Ecological specificity of arbuscular mycorrhizae: evidence from foliar- and seed-feeding insects

Arbuscular mycorrhizal (AM) fungi have a variety of effects on foliar-feeding insects, with the majority of these being positive, although reports of negative and null effects also exist. Virtually all previous experiments have used mobile insects confined in cages and have studied the effects of one, or at most two, species of mycorrhizae on one species of insect. The purpose of this study was to introduce a greater level of realism into insect-mycorrhizal experiments, by studying the responses of different insect feeding guilds to a variety of AM fungi. We conducted two experiments involving three species of relatively immobile insects (a leaf-mining and two seed-feeding flies) reared in natural conditions on a host (Leucanthemum vulgare). In a field study, natural levels of AM colonization were reduced, while in a phytometer trial, we experimentally colonized host plants with all possible combinations of three known mycorrhizal associates of L. vulgare. In general, AM fungi increased the stature (height and leaf number) and nitrogen content of plants. However, these effects changed through the season and were,dependent on the identity of the fungi in the root system. AM fungi increased host acceptance of all three insects and larval performance of the leaf miner, but these effects were also season- and AM species-dependent. We suggest that the mycorrhizal effect on the performance of the leaf miner is due to fungal-induced changes in host-plant nitrogen content, detected by the adult fly. However, variability in the effect was apparent, because not all AM species increased plant N content. Meanwhile, positive effects of mycorrhizae were found on flower number and flower size, and these appeared to result in enhanced infestation levels by the seed-feeding insects. The results show that AM fungi exhibit ecological specificity, in that different. species have different effects on host-plant growth and chemistry and the performance of foliar-feeding insects. Future studies need to conduct experiments that use ecologically realistic combinations of plants and fungi and allow insects to be reared in natural conditions.

Modelling the partitioning of evapotranspiration in a maize-sunflower intercrop

The primary purpose of this study was to model the partitioning of evapotranspiration in a maize-sunflower intercrop at various canopy covers. The Shuttleworth-Wallace (SW) model was extended for intercropping systems to include both crop transpiration and soil evaporation and allowing interaction between the two. To test the accuracy of the extended SW model, two field experiments of maize-sunflower intercrop were conducted in 1998 and 1999. Plant transpiration and soil evaporation were measured using sap flow gauges and lysimeters, respectively. The mean prediction error (simulated minus measured values) for transpiration was zero (which indicated no overall bias in estimation error), and its accuracy was not affected by the plant growth stages, but simulated transpiration during high measured transpiration rates tended to be slightly underestimated. Overall, the predictions for daily soil evaporation were also accurate. Model estimation errors were probably due to the simplified modelling of soil water content, stomatal resistances and soil heat flux as well as due to the uncertainties in characterising the 2 micrometeorological conditions. The SW’s prediction of transpiration was most sensitive to parameters most directly related to the canopy characteristics such as the partitioning of captured solar radiation, canopy resistance, and bulk boundary layer resistance.

The occurrence and economic impact of plasmodiophora brassicae and clubroot disease

The significance of Plasmodiophora brassicae Woronin and clubroot disease which it incites in members of the family Brassicaceae is reviewed as the focus for this special edition of the Journal of Plant Growth Regulation. This is a monographic treatment of recent research into the pathogen and disease; previous similar treatments are now well over half a century old. Vernacular nomenclature of the disease indicates that it had a well-established importance in agriculture and horticulture from at least the Middle Ages onward in Europe and probably earlier. Subsequently, the pathogen probably spread worldwide as a result of transfer on and in fodder taken by colonists as livestock feed. It is a moot point, however, whether there was much earlier spread by P. brassicae into China and subsequently Japan as Brassica rapa (Chinese cabbage and many variants) colonized those lands in archaeological time. Symptoms, worldwide distribution, and economic impact are briefly described here to provide a basis for understanding subsequent papers. Clubroot disease devastates both infected field and protected vegetable and agricultural Brassica crops. Particular importance is placed on recent reports of crop losses in tropical countries, albeit where the crops are grown in cooler altitudes, and in the Canadian prairie land canola crops. The latter is of enormous importance because this crop is the single most important and essential source of vegetable oils used in human foodstuffs and in industrial lubricants where mineral oils are inappropriate.

