Eckhard George

Effect of arbuscular mycorrhizal colonization and two levels of compost supply on nutrient uptake and flowering of pelargonium plants

Two challenges frequently encountered in the production of ornamental plants in organic horticulture are: (1) the rate of mineralization of phosphorus (P) and nitrogen (N) from organic fertilizers can be too slow to meet the high nutrient demand of young plants, and (2) the exclusive use of peat as a substrate for pot-based plant culture is discouraged in organic production systems. In this situation, the use of beneficial soil microorganisms in combination with high quality compost substrates can contribute to adequate plant growth and flower development. In this study, we examined possible alternatives to highly soluble fertilizers and pure peat substrates using pelargonium (Pelargonium peltatum L’Her.) as a test plant. Plants were grown on a peat-based substrate with two rates of compost addition and with and without arbuscular mycorrhizal (AM) fungi. Inoculation with three different commercial AM inocula resulted in colonization rates of up to 36% of the total root length, whereas non-inoculated plants remained free of root colonization. Increasing the rate of compost addition increased shoot dry weight and shoot nutrient concentrations, but the supply of compost did not always completely meet plant nutrient demand. Mycorrhizal colonization increased the number of buds and flowers, as well as shoot P and potassium (K) concentrations, but did not significantly affect shoot dry matter or shoot N concentration. We conclude that addition of compost in combination with mycorrhizal inoculation can improve nutrient status and flower development of plants grown on peat-based substrates.

Colonisation with the arbuscular mycorrhizal fungus Glomus mosseae (Nicol. & Gerd.) enhanced phosphorus uptake from dry soil in Sorghum bicolor (L.)

The aim of the present study was to quantify the contribution of AMF to phosphorus (P) nutrition of the host plant when the P availability in the soil was limited by drought. To investigate the potential of AMF hyphae in taking up P from dry soil, mycorrhizal [+M] and nonmycorrhizal [−M]Sorghum bicolor L. plants were grown in a vertical split root system that consisted of two compartments placed upon one the other. The upper compartment was filled with well fertilised soil and the plant roots were allowed to grow into the lower compartment through a perforated bottom. The lower compartment was filled with an expanded clay substrate and nutrient solution, to supply the plants with water and all nutrients except P. The soil in the upper compartments was either dried [−W] or kept moist [+W] during a period of four weeks before harvest.The total plant P content did not differ significantly between the [−M] and the [+M] plants within the [+W] treatment. In contrast, the P content of the [+M] plants was almost twice as high as the [−M] plants when the soil in the upper compartment was dried. The concentrations of all elements except P in plant shoot tissue were sufficient for adequate plant growth. Phosphorus concentrations in the shoots of [−M/−W] plants indicated P deficiency, and these plants also had significantly lower dry matter and transpiration compared to the plants in all other treatments.From the results of the present experiment it can be concluded that mycorrhizal colonisation seems to be particularly benefical to P uptake from dry soil

Impact of Douglas-fir and Scots pine seedlings on plagioclase weathering under acidic conditions

The weathering of soil minerals in forest ecosystems increases nutrient availability for the trees. The rate of such weathering and its relative contribution to forest tree nutrition, is a major issue when evaluating present and potential forest stand productivity and sustainability. The current paper examines the weathering rate of plagioclase with and without Douglas-fir or Scots pine seedlings, in a laboratory experiment at pH 3–4 and 25 °C. All nutrients, with the exception of Ca, were supplied in sufficient amounts in a nutrient solution. The objective of the experiment was to evaluate the potential of trees to mobilise Ca from the mineral plagioclase that contained 12% of Ca. Amounts of nutrients supplied in the nutrient solution, amounts accumulated in the living tissue of the seedlings and amounts leached from the experimental vessels, were measured. A weathering balance, accounting for leached + accumulated − supplied amounts, was established. Bio-induced weathering, defined as the weathering increase in the presence of trees, relative to the weathering rate without trees (geochemical weathering; control vessels), under the present experimental conditions, explained on average, 40% of total weathering (biological + geochemical). These conditions appeared more beneficial to Scots pine (higher relative growth rate, higher Ca incorporation) than to Douglas-fir.

Nutrient Uptake: The Arbuscular Mycorrhiza Fungal Symbiosis as a Plant Nutrient Acquisition Strategy

Symbiotic association of roots with arbuscular mycorrhizal (AM) fungi is a very widespread strategy by which plants facilitate their acquisition of mineral elements from the soil. Studies employing sophisticated methodology in the fields of in vitro culture of AM colonized roots, microscopy, isotope labeling and molecular biology have shed light into the physiology of AM fungal nutrient uptake, transport and delivery to the host plant. In addition to the direct contribution to element uptake via the symbiotic pathway, AM mycelia have also been shown to affect root morphology and functioning, as well as mycorrhizosphere soil properties. This may lead to indirect effects of the AM association on plant nutrient availability and uptake. With their thin diameter, AM hyphae might be able to access smaller soil pores, and better compete with soil microbes for nutrient resources, compared with plant roots. Alone or in collaboration with associated hyphosphere bacteria, AM mycelia might also promote chemical mobilization of nutritional elements from sparingly plant available resources. Similar with plant root systems, AM mycelia appear to differ considerably in their architecture and physiological activities depending on their genotype. Whether such inherent traits represent different strategies in nutrient acquisition in collaboration with functionally compatible host roots, still remains speculative. Not much is known about how the AM fungal symbiosis is integrated into particular plant nutrient acquisition strategies, but it can be assumed that individual symbiotic strategies are highly diverse. The AM mycelium might assist the roots in spatial and/or chemical soil nutrient resource exploitation in a complementary and/or synergistic way. Knowledge about what factors determine the extent of functional compatibility between individual plant nutrient acquisition strategies and certain AM fungal traits would contribute to our understanding of ecosystem functioning, and might assist further development of mycorrhiza technology for plant production.

