Roots Of Resistant Varieties Research Articles

Resistance to the Weevils Cylas puncticollis and Cylas brunneus Conferred by Sweetpotato Root Surface Compounds

Seven resistant varieties of sweetpotato were compared with three susceptible varieties in field trials and laboratory bioassays and showed that resistance was an active process rather than an escape mechanism, as field resistant varieties also had reduced root damage and oviposition compared with susceptible varieties in the laboratory. Liquid chromatography-mass spectrometry (LC-MS) of root surface and epidermal extracts showed significant variation in the concentration of hexadecyl, heptadecyl, octadecyl, and quinic acid esters of caffeic and coumaric acid, with higher concentrations correlated with resistance. All compounds were synthesized to enable their positive identification. Octadecyl coumarate and octadecyl caffeate applied to the surface of susceptible varieties in laboratory bioassays reduced feeding and oviposition, as observed on roots of resistant varieties, and therefore are implicated in weevil resistance. Segregating populations from breeding programs can use these compounds to identify trait loci for resistance and enable the development of resistant varieties.

The distribution of cucumber mosaic virus in resistant and susceptible plants of pepper

Immunofluorescence microscopy, azur A staining of viral inclusion bodies, and ELISA tests revealed differences between resistant and susceptible decapitated plants of pepper for virus distribution through the plant but not for virus multiplication and spread in the artificially inoculated leaves. Within 7 to 10 days after inoculation, viral antigens were detected with ELISA tests in all organs of the susceptible plants; only in inoculated leaves of highly resistant plants; and in inoculated leaves, primary stems, and primary roots of partially resistant plants. Immunofluorescence microscopy revealed that infection in petioles, stems, and roots of resistant varieties was restricted to one or two phloem bundles and did not spread to neighbouring tissues or to other plant organs. However, in some partially resistant plants the virus spread lately to other tissues and organs, resulting in a delayed systemic infection and mosaic symptoms on one or two axillary shoots.

The Dynamics of Amino Acid Variations During Pathogenesis and Host Resistance

On the analysis of conclusions, arrived at by different workers regarding amino acid status during pathogenesis and resistance, certain basic trends have emerged. They are as follows: 1. Virus-infected tissues do not show any definite pattern of variation in their amino acid constituents. 2. The roots of susceptible plants, by exuding some nutritionally needed amino compounds in the rhizosphere, evoke fungal pathogenicity, especially of the vascular wilt diseases. Contrary to that, roots of resistant varieties, by secreting inhibitory amino acids, restrain inoculum build-up and retain resistance. 3. Inhibitory potentiality of a particular amino acid maintains resistance in the shoots, too. 4. In the leaves, during fungal pathogenesis presence, increase and depletion of amino acids are more rhythmic than uniform. Even toxins and varied nycto temperature s have their own autonomous effects. There is no definite pattern of the cause and effect of relationship between resistance and susceptibility of the host on one hand and amino acid status of the leaves on the other. 5. Pathogenesis of the leaves due to bacteria have almost the same fluctuating design of their amino acid constituents, as evinced during fungal pathogenesis. 6. Amino acid component of histone and non-histone protein of normal tissue and crown gall tumour tissue do not differ much. In other tumours of bacterial origin, however, tumour tissue has specific amino compounds, absent from normal tissue. 7. In fruits the activity during pathogenesis causes either depletion or loss of amino acids, due to their nutritional utilization by the pathogen. This could also be due to their incorporation during protein synthesis. The increase in the amount of individual amino acids, however, may be due to proteolytic breakdown of the host protein activated by pathogenic enzymes. The myriad conclusions regarding the amino acid status during disease development only accentuate the necessity of investigating this problem in its totality, taking into account the concurrent and concomitant activities leading to synthesis of proteins and phenolics as well as their enzymatic breakdown. Die Auswertung von Versuchsergebnissen, den Status der Aminosäuren in erkrankten und resistenten Wirtspflanzen betreffend, ergibt folgende Trends: 1. Virus-infizierte Gewebe zeigen keine definitive Abweichung in der Zusammensetzung der Aminosäuren. 2. Die Wurzeln empfindlicher Pflanzen fördern Pilzerkrankungen, vor allem Welkekrankheiten im Gefäßsystem, indem sie verschiedene für die Ernährung wichtige Aminoverbindungen ausscheiden. Die Wurzeln resistenter Pflanzen scheiden inhibierende Aminosäuren aus, die einen Befall verhindern. 3. Die Fähigkeit einiger Aminosäuren zur Inhibition erhält auch die Resistenz der Sprosse. 4. In den Blättern verlaufen Zunahme und Entleerung von Aminosäuren bei pilzlichen Erkrankungen nicht gleichmäßig, sondern vielmehr rhythmisch. Eine Beziehung zwischen Empfindlichkeit und Resistenz einerseits und Aminosäurestatus der Blätter andererseits läßt sich nicht ableiten. 5. Für bakterielle Erkrankungen gilt Ähnliches. 6. Die Zusammensetzung der Aminosäuren in Histonen und Nicht-Histonen von normalen und erkrankten (Wurzelkropf) Geweben unterscheidet sich nur wenig. Doch gibt es davon auch Abweichungen. 7. In Früchten verursacht Krankheitsbefall Erschöpfung oder Verlust von Aminosäuren, abhängig von der Nutzung als Nahrungsquelle des Pilzes. Die Erhöhung des Gehaltes an speziellen Aminosäuren kann auf Proteolyse des Eiweißes des Wirtes zurückgeführt werden, welche durch Enzyme des Krankheitserregers verursacht wird. Die Vielzahl der Meinungen hinsichtlich des Aminosäurestatus bei Krankheitsbefall verweist auf die Notwendigkeit, dieses Problem in seiner Gänze zu untersuchen unter Beachtung konkurrierender und begleitender Aktivitäten.

