Photosynthesis is the basic function of plants. Anything which interferes with it will impair the health of the plant. Chlorosis (yellowing) and Necrosis (browning and death) are the most obvious symptoms that photosynthesis is being impaired. If it continues long enough even mild photosynthetic impairment will lead to debilitation and increased susceptibility to lesser parasites/pathogens.

• Respiration of diseased plants
  

  It is generally recognized that most, if not all, infectious diseases increase respiration. This is actually a general reaction of the plant in response to most types of harmful stimuli. Studies concerning respiration are often complicated by he fact that except for viruses, pathogens also undergo respiration. This may lead to an error if the total respiration of the host and pathogen is measures. However, sophisticated studies indicate that most of the increased respiration comes from the affected host tissue. The increase of respiration of wounded plant tissue has long been known. Therefore, it is nor surprising that all forms of plant injury result in respiration increase.
  

  In the susceptible host, affected tissues generally undergo a slight increase in temperature as a result of infection. This is associated with an increased oxygen up-take and an enhanced activity of respiratory enzymes. The is usually an accumulation of compounds around points of infection. THis concentration and movement is an active process that requires some energy, and consequently, it is dependent upon respiration. FUrthermore, in an number os diseases, especially those involving obligate parasites, there is an increase in dry weight of the host that is correlated with increase in respiration. This occurs in the early stages of infection, prior to the production of symptoms. This increase of dry weight can only ex explained by an increase in synthesis by the host, which also require respiration.
  

  Powdery mildews have provided excellent experimental models to study the effect of pathogens on respiration. In response to infection by these fungi, increased respiration can be detected/measured in the tissue. When the powdery mildew fungus is stripped off the surface of the leaf, the increased respiration continues. Because powdery mildews penetrate only the epidermal cells and then only to the extent of haustorial formation and membrane contact, it has been concluded that these experiments prove that it is the host respiration that increases in diseased tissue and the increased in respiration is not due solely to pathogen metabolism.

• Effects of pathogens on permeability of cell membranes

  Changes in cell membrane permeability are often the first detectable responses of cell to infection by pathogens, to most host-specfic and several nonspecific toxins. to certain pathogen enzymes, and to certain toxic chemicals, such as air pollutants. The most commonly observed effect of changes in cell membrane permeability is the loss of electrolytes. In some very simple and elegant work by Dr. L.D. Dunkle, he demonstrated that the early effects of periconia toxin on sorghum seedlings was a loss of calcium and potassium ions. These type of losses may be measured by a simple conductivity bridge and with greater sophistication and discrimination by specific-ion electrodes. Using specific ion electrodes, Dr. Dunkle was able to demonstrate that varieties of sorghum that were resistant to the fungus Periconia circinata were also resistant to ion loss when a toxin prepared from culture filtrates was added to hydroponic solution. On the other hand, susceptible sorghum varieties succumbed to both the fungus or its toxin.

  Point to Ponder: This elegant experiment by Dr. Dunkle conclusively proves that it is the production of periconia toxin by the fungus that renders the parasite a pathogen. Or does it? Are there other possibilities?

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This page is authored and maintained by:

Dr. J.E. Partridge, Department of Plant Pathology, University of Nebraska-Lincoln

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Copyright (C)2008 J.E. Partridge, University of Nebraska-Lincoln. All Rights Reserved.