P.G. Ayres

A cellulase complex in culture filtrates of Rhynchosporium secalis (barley leaf blotch)

Rhynchosporium secalis (Oudem.) J. J. Davis grew in liquid culture media that contained single soluble or insoluble cellulosic sources of carbon, liberating into the medium a complex of cellulase enzymes. The complex was separated by gel filtration followed by ion-exchange chromatography into six components, three of high molecular weight and three of low molecular weight. One of the high molecular weight components showed C 1 cellulase activity. Alone it had no effect on insoluble cellulose, but in combination with other components it caused a synergistic increase in the hydrolysis of the substrate.

Effect of water potential of pea leaves on spore production by Erysiphe pisi (powdery mildew)

Water stress imposed on field peas by the cessation of soil watering at different times before or after inoculation with Erysiphe pisi DC. caused decreased mycelial growth and spore production as leaf-water potential, measured at midday, decreased from −4.5 + 0.5 bar (soil water at field capacity) to the permanent wilting point (− 9.5 + 1.0 bar). Similar results were obtained with whole plants grown in nutrient solution, and with leaf disks, if stress was rapidly imposed as sporulation began.

Effects of plant age and water stress on production of conidia by Erysiphe graminis f.sp. hordei examined by non-destructive sampling

A spore trap was designed which enabled repeated collection of conidia to be made from the same individual colonies of Erysiphe graminis f.sp. hordei throughout their development. It was found that in a fully susceptible barley cultivar/fungal isolate combination, the commencement of sporulation was delayed, and the number of conidia produced per day was reduced, by increased plant age and water stress. In well-watered plants cumulative sporulation up to the thirteenth day after inoculation was reduced by 64 % on adult plants compared with seedlings. The corresponding figures for water-stressed seedlings and adult plants were 74 and 77 % respectively.

Conidial germination and germ-tube growth of Erysiphe pisi in relation to visible light and its transmission through pea leaves

Germination of conidia of Erysiphe pisi, and germ-tube growth, are sensitive to low levels of irradiance at wavelengths within the visible spectrum, especially if conidia come from plants previously held in darkness for 24 h. At similar irradiances, germination is higher on the lower than on the upper epidermis of pea leaves illuminated from above. Germination, but not germ-tube growth, in vitro is stimulated by light transmitted through pea leaves. Development of conidia in vitro, in light transmitted through selected interference filters, is stimulated by bands of peak wavelengths 430 and 496 nm, inhibited by light of peak 551 nm, and insensitive to light of longer wavelengths. The significance of the responses is discussed in relation to the light transmitting properties of pea leaves which alter after mildew infection.

Plant-fungus water relations affect carbohydrate transport from pea leaf to powdery mildew (Erysiphe pisi) mycelium

The effects that water relations may have on the net uptake of carbohydrate by powdery mildew mycelium were investigated. Strips of pea leaves infected by the powdery mildew fungus (Erysiphe pisi) were fed [ 14 C]sucrose. Net uptake into leaf (plus haustoria) and mycelial fractions were measured separately. An examination of experimental variables governing uptake showed that total uptake was approximately linear over a 3 h pulse period, about 12% of the total being in the mycelium. This increased to 26% during a 3 h chase period on unlabelled medium. Uptake by both leaf and mycelium was generally insensitive to pH in the range 4.8–6.8. Over a range of external sucrose concentrations uptake by both leaf and mycelium was biphasic, the pattern suggesting that uptake to the fungus was predominantly active in the range 0.05–10.0 mmol dm −3 . The fungus apparently had a higher affinity for sucrose, K m 7.65 mmol dm −3 , than did the leaf, K m 23.52 mmol dm −3 , and on a dry-weight basis took up more sucrose than the leaf. Water stress, both of plants from which strips were taken and during incubation, increased the amount of sucrose taken up by the leaf but decreased the percentage of total 14 C taken up that was found in the mycelium. In combination, pre-incubation and incubation stress had additive, negative effects on transport to the mycelium. Uptake by leaf and mycelium were stimulated by air movement and the proportion of total uptake in the mycelium rose to 38% in the most rapidly moving air. It is proposed that air movement stimulated transpiration which, in turn, promoted the development of turgor pressure gradients driving carbon transport by mass flow into the mycelium. Water stress inhibits the development of such turgor pressure gradients and in this way it could exacerbate the stress-induced inhibition of carbohydrate transport to the mycelium that was observed in leaf strips in still air.

