The Colletotrichum gloeosporioides species complex

The conditions of learning

The challenge of problem-based learning

Phosphorus (P) is an essential plant nutrient and one of the most limiting in natural habitats as well as in agricultural production world-wide. The control of P acquisition efficiency and its subsequent uptake and translocation in vascular plants is complex. The physiological role of key cellular structures in plant P uptake and underlying molecular mechanisms are discussed in this review, with emphasis on phosphate transport across the cellular membrane at the root and arbuscular-mycorrhizal (AM) interfaces. The tools of molecular genetics have facilitated novel approaches and provided one of the major driving forces in the investigation of the basic transport mechanisms underlying plant P nutrition. Genetic engineering holds the potential to modify the system in a targeted way at the root-soil or AM symbiotic interface. Such approaches should assist in the breeding of crop plants that exhibit improved P acquisition efficiency and thus require lower inputs of P fertilizer for optimal growth. Whether engineering of P transport systems can contribute to enhanced P uptake will be discussed.

Functional biology of plant phosphate uptake at root and mycorrhiza interfaces

The Colletotrichum acutatum species complex

Two reliable small-plant bioassays were developed using tissue-cultured banana, resulting in consistent symptom expression and infection by Fusarium oxysporum f. sp. cubense (Foc). One bioassay was based on providing a constant watertable within a closed pot and the second used free-draining pots. Culture medium for spore generation influenced infectivity of Foc. Inoculation of potted banana by drenching potting mix with a conidial suspension, consisting mostly of microconidia, few macroconidia and no chlamydospores, generated from one-quarter-strength potato dextrose agar + streptomycin sulfate, resulted in inconsistent infection. When a conidial suspension that consisted of all three spore types, microconidia, macroconidia and chlamydospores, prepared from spores generated on carnation leaf agar was used, all plants became infected, indicating that the spore type present in conidial suspensions may contribute to inconsistency of infection. Inconsistency of infection was not due to loss of virulence of the pathogen in culture. Millet grain precolonised by Foc as a source of inoculum resulted in consistent infection between replicate plants. Sorghum was not a suitable grain for preparation of inoculum as it was observed to discolour roots and has the potential to stunt root growth, possibly due to the release of phytotoxins. For the modified closed-pot system, a pasteurised potting mix consisting of equal parts of bedding sand, perlite and vermiculite plus 1 g/L Triabon slow release fertiliser was suitable for plant growth and promoted capillary movement of water through the potting mix profile. A suitable potting mix for the free-draining pot system was also developed.

Development of a small-plant bioassay to assess banana grown from tissue culture for consistent infection by Fusarium oxysporum f. sp. cubense

Problem-based learning (PBL) is a powerful instructional approach. By working through assessable complex problem-solving tasks learners can be encouraged to actively engage in investigation and inquiry and to use high level cognitive thought processes to solve real-life problems in professional contexts. A critical element of a successful PBL design is the inclusion of instructional support, such as scaffolding, to guide and assist the learner through the reasoning process that is crucial to successful problem-solving. The e-learning tool ‘Challenge FRAP’ (Form for the Recording of the Analysis of Problems) is client-based public domain authoring software which facilitates the use of scaffolding, the provision of progressive feedback and can promote student reflection at key decision-making points. This paper illustrates the benefits of such an e-learning scaffolding tool through two PBL case studies; one group-based PBL task in science and technology and one self-directed PBL task in plant pathology.

Enhancing Problem-Based Learning Designs with a Single E-Learning Scaffolding Tool: Two Case Studies Using Challenge FRAP