Plasmodiophora brassicae in its Environment

Plasmodiophora brassicae Wor. is viewed in this article from the standpoint of a highly evolved and successful organism, well fitted for the ecological niche that it occupies. Physical, chemical, and biological components of the soil environment are discussed in relation to their effects on the survival, growth, and reproduction of this microbe. It is evident that P. brassicae is well equipped by virtue of its robust resting spores for survival through many seasonal cycles. Germination is probably triggered as a result of signals initiated by root exudates. The resultant motile zoospore moves rapidly to the root hair surface and penetration and colonization follow. The short period between germination and penetration is one of greatest vulnerability for P. brassicae. In this phase survival is affected at the very least by soil texture and structure; its moisture; pH; calcium, boron, and nitrogen content; and the presence of active microbial antagonists. These factors influence the inoculum potential (sensu Garrett, 1956) and its viability and invasive capacity. There is evidence that these effects may also influence differentially the survival of some physiologic races of P. brassicae. Considering the interaction of P. brassicae with the soil environment from the perspective of its biological fitness is an unusual approach; most authors consider only the opportunities to destroy this organism. The approach adopted here is borne of several decades spent studying P. brassicae and the respect that has been engendered for it as a biological entity. This review stops at the point of penetration, although some of the implications of the environment for successful colonization are included because they form a continuum. Interactions with the molecular and biochemical cellular environment are considered in other sections in this special edition.

The regulation of sweet cherry fruit abscission by polar auxin transport

Inconsistency of cropping is an important problem for UK sweet cherry production. Premature fruit abscission in Prunus can reduce yields severely, however, the environmental cues and hormonal signals that trigger abscission have not been identified. Auxin (IAA) is known to delay abscission by reducing the sensitivity of cells in the abscission zone to ethylene, a promoter of abscission. Therefore, the capacity for polar auxin transport (PAT) through sweet cherry pedicels was examined in relation to fruit abscission. Cherry ‘spurs’ (short shoots) with similar leaf areas and different fruit numbers were phloem-girdled to restrict assimilate movement. Abscission from spurs with many fruit (eight or more) occurred within 14 days of girdling, whereas abscission from spurs with few (two) fruit was minimal. The pedicels’ capacity for PAT in spurs with different fruit numbers was determined 1, 3 and 9 days after girdling (DAG). Fruit were analysed for endogenous IAA concentration 3, 5, 7 and 9 DAG. PAT inhibitors 2,3,5-triiodobenzoic acid or 1-N-naphthylphtalamic acid were applied to pedicels of fruit not expected to abscise, i.e. on spurs with few fruit. The effect of these inhibitors on fruit abscission was determined 14 DAG. The proportion of the transported [3H]-IAA was lower from the outset in pedicels from spurs with many fruit. By 9 DAG, symptoms of fruit abscission were apparent and 40% less [3H] -IAA was transported through pedicels on spurs with many fruit. Fruit endogenous IAA concentrations were similar in the two groups of spurs. Application of PAT inhibitors shortly after girdling increased fruit abscission by 30%. The results suggest that although a decline in PAT is not the only cause of fruit abscission, the maintenance of PAT contributes to fruit retention.

Cupins: the most functionally diverse protein superfamily?

The cupin superfamily of proteins, named on the basis of a conserved β-barrel fold (‘cupa’ is the Latin term for a small barrel), was originally discovered using a conserved motif found within germin and germin-like proteins from higher plants. Previous analysis of cupins had identified some 18 different functional classes that range from single-domain bacterial enzymes such as isomerases and epimerases involved in the modification of cell wall carbohydrates, through to two-domain bicupins such as the desiccation-tolerant seed storage globulins, and multidomain transcription factors including one linked to the nodulation response in legumes. Recent advances in comparative genomics, and the resolution of many more 3-D structures have now revealed that the largest subset of the cupin superfamily is the 2-oxyglutarate-Fe2+ dependent dioxygenases. The substrates for this subclass of enzyme are many and varied and in total amount to probably 50–100 different biochemical reactions, including several involved in plant growth and development. Although the majority of enzymatic cupins contain iron as an active site metal, other members contain either copper, zinc, cobalt, nickel or manganese ions as a cofactor, with each cofactor allowing a different type of chemistry to occur within the conserved tertiary structure. This review discusses the range of structures and functions found in this most diverse of superfamilies.

Graphical tracking systems revisited: a practical approach to computer scheduling in horticulture

Graphical tracking is a technique for crop scheduling where the actual plant state is plotted against an ideal target curve which encapsulates all crop and environmental characteristics. Management decisions are made on the basis of the position of the actual crop against the ideal position. Due to the simplicity of the approach it is possible for graphical tracks to be developed on site without the requirement for controlled experimentation. Growth models and graphical tracks are discussed, and an implementation of the Richards curve for graphical tracking described. In many cases, the more intuitively desirable growth models perform sub-optimally due to problems with the specification of starting conditions, environmental factors outside the scope of the original model and the introduction of new cultivars. Accurate specification for a biological model requires detailed and usually costly study, and as such is not adaptable to a changing cultivar range and changing cultivation techniques. Fitting of a new graphical track for a new cultivar can be conducted on site and improved over subsequent seasons. Graphical tracking emphasises the current position relative to the objective, and as such does not require the time consuming or system specific input of an environmental history, although it does require detailed crop measurement. The approach is flexible and could be applied to a variety of specification metrics, with digital imaging providing a route for added value. For decision making regarding crop manipulation from the observed current state, there is a role for simple predictive modelling over the short term to indicate the short term consequences of crop manipulation.

