Transpiration in potato plants infected with Verticillium spp

Potato plants (cv. King Edward) infected with Verticillium albo-atrum and with V. dahliae transpired more slowly than healthy plants; this difference increased as the disease progressed. Diurnal fluctuations in transpiration were smaller in infected plants than in controls because infection markedly reduced water loss during the normal daytime peak period. Transpiration at night was unaffected by infection.
Both stomatal and cuticular transpiration of single, detached leaves were reduced by infection. A linear correlation was obtained between 'water saturation deficit' and transpiration rate in both diseased and healthy plants until the leaves wilted, suggesting that reductions in the stomatal rate are a consequence of the greater water deficits found in diseased plants, the differences in cuticular rates probably being due to anatomical differences between healthy and diseased leaves.
Close parallels between transpiration and water deficit indicate that in diseased plants water loss is largely determined by leaf water content. Thus wilting, commonly seen as a symptom of infection, is not the result of excessive water loss but follows a reduction in the supply of water to the leaves.
The author thanks Professor I. Isaac of this department and Dr G. C. Evans of the Botany School, Cambridge for their advice. The research was sponsored by the Potato Marketing Board.     

Effect of main stem number and lateral stem development in potato plants infected with Verticillium albo-atrum and V. dahliae

‘Early-dying’ disease was examined in potato plants (King Edward) with varying numbers of main stems and lateral stems. Infection with Verticillium affected neither the number of main stems produced nor the stem number/ lateral number/yield inter-relationships, but did slightly reduce lateral development. The host growth pattern, however, markedly influenced the severity of disease: infected plants with a single main stem or a few, compared with those having many main stems, showed delayed symptom expression and also produced many laterals which further reduced disease severity and increased longevity. Thus yield reductions as a result of infection are likely to be smaller in single-stemmed plants showing considerable lateral development than in those plants with many main stems and few or no laterals.     

Leaf-area development in King Edward potato plants inoculated with Verticillium albo-atrum and V. dahliae

Leaf-area development in King Edward potato plants infected with Verticillium albo-atrum and V. dahliae was examined both in plants with a normal growth pattern and in those where maturity had been artificially delayed. Methods are described for producing uniform, single-stemmed, initially disease-free host plants, and for measuring their total and green leaf areas throughout their development. Under both growth conditions the pathogens had no apparent effect upon the initiation of new leaves on the main axis of the plant, but they did influence their subsequent development. During the growing period the pathogens caused stunting, thus preventing the production of the potential maximum leaf area, while at maturity the chlorosis and necrosis of the diseased leaves and their premature fall reduced functional leaf area. In diseased plants in which maturity had been delayed, stunting at the apices was more apparent: internode length, leaf petiolar axis length and leaf area were all smaller than in healthy plants, the greatest reductions being shown in leaf area. Cells in the stunted leaves were fewer and smaller than those in healthy leaves. A direct result of leaf-area reduction was the development of smaller tubers, with consequent reduction in the fresh weight, and some reduction in tuber number. V. albo-atrum invariably proved to be more virulently pathogenic than V. dahliae; the use of an average Verticillium index was shown to be a reliable method for estimating relative virulence since it reflected both leaf area and yield reductions. Delaying host maturity and thus lengthening the period of extension growth conferred some resistance on plants infected with V. dahliae; symptom progression was stopped after its initial expression, and consequently leaf area was increased. This form of resistance was not shown in the plants inoculated with V. albo-atrum.     

Studies on the nature of resistance in tomato plants to Verticillium albo-atrum

Extracts of healthy resistant and of healthy susceptible plants of tomato had the same effect on growth of Verticillium albo-atrum in vitro. Tracheal saps from resistant and from susceptible plants showed no difference in their effect on spore germination and mycelial growth of V. albo-atrum. Cuttings from resistant plants, inoculated with V. albo-atrum and fed with low concentrations of casamino acids, or glucose, at first wilted more than controls but soon recovered. Continuous treatment with dilute ethanol solutions for 2 weeks induced marked wilting in inoculated cuttings of resistant plants: treatment for shorter periods caused less severe symptoms, from which cuttings recovered slowly. Metabolic inhibitors did not break resistance of cuttings, but the pathogen survived longer in cuttings treated with sodium diethyldithiocarbamate, salicylaldoxime or 8-hydroxyquinoline than in controls. When one end of segments of stems of resistant plants was inoculated with the pathogen, and 48 h later the uninoculated end was placed near a colony of V. albo-atrum on agar, growth of the fungus colony towards the stem segment was sometimes inhibited. There was no such inhibition when segments from susceptible plants were used. Both tracheal sap and diffusates from segments of inoculated resistant plants supported less growth of germ tubes of V. albo-atrum than sap and diffusates from uninoculated plants. These differences were not obtained with the susceptible variety and production of fungitoxic substances in resistant plants after infection is inferred.     

