B.G. Lewis

Behaviour of uredospore germ-tubes of Puccinia graminis tritici in relation to the fine structure of wheat leaf surfaces

Studies of wheat leaf surfaces by transmission and scanning electron microscopy have revealed a regular lattice of wax crystals which overlies and entirely covers the cuticle. When uredospore germ-tubes of Puccinia graminis Pers. f.sp. trictici Erikss. & Henn. contact this lattice, they become orientated parallel to the short axis of the leaf. It is suggested that thigmotropic responses of the germ-tube to the lattice is the orientation mechanism which maximizes the probability of the hypha contacting a stoma. A technique for preparing fragile biological specimens for the scanning electron microscope is described.

Uptake of potassium by the developing sporangiophore of Phycomyces blakesleeanus

Movement of potassium from somatic hyphae into sporangiophore initials (stages I and II) involved bulk flow induced by evapotranspiration. A progressive decrease in water uptake by the sporangiophore after stage I was associated with a decrease in permeability of the surface, particularly of the upper part of the structure. The rate of uptake of potassium declined more rapidly than water uptake, reaching a minimum during stages II and III, and subsequently increased. Distribution of potassium during the later phases of development was not uniform throughout the sporangiophore. The concentration in the apex was maintained at a constant value of approximately 125 mmol l −1 irrespective of environmental water activity, whereas the concentration in the base was higher, more variable and dependent on environmental water activity. Movement of potassium from base to apex of the sporangiophore was associated with a second type of cytoplasmic streaming which was independent of atmospheric water activity.

Survival of chlamydospores and subsequent development of Mycocentrospora acerina in soil.

In field plots of fen peat soil, chlamydospores of Mycocentrospora acerina on nylon mesh showed a negligible decline in viability over a 2-year period. When infected pieces of carrot tissue were buried, most of the tissues disintegrated within a few weeks and clumps of chlamydospores remained in an amorphous matrix of lignified cell walls and periderm. In the absence of living host roots, these chlamydospores usually remained quiescent in soil and any germ-tubes rapidly lysed; even with a food-base of carrot tissue, growth through soil was negligible. However, roots of carrot seedlings stimulated chlamydospore germination, caused positive tropic reponses of the germ-tubes, and promoted mycelial growth. If chlamydospores were exhumed and placed on the soil surface in high humidity conditions, they usually formed a short germ-tube with a terminal conidium which was readily detached when immersed in water. Implications of these results are discussed in relation to the pre-harvest infection of susceptible crops.

Mechanism of translocation of potassium in sporangiophores of Phycomyces blakesleeanus in an aqueous environment

In sporangiophores of Phycomyces blakesleeanus Burgeff bridging aqueous phases of different potassium concentration, gradients of potassium concentration and electrical potential were generated intracellularly and caused movement of potassium in a direction towards the aqueous phase of lower potassium concentration. Measured rates of potassium movement agreed closely with rates predicted from the Nernst-Planck diffusion equation. It appears that bulk flow of cytoplasm does not make a significant contribution to the translocation of potassium under the experimental conditions.

Mechanism of translocation of potassium in mycelium of Phycomyces Blakesleeanus in an aqueous environment

Measurements of rates of translocation of potassium in mycelium of Phycomyces blakesleeanus suggest that the movement of potassium in this system may occur by diffusion alone. It is not necessary to involve bulk flow of cytoplasm as a contributory factor. The forces causing the translocation of potassium are the intracellular concentration gradient of potassium and an intracellular electrical potential gradient. The latter gradient is the sum of two contributions: one is an indigenous electric potential gradient in the mycelium (negative towards the apices); the other is an induced potential gradient caused by the extracellular concentration gradient of potassium.

Infection of carrot plants by Mycocentrospora acerina

In field plot experiments, Mycocentrospora acerina delayed emergence and caused ‘damping-off’ and death of emerged carrot seedlings, substantially reducing the population density. Surviving plants were often stunted and usually senesced earlier than healthy ones. The fungus was one of the main primary colonizers of the root surfaces where it developed extensively as hyaline mycelium and then formed pigmented mycelium and numerous chlamydospores as the tap root developed. These remained attached to the roots during harvest and were the main source of inoculum for infection during storage. Leaf lesions, bearing conidia of M. acerina , were also observed towards the end of the growing season but appeared to be relatively unimportant in the build up of the fungus in the field or in storage.

Behaviour of Mycocentrospora acerina on periderm and wounded tissues of carrot roots

Development of Mycocentrospora acerina on harvested carrot roots was studied. In early storage, all prepenetration stages of the fungus were suppressed on the periderm surface; many chlamydospores remained ungerminated but viable for at least 360 days. An inhibitory principle diffused out of periderm into water or agar films. On wound surfaces, inhibition, which was inversely correlated with the depth of wound, affected spore germination only. Attempted penetration in early storage was followed by a cellular reaction resembling a hypersensitive response. Inhibitory effects were greater at 3–5° than at 15°C.

Responses of grass and maize roots to invasion by Gaeumannomyces graminis and Phialophora radicicola

The ability of Gaeumannomyces graminis (Sacc.) Arx & Olivier var. tritici Walker, Phialophora radicicola Cain var. graminicola Deacon and P. radicicola var. radicicola to invade roots of Zea mays L. and five species of grass was investigated. All three fungi were restricted to the outer cortex of maize roots but were able to cross the whole cortex of grass roots. Subsequently, G. graminis var. tritici consistently progressed into the stele of grass roots, whereas P. radicicola var. radicicola only did so occasionally and P. radicicola var. graminicola was arrested at the endodermis. Effects of these interactions on the populations of these fungi are discussed.