop growth and is strongly related to stress tolerance and water-use efficiency (WUE). A major physiological effect of K deficiency is the inhibition of net CO2 assimilation (AN) during photosynthesis. Whether this reduction originates from limitations either to photochemical energy conversion or biochemical CO2 fixation or from a limitation to CO2 diffusion through stomata and the leaf mesophyll is debated. In this study, limitations to photosynthetic carbon gain of sunflower (Helianthus annuus L.) under K deficiency and PEG- induced water deficit were quantified and their implications on plant- and leaf-scale WUE (WUEP, WUEL) were evaluated. Results show that neither maximum quantum use efficiency (Fv/Fm) nor in-vivo RubisCo activity were directly affected by K deficiency and that the observed impairment of AN was primarily due to decreased CO2 mesophyll conductance (gm). K deficiency additionally impaired leaf area development which, together with reduced AN, resulted in inhibition of plant growth and a reduction of WUEP. Contrastingly, WUEL was not affected by K supply which indicated no inhibition of stomatal control. PEG-stress further impeded AN by stomatal closure and resulted in enhanced WUEL and high oxidative stress. It can be concluded from this study that reduction of gm is a major response of leaves to K deficiency, possibly due to changes in leaf anatomy, which negatively affects AN and contributes to the typical symptoms like oxidative stress, growth inhibition and reduced WUEP.

Fast responses of metabolites in Vicia faba L. to moderate NaCl stress

Salt stress impairs global agricultural crop production by reducing vegetative growth and yield. Despite this importance, a number of gaps exist in our knowledge about very early metabolic responses that ensue minutes after plants experience salt stress. Surprisingly, this early phase remains almost as a black box. Therefore, systematic studies focussing on very early plant physiological responses to salt stress (in this case NaCl) may enhance our understanding on strategies to develop crop plants with a better performance under saline conditions. In the present study, hydroponically grown Vicia faba L. plants were exposed to 90 min of NaCl stress, whereby every 15 min samples were taken for analyzing short-term physiologic responses. Gas chromatography-mass spectrometry-based metabolite profiles were analysed by calculating a principal component analysis followed by multiple contrast tests. Follow-up experiments were run to analyze downstream effects of the metabolic changes on the physiological level. The novelty of this study is the demonstration of complex stress-induced metabolic changes at the very beginning of a moderate salt stress in V. faba, information that are very scant for this early stage. This study reports for the first that the proline analogue trans-4-hydroxy-L-proline, known to inhibit cell elongation, was increasingly synthesized after NaCl-stress initiation. Leaf metabolites associated with the generation or scavenging of reactive oxygen species (ROS) were affected in leaves that showed a synchronized increase in ROS formation. A reduced glutamine synthetase activity indicated that disturbances in the nitrogen assimilation occur earlier than it was previously thought under salt stress.

The Activity of Nodules of the Supernodulating Mutant Mtsunn Is not Limited by Photosynthesis under Optimal Growth Conditions

Legumes match the nodule number to the N demand of the plant. When a mutation in the regulatory mechanism deprives the plant of that ability, an excessive number of nodules are formed. These mutants show low productivity in the fields, mainly due to the high carbon burden caused through the necessity to supply numerous nodules. The objective of this study was to clarify whether through optimal conditions for growth and CO2 assimilation a higher nodule activity of a supernodulating mutant of Medicago truncatula (M. truncatula) can be induced. Several experimental approaches reveal that under the conditions of our experiments, the nitrogen fixation of the supernodulating mutant, designated as sunn (super numeric nodules), was not limited by photosynthesis. Higher specific nitrogen fixation activity could not be induced through short- or long-term increases in CO2 assimilation around shoots. Furthermore, a whole plant P depletion induced a decline in nitrogen fixation, however this decline did not occur significantly earlier in sunn plants, nor was it more intense compared to the wild-type. However, a distinctly different pattern of nitrogen fixation during the day/night cycles of the experiment indicates that the control of N2 fixing activity of the large number of nodules is an additional problem for the productivity of supernodulating mutants.

Optimized potassium nutrition improves plant-water-relations of barley under PEG-induced osmotic stress

AimsWater use efficiency (WUE) of crop plants is an important plant trait for maintaining high yield in water limited areas. By influencing osmoregulation of plants, potassium (K) plays a critical role in stress avoidance and adaptation. However, whole plant physiological mechanisms modulated by K supply in respect of plant drought tolerance and water use efficiency are not well understood. In the present study, growth, development and transpiration dynamics of two barley cultivars were evaluated with and without PEG-induced osmotic stress using an automated balance system and image based leaf area determination.MethodsExperiments were conducted to study the effects of varied K supply under different osmotic stress treatments on a wide range of morphological, biochemical and physiological characteristics of barley plants such as leaf area development, daily whole plant transpiration rate (DTR), stomatal conductance (gs), assimilation rate (AN), biomass and leaf water use efficiency (WUE) as well as foliar abscisic acid (ABA) concentrations. Two barley cultivars (cv. Sahin-91 and cv. Milford) were treated with two K supply levels (0.04 and 0.8 mM K) and osmotic stress induced by polyethylene glycol 6000 (PEG) for a period of 9 days (in total 48 days experiment) in the hydroponic plant culture (non-PEG and + 20% PEG ).ResultsWithout PEG, low-K supply depressed dry matter (DM) by almost 60% averaged across both cultivars. Under osmotic stress (+PEG), total leaf area was reduced by almost 70% in low-K compared to adequate-K plants. Low K concentration under PEG stress was correlated with higher ABA concentration and was correlated with lower leaf- and whole plant transpiration rate. Biomass-WUE under low K supply decreased significantly in both barley cultivars, to a greater extent in cv. Milford under osmotic stress. However, leaf-WUE was not affected by K supply in the absence of osmotic stress.ConclusionsIt was suggested that reduced biomass-WUE in low-K treated barley plants was not related to inefficient stomatal control under K deficiency, but instead due to reduced assimilation rate. It was further hypothesized that under low K supply, a number of energy consuming activities reduce biomass-WUE, which are not distinguished by measuring leaf-WUE. This study showed that low K supply under osmotic stress increases foliar ABA concentration thereby decreasing plant transpiration.