Water or salt stress increases infectivity of Erysiphe pisi conidia taken from stressed plants

Conidia of Erysiphe pisi were taken from pea plants (donors) subjected either to water stress, by withholding water from the soil, or to salt stress, by addition of NaCl solutions to the soil, or from unstressed control plants, and were inoculated on to leaves of similarly stressed or control plants (recipients). Germination of conidia, appressorium formation and infectivity, and the production of elongating secondary hyphae (ESH) by germinated spores, were observed under the light microscope at intervals up to 48 h after inoculation. Conidia from controls completed germination by 8 h and ESH formation by 24 h after inoculation, but conidia from stressed plants continued their development up to 48 h. Conidia from water-stressed donors had germinated more by the end of the experiment than conidia from control donors, but conidia from salt-stressed donors had only reached the same levels as those from controls. Neither water nor salt stress in donors or recipients affected the formation of appressoria by germinated spores. Both water and salt stress of donors significantly stimulated ESH formation from germinated conidia. The only significant effect of stress of recipients was that salt stress stimulated the germination of conidia. Water or salt stress of the donor plant may thus affect the infectivity of spores produced by mildew colonies on that plant. Such changes in infectivity, although small, may have a considerable cumulative effect over several generations of a polycyclic epidemic in the field.

Effects of materials from oak leaf surfaces on germination of Microsphaera alphitoides conidia

Germination of conidia of Microsphaera alphitoides is poorer on the surface of oak leaves than in vitro . Aqueous washings of oak leaves were extracted in diethyl ether and separated by thinlayer chromatography. Two inhibitory substances were isolated, tests on which indicated that they were probably phenolic and were combined, possibly, with a lipidic moiety. The substances could not be unequivocally identified. Although prolonged immersion in chloroform, for 40 s or more, released inhibitory substances from leaves, surface waxes extracted by a brief, 10 s immersion had no effect on germination. Earlier reports that t.1.c. of the whole wax leads to the separation of fractions that stimulate germination could not be confirmed.

Uptake of 14C-labelled d-uronic acids by Sclerotinia fructigena mycelium

A study was made of the uptake of 14C-labelled glucuronic and galacturonic acids by mycelium of Sclerotinia fructigena Aderh. & Ruhl. Net uptake of glucuronic acid was slower and declined sooner than uptake of galacturonic acid. Uptake of both acids was not affected by the degree of dissociation of the acid in the pH range 4.1–4.9. Uptake of both acids was only sensitive in small part to temperature, and was directly proportional to the external concentration of the acid over the range 0.01–1.00 % (w/v). Uptake of labelled glucuronic acid was inhibited when the mycelium was pre-incubated in unlabelled acid for either 20 min or 3 days. Uptake of galacturonic acid was stimulated by similar pre-incubation treatments, though only slightly by the 20 min pre-incubation, suggesting that, for both acids, efflux is not a significant factor determining rates of net uptake. Each uronic acid did not inhibit the uptake of the other. It is concluded that uptake of both acids is largely, if not wholly, a passive process, but that the rate of uptake is modified by active assimilation of the uronic acids into cell metabolism, as indicated by the inhibition of uptake caused by metabolic inhibitors, and the stimulation of uptake caused by the presence of a second carbohydrate in the incubation medium.

Volume, water and energy contents of conidia of Erysiphe pisi from water-stressed pea plants

The mean volume of conidia of Erysiphepisi produced from mycelium on peas subjected to water stress was similar to that of conidia from a well-watered source. Conidia from stressed plants had, however, a lower water content and, as measured by bomb calorimetry, a higher total energy content per conidium.