Rhizosphere effects on ion concentrations near different root zones of Norway spruce (Picea abies (L.) Karst.) and root types of Douglas-fir (Pseudotsuga menziesii L.) seedlings

Micro-suction cups were installed in split-root rhizotrons to investigate changes of ion concentrations pertaining to different root zones (Norway spruce) or various root types (Douglas-fir). Plant seedlings were grown in mineral soils fertilised with a mix of KMgCa or unfertilised. In Norway spruce, ions accumulated mostly in the rhizosphere near root tips in fertilised soil. Cations (Fe3+/Mn2+, Na+) and anions (Cl-, SO42-) were depleted in basal root areas in unfertilised soil. In Douglas-fir, ion accumulations (except K in unfertilised soil) occurred in rhizosphere of current-year suberized roots in both fertilised and unfertilised soils. Ion concentrations in rhizosphere of one-year-old suberized roots were highest compared to those of newly-grown or current-year roots in fertilised soil. These data demonstrate that soil solution chemistry clearly differs in rhizosphere of different segments of single roots and various root types. Ion accumulation in rhizosphere may be due to high mass flow transport of ions to root surfaces, and the accumulation of ions in rhizosphere was more distinct for root tips.

Root proliferation of Norway spruce and Scots pine in response to local magnesium supply in soil.

Nutrient sources in soils are often heterogeneously distributed. Although many studies have examined the root responses to local N and P enrichments in the soil, less research was conducted on root responses to Mg patches. New roots of pre-grown Mg-insufficient and Mg-sufficient plants of Norway spruce (Picea abies [L.] Karst.) and Scots pine (Pinus sylvestris L.) seedlings were allowed to grow into four other pots of equal size, which were placed under the tree-bearing pot. Soils in the lower pots were either unfertilised, or supplied with Mg, or NPK or a mixture of NPKMg sources. Plants were harvested after 9 months of growth. Compared to the corresponding controls (Mg versus unfertilised and NPKMg versus NPK), Mg additions did not have a significant effect on either root dry matter, total root length (TRL) or specific root length (SRL), irrespective of tree species and plant Mg nutritional status. In contrast, NPK and NPKMg additions significantly increased the root dry matter and TRL in the nutrient-rich soil patch, and decreased SRL in Norway spruce. However, the observed root morphological changes did not occur in Scots pine. Root Mg concentrations were increased in Mg-rich soil patches, but those accumulations varied with tree species. Mg accumulation in a marked patch was measured only in newly grown roots of Mg-sufficient Norway spruce, whereas a more homogenous distribution of Mg concentration was observed for all newly grown roots in Mg-insufficient trees in the four soil treatments. In Scots pine, Mg accumulations occurred in both Mg-insufficient and Mg-sufficient plants. These results suggest that Mg patches in the soil may not lead to a local increase in root growth, but to Mg uptake and root Mg accumulation. Tree roots react differently to Mg patches in comparison to their response to N or P patches in the soil.

Effect of the ectomycorrhizal fungus Paxillus involutus on growth and cation (potassium, calcium, and magnesium) nutrition of Pinus sylvestris L. in semi-hydroponic culture.

A semi-hydroponic culture was used to compare growth and cation nutrition of mycorrhizal (Paxillus involutus) and non-mycorrhizal Scots pine seedlings. When roots and hyphae grew together, concentrations and contents of macronutrients in needles and roots were not significantly different between mycorrhizal and non-mycorrhizal plants. When grown in two separate compartments, root potassium (K) concentrations, concentrations and contents of calcium (Ca) in needles and roots, needle nitrogen (N) concentrations, total N content and contents of root K and Mg were significantly reduced in mycorrhizal plants. Whereas 15N abundance increased in roots of mycorrhizal plants. The results indicated that the extraradical mycelium of the fungus strain used was able to transport N to the plant but did not contribute to long-term cation uptake and growth of host plants. An insufficient supply of macro-elements [N, phosphorus (P)] may account for the reduced growth of mycorrhizal plants and the differences in cation uptake between mycorrhizal and non-mycorrhizal plants.

Growth and mineral nutrient acquisition of mycorrhizal soybean grown on a calcareous soil under drought stress conditions

Plants grown on calcareous soils under drought conditions often have reduced productivity because of limited availability of mineral nutrients and water in soil. The objective of this research was to study the effect of mycorrhizal colonization on the growth and nutrient acquisition of soybean (Glycine max (L.) Merill cv. Marble-Arrow) plants under drought conditions. The study tested specifically the hypothesis that arbuscular mycorrhizal fungi can increase plant nutrient uptake even at a relatively high supply of soil P, and that this effect is relatively greater in dry soil than in weIl watered soil. At harvest, plants grown under drought conditions had decreased total biomass compared to weIl watered plants. Arbuscular mycorrhizal (AM) plants under drought conditions had significantly higher P and Zn concentrations and contents in shoots compared to non-mycorrhizal (nonAM) plants. In contrast, shoot P concentrations and total P content were not increased in AM plants under weIl watered conditions. Improved drought tolerance of AM soybean plants can be due to increased uptake of immobile ions such as P and Zn.