A study of fusarium wilt resistance in tomato plants
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"Several strains of tomatoes, Lycopersicon esculentum, Lycopersicon pimpinellifolium, and hybrids of the two, were subjected to races 1 and 2 of Fusarium oxysporum f. lycopersici. The method of treatment was to excise a lateral root (in most cases), expose the wounded portion to some preparation of the fungus, either a mycelium pad, a cell free suspension of Richard's solution in which the fungus had been grown for three weeks, or a homogenate prepared by grinding in a Waring Blender a mycelium mat along with the Richard's solution in which it had been grown for three weeks. After various periods of exposure to the fungus preparation the plant roots, and in some cases the stems Just above the first internode, were dissected and portions of the primary root (and stem) were furthered sectioned, free hand, and microchemical tests were run on the fresh tissues. In all cases several serial sections were taken and any given test was considered unsatisfactory unless all the sections gave identical results. The xylem reactions were of prime interest in this study. Since one aim of the research was to elaborate further the findings of Finley concerning plugging material found within xylem elements special care was taken to seek out and attempt to identify the compounds comprising this obstruction. Two sorts of tests were run: the first series were intended to identify large organic groups of material present, and the second series was designed to give some idea of inorganic ions and radicals that may be found in the xylem. During the course of the investigation it became evident that two effects wore being expressed using the Finley technique: i.e., a wound response was being expressed in the same general area of the primary root that any sort of resistance reaction capable of being produced by the plant was also realised. In an effort to circumvent this difficulty Haymaker’s technique of utilising an almost undisturbed root system and still obtaining the resistance reaction was adopted. The general indications from the microchemical tests of the roots from a histological aspect were remarkably consistent. The basic distribution of pentoses and pentosans and of the polyphenols identified as such by the Hoepfner-Vorsats test seemed to correlate exactly with the degree of resistance expressed by the plants. In metabolically active cells surrounding the mature xylem these substances, in resistant plants, seemed to occur in amounts far exceeding those found in susceptible plants. Furthermore, plants shown to be capable of developing plugging within parasitised xylem elements were invariably shown to contain the above described distribution of pentoses, pentosans, and poly-phenols. The plugging material produced by almost all tomato plants subsequent to excision of a lateral root was identified tentatively as Molisch's and Tunmann’s wound gum. This material was produced at such a time as to be beyond consideration as a basis of Fusarium resistance, but acted solely as a wound protecting cicatrix. However the so-called resistant plants were observed to produce further plugging of quite a different nature (in this case, apparently a pectic gel) far in advance of the fungus invasion. Furthermore, simultaneously a sudden flush of polyphenols appeared in the active xylem sheathing cells. It was tentatively concluded that these latter effects, the intracellular pectic gelling and the surrounding mobilisation were active expressions of a resistance reaction capable of discouraging further advance of the fungus, and inhibiting any expression of pathogenicity of those hyphae already present in the host. Since it was also noted that living Fusarium did often occur within the plant body of a healthy host, it is suggested that these plants be referred to as Fusarium tolerant, rather than Fusarium resistant."--Summary.
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