Developing protocols for nursery based scheduling trials

Most existing crop scheduling models are cultivar specific and are developed using academic resources. As such they rarely meet the particular needs of a grower. A series of protocols have been created to generate effective schedules for a changing product range using data generated on site at a commercial nursery. A screening programme has been developed to help determine a cultivar's photoperiod sensitivity and vernalisation requirement. Experimental conditions were obtained using a cold store facility set to 5degreesC and photoperiod cloches. Eight and 16 hour photoperiod treatments were achieved at low cost by growing plants in cloches of opaque plastic with a motorised rolling screen. Natural light conditions were extended where necessary using a high pressure sodium lamp. Batches of plants were grown according to different schedules based on these treatments. The screening programme found Coreopsis grandiflora 'Flying Saucers' to be a long day plant. Data to form the basis of graphical tracks was taken using variations on commercial schedules. The work provides a nursery based approach to the continuous improvement of crop scheduling practises.

Glomus intraradices and Gigaspora margarita arbuscular mycorrhizal associations differentially affect nitrogen and potassium nutrition of Plantago lanceolata in a low fertility dune soil

Two controlled microcosm experiments aimed at a critical re-assessment of the contributions of divergent arbuscular mycorrhizal (AM) fungi to plant mineral nutrition were established that specifically targeted Plantago lanceolata–Glomus intraradices (B.B/E) and –Gigaspora margarita (BEG 34) symbioses developed in a native, nutrient limited, coastal dune soil. Plant tissue nitrogen (N), phosphorus (P) and potassium (K) status as well as plant growth parameters and levels of mycorrhizal colonization were assessed at harvest. In addition to the general well-established mycorrhizal facilitation of P uptake, the study was able to demonstrate a G. intraradices-specific contribution to improved plant nitrogen and potassium nutrition. In the two respective experiments, G. intraradices-inoculated plants had 27.8% and 40.8% more total N and 55.8% and 23.3% more total K when compared to Gi. margarita inoculated counterparts. Dissimilar overall contribution of the two isolates to plant nutrition was identified in AM-genus specific differences in plant tissue N:P:K ratios. G. intraradices inoculated and non-mycorrhizal plants generally exhibited N:P:K ratios indicative of P limitation whereas for Gi.margarita mycorrhizal plants, corresponding ratios strongly implied either N or K limitation. The study provides further evidence highlighting AM functional biodiversity in respect to plant nutrient limitation experienced by mycorrhizal P. lanceolata in an ecologically relevant soil system.

Simulating the impacts of global warming on wheat in China using a large area crop model

Temperature is one of the most prominent environmental factors that determine plant growth, devel- opment, and yield. Cool and moist conditions are most favorable for wheat. Wheat is likely to be highly vulnerable to further warming because currently the temperature is already close to or above optimum. In this study, the impacts of warming and extreme high temperature stress on wheat yield over China were investigated by using the general large area model (GLAM) for annual crops. The results showed that each 1±C rise in daily mean temperature would reduce the average wheat yield in China by about 4.6%{5.7% mainly due to the shorter growth duration, except for a small increase in yield at some grid cells. When the maximum temperature exceeded 30.5±C, the simulated grain-set fraction declined from 1 at 30.5±C to close to 0 at about 36±C. When the total grain-set was lower than the critical fractional grain-set (0.575{0.6), harvest index and potential grain yield were reduced. In order to reduce the negative impacts of warming, it is crucial to take serious actions to adapt to the climate change, for example, by shifting sowing date, adjusting crop distribution and structure, breeding heat-resistant varieties, and improving the monitoring, forecasting, and early warning of extreme climate events.

Elevated atmospheric CO2 changes phosphorus fractions in soils under a short rotation poplar plantation (EuroFACE)

Future high levels of atmospheric carbon dioxide (CO2) may increase biomass production of terrestrial plants and hence plant requirements for soil mineral nutrients to sustain a greater biomass production. Phosphorus (P), an element essential for plant growth, is found in soils both in inorganic and in organic forms. In this work, three genotypes of Populus were grown under ambient and elevated atmospheric CO2 concentrations (FACE) for 5 years. An N fertilisation treatment was added in years 4 and 5 after planting. Using a fractionation scheme, total P was sequentially extracted using H2O, NaOH, HCl and HNO3, and P determined as both molybdate (Mo) reactive and total P. Molybdate-reactive P is defined as mainly inorganic but also some labile organic P which is determined by Vanado-molybdophosphoric acid colorimetric methods. Organic P was also measured to assess all plant available and weatherable P pools. We tested the hypotheses that higher P demand due to increased growth is met by a depletion of easily weatherable soil P pools, and that increased biomass inputs increases the amount of organic P in the soil. The concentration of organic P increased under FACE, but was associated with a decrease in total soil organic matter. The greatest increase in the soil P due to elevated CO2 was found in the HCl-extractable P fraction in the non-fertilised treatment. In the NaOH-extractable fraction the Mo-reactive P increased under FACE, but total P did not differ between ambient and FACE. The increase in both the NaOH- and HCl-extractable fractions was smaller after N addition. The results showed that elevated atmospheric CO2 has a positive effect on soil P availability rather than leading to depletion.We suggest that the increase in the NaOH- and HCl-extractable fractions is biologically driven by organic matter mineralization, weathering and mycorrhizal hyphal turnover.