Host/parasite relations up to the time of tuber initiation in potato plants infected with Verticillium spp

The growth pattern of potato plants, infected with V. albo-atrum and V. dahliae, was studied, using growth-analysis techniques, during the initial stages of development up to tuber initiation. During the first 5–6 weeks of growth the morphology of the infected plants was not affected, but the distribution of dry weight materials between various plant organs and the growth rates of the whole plants were affected much sooner. In contrast to healthy plants, those infected exhibited lower specific leaf areas, higher leaf weight ratios and higher leaf area ratios, and, under dry conditions, lower relative growth rates and lower unit leaf rates. Thus during the incubation phase of the disease, although effects of infection are not immediately apparent, Verticillium pathogens have a considerable influence upon the development of the host.     

Phenylalanine Ammonia-Lyase from Tomato Cell Cultures Inoculated with Verticillium albo-atrum

Tomato (Lycopersicon esculentum Mill.) cell suspension cultures accumulated wall-bound phenolic materials in response to inoculation with Verticillium albo-atrum Reinke et Berth. in a fashion analogous to that observed in whole plants. Both monomeric and polymeric materials were recovered. Deposition of phenolics into the cell walls of inoculated tomato cell cultures was inhibited by the phenylalanine ammonia-lyase (PAL) inhibitor, 2-amino-2-indanephosphate. Tomato PAL activity was induced over 12-fold by fungal inoculation, with a concomitant increase in the corresponding mRNA. The enzyme was purified >3400-fold, to apparent homogeneity, by anion-exchange chromatography, chromatofocusing, and gel filtration. The holoenzyme had a molecular mass of 280 to 320 kilodaltons, comprising 74-kilodalton subunits, and displayed an isoelectric point of 5.6 to 5.7. Induced PAL displayed apparent Michaelis-Menten kinetics (K_m = 116 micromolar) and was not appreciably inhibited by its product cinnamic acid. Chromatographic analysis did not reveal multiple forms of the enzyme in either inoculated or uninoculated cultures.     

Elicitation of Ethylene by Verticillium albo-atrum Phytotoxins in Potato

Petioles from a susceptible cultivar (Désirèe) of Solanum tuberosum treated with a low‐molecular mass toxin, separated from culture fluid of Verticillium albo‐atrum, produced greater quantities of ethylene than did petioles of a tolerant cultivar (Home Guard). Pretreatment of leaflets from cv. Désirèe with silver thiosulphite, which inhibits perception of ethylene, prevented the chlorosis and necrosis normally associated with exposure to the toxin. Similarly, application of aminoethoxyvinylglycine (AVG) an inhibitor of aminocyclopropane‐1‐carboxylic acid (ACC) synthase, to petioles of cv. Désirèe reduced toxin‐induced ethylene synthesis and symptom development. The data indicate that, in part, Verticillium‐toxin acts through induction of ethylene biosynthesis in the host tissues, and different responses of susceptible and tolerant potato cultivars to V. albo‐atrum are the result of differential production of ethylene.     

Factors influencing infection of the tomato by Verticillium albo-atrum

Infection of tomato plants by Verticillium albo-atrum was encouraged by application of nitrogenous manures. Application of phosphate had no significant effect on the progress of the disease, but a deficiency of potash tended to encourage it. Steam-sterilized soil inoculated with Verticillium immediately after treatment produced a very high total of infected plants. When inoculation of the steamed soil by Verticillium was delayed for 17 days or longer after steaming, the steamed soil was no more favourable for development of the disease than untreated soil. Spread of Verticillium from the roots of an infected plant to those of neighbouring healthy plants was hastened by killing the infected plant.     

Growth Regulator Changes in Cotton Associated with Defoliation Caused by Verticillium albo-atrum

Cotton plants, variety Acala 4-42 family 77 (Gossypium hirsutum L.,), were stem puncture-inoculated with either a defoliating isolate (T9) or a nondefoliating isolate (SS4) of Verticillium albo-atrum (Reinke and Berth.). As symptoms developed, growth regulators were assayed in diseased plants to discern their importance in the disease syndrome.