The Influence of Biochar on Growth of Lettuce and Potato

The growth response of 2 vegetable crops to 5 rates of applied phosphorus (P) in the presence or absence of an existing network of extraradical mycorrhizal mycelium was determined in 2 greenhouse pot experiments (Expt 1, autumn-winter; Expt 2, summer-autumn) using a low-P growth medium (6 or 5 mg NaHCO3-extractable P/kg for Expt 1 or 2, respectively). In both experiments, capsicum (Capsicum annuum L.) and tomato (Lycopersicon esculentum Mill.) plants were grown at 0 (P-1), 9.2 (P-2), 27.5 (P-3), 82.5 (P-4), or 248 (P-5) mg P/kg oven-dry soil (spot-placed at sowing) within a nylon mesh (pore size 44 mu m) The mesh excluded roots from the original sunflower (Helianthus annuus L.) host plants, to which either live (VAM+) or killed (VAM-) mycorrhizal [Glomus etunicatum Becker & Gerdemann and Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe] inoculum was added at sowing. The mesh did allow fungal hyphae to grow into the growth medium contained by the mesh. Whereas VAM+ plants generally had higher P concentrations in index tissues than VAM- plants at low P rates, a concomitant increase in dry matter yield was restricted to the P-1 rate. At P-1 in Expt 2, the increase in the dry weight of whole plants as a result of VAM colonisation was as large as 91.7-fold and 17.9-fold for capsicum and tomato, respectively. Root starch analysis indicated that the lower dry matter yields of VAM+ plants than of VAM- plants at greater than or equal to P-2 could be attributed to insufficient photosynthate production by VAM+ plants to meet the carbon (C) demand of both host and endophytes within the relatively low-light environment of the greenhouse (average daily solar irradiance of 8.4 MJ/m(2) for Expt 1 and 13.4 MJ/m(2) for Expt 2). The growth response of vegetable crops grown within the greenhouse from colonisation by an established mycorrhizal mycelium appears to depend on a critical balance of P and C supply; i.e. at P-1, P was more limiting than C, and the increased uptake of P as a result of colonisation of plant roots by VAM resulted in a growth response. At higher P rates, C was more limiting than P due to low light in the greenhouse, and the additional demand for photosynthate imposed by the endophytes on the host resulted in a growth depression relative to non-mycorrhizal plants.

Effects of mycorrhizae, established from an existing intact hyphal network, on the growth response of capsicum (

This field trial determined the importance of mycorrhizae [Acaulospora mellea Spain & Schenck, Gigaspora margarita Becker & Hall, Glomus clarum Nicolson & Schenck, Glomus etunicatum Becker & Gerdemann, and Scutellospora pellucida (Nicol. & Schenck) Walker & Sanders], established from an existing intact network of vesicular-arbuscular mycorrhizae (VAM) propagules, for production of capsicum (Capsicum annuum L. cv. Target) in a low phosphorus (less than or equal to 14 mg NaHCO3-extractable P/kg) Typic Paleudalf. The mycorrhizal network was formed previously with sweetcorn (Zea mays L. cv. Snosweet) plants grown in distinct rows. A narrow band of milled superphosphate was applied at 0 (P-1), 5 (P-2), 15 (P-3), 45 (P-4), or 135 (P-5) kg P/ha to plots that were either fumigated (VAM-) or not fumigated (VAM+). The higher (P < 0.05) concentrations of P in the youngest mature leaf(blade plus petiole) of VAM+ than those of VAM- capsicum plants at P-1, P-2, and P-3 coincided with a greater (P < 0.05) weight of marketable fruit of VAM+ than of VAM- plants; this finding confirmed the importance of VAM to the enhanced P nutrition of capsicum at low P levels. In the non-fumigated soil (1061 non-dormant infective VAM propagules/g air-dry soil), the inverse sine transformed means of percentage colonisation of roots were generally unaffected by P application, indicating that addition of P may not reduce VAM colonisation of roots if the inoculum potential of the soil is high. An alternative hypothesis to account for the relatively undiminished mycorrhizal colonisation of VAM+ plants at high P rates may relate to a reduction in colonisation for roots growing within the narrow band of applied P, but not for roots growing in the much larger volume of soil outside of this localised P zone. The absence of reduced yields and of lower starch concentrations of roots of VAM+ relative to VAM- plants suggested that photosynthate production was surplus to the requirements of the plant and fungus for the irradiance encountered during the trial. The gross margin for VAM-plants was maximal at P-5 ($AU3340/ha), and this amount was similar to the margin for VAM+ plants grown at all treatments except pi, which had a lower gross margin. As a substitute for P, mycorrhizae have limited potential in intensive vegetable production systems since the cost of P is low compared with total costs. However, other benefits of VAM such as decreased susceptibility to disease and improved structure of the soil need to be fully assessed to determine the full benefit of mycorrhizae in such systems.

Victor J. Galea

Papers

Development of a small-plant bioassay to assess banana grown from tissue culture for consistent infection by Fusarium oxysporum f. sp. cubense

Enhancing Problem-Based Learning Designs with a Single E-Learning Scaffolding Tool: Two Case Studies Using Challenge FRAP

The Influence of Biochar on Growth of Lettuce and Potato

Effects of mycorrhizae, established from an existing intact hyphal network, on the growth response of capsicum (

Effects of a network of mycorrhizae on capsicum (Capsicum annuum L.) grown in the field with five rates of applied phosphorus