The effects of organic and conventional fertilizers on cereal aphids and their natural enemies

Aphids are important pests of spring cereals and their abundance and the impact of their natural enemies may be influenced by fertilizer regime.2We conducted a 2-year field study investigating the effects of organic slow-release and conventional fertilizers on cereal aphids, hymenopteran parasitoids and syrphid predators and considered how the effects of fertilizers on barley morphology and colour might influence these species.3Barley yield was greater in conventionally fertilized pots. Barley morphology was also affected by treatment: vegetative growth was greater under conventional treatments. Barley receiving organic fertilizers or no fertilizer was visually more attractive to aphids compared with plants receiving conventional fertilizers.4Aphids were more abundant in conventionally fertilized barley but the reason for this increased abundance was species specific. Metopolophium dirhodum was responding to fertilizer effects on plant morphology, whereas Rhopalosiphum padi was sensitive to the temporal availability of nutrients.5Syrphid eggs were more numerous in conventionally fertilized pots, whereas the response of parasitoids appeared to be dependent on the abundance of aphids, although the number of parasitoid mummies was low in both years.6This research shows that the fertilizer treatment used can affect numerous characteristics of plant growth and colour, which can then influence higher trophic levels. This knowledge might be used to make more informed fertilizer application choices.

Diversity in expression of phosphorus (P) responsive genes in Cucumis melo L.

Background: Phosphorus (P) is a major limiting nutrient for plant growth in many soils. Studies in model species have identified genes involved in plant adaptations to low soil P availability. However, little information is available on the genetic bases of these adaptations in vegetable crops. In this respect, sequence data for melon now makes it possible to identify melon orthologues of candidate P responsive genes, and the expression of these genes can be used to explain the diversity in the root system adaptation to low P availability, recently observed in this species. Methodology and Findings: Transcriptional responses to P starvation were studied in nine diverse melon accessions by comparing the expression of eight candidate genes (Cm-PAP10.1, Cm-PAP10.2, Cm-RNS1, Cm-PPCK1, Cm-transferase, Cm-SQD1, Cm-DGD1 and Cm-SPX2) under P replete and P starved conditions. Differences among melon accessions were observed in response to P starvation, including differences in plant morphology, P uptake, P use efficiency (PUE) and gene expression. All studied genes were up regulated under P starvation conditions. Differences in the expression of genes involved in P mobilization and remobilization (Cm-PAP10.1, Cm-PAP10.2 and Cm-RNS1) under P starvation conditions explained part of the differences in P uptake and PUE among melon accessions. The levels of expression of the other studied genes were diverse among melon accessions, but contributed less to the phenotypical response of the accessions. Conclusions: This is the first time that these genes have been described in the context of P starvation responses in melon. There exists significant diversity in gene expression levels and P use efficiency among melon accessions as well as significant correlations between gene expression levels and phenotypical measurements.

Sucrose transport in the phloem: integrating root responses to phosphorus starvation

Sugars in plants, derived from photosynthesis, act as substrates for energy metabolism and the biosynthesis of complex carbohydrates, providing sink tissues with the necessary resources to grow and to develop. In addition, sugars can act as secondary messengers, with the ability to regulate plant growth and development in response to biotic and abiotic stresses. Sugar-signalling networks have the ability to regulate directly the expression of genes and to interact with other signalling pathways. Photosynthate is primarily transported to sink tissues as sucrose via the phloem. Under phosphorus (P) starvation, plants accumulate sugars and starch in their leaves. Increased loading of sucrose to the phloem under P starvation not only functions to relocate carbon resources to the roots, which increases their size relative to the shoot, but also has the potential to initiate sugar-signalling cascades that alter the expression of genes involved in optimizing root biochemistry to acquire soil phosphorus through increased expression and activity of inorganic phosphate transporters, the secretion of acid phosphatases and organic acids to release P from the soil, and the optimization of internal P use. This review looks at the evidence for the involvement of phloem sucrose in co-ordinating plant responses to P starvation at both the transcriptional and physiological levels.