The population genetics of the potato cyst‐nematode, Heterodera rostochiensis: mathematical models to simulate the effects of growing eelworm ‐resistant potatoes bred from Solatium tuberosum ssp. andigena

SUMMARYTo follow population changes when potato varieties with resistance to Heterodera rostochiensis derived from Solanum tuberosum ssp. andigena were grown on infested land, a computer programme was written including three mathematical relationships: (1) a law relating multiplication to pre‐cropping density; (2) two mathematical models of inheritance of ability to overcome resistance; (3) a law relating the proportion of larvae able to become female to pre‐cropping population density. The programme also included four parameters: (1) the maximum possible reproductive rate; (2) the fraction of the population (eggs) not participating in reproduction when potatoes are grown and carried over to the following year unchanged; (3) the fraction carried over annually when other crops are grown; (4) the frequency of larvae able to become female in the population initially. Population density was measured relative to the equilibrium density and was therefore independent of the units in which density is usually measured.After supplying a range of parameters for all the above to include those likely to be encountered in practice, the changes expected (a) in the frequency of larvae able to become female in the roots of resistant varieties and (b) in population density were computed for resistant varieties grown continuously or alternately with susceptible varieties in crop rotations of different lengths. Because well established field populations are relatively dense, observed reproductive rates are small and rarely approach the maximum possible. Reproductive rate is therefore a relatively unimportant determinant of genetic change. The fraction of the population carried over to the following year is more important because it affects the length of a crop rotation necessary to make loss of potato yield acceptable, determines what the multiplication rate will be and influences the speed of genetic change by providing a reservoir of initial type males which backcross with any genetically different females that may develop on the roots of resistant plants.No experiment seems to have been done specifically to determine the parameters needed to calculate population changes. Some values can be obtained from the literature but mostly they must be guessed.When the law relating the proportion of larvae able to become female to pre‐cropping population density was included in the computations, it had little effect initially but later, after several generations, it delayed genetic change.Two field experiments, one by Huijsman (1961) another by Williams (1958) and Cole & Howard (1962 a), provide some of the variables needed to compute trends in population density. Best fitting variables were computed for the data in these experiments by the method of maximum likelihood. The computed parameters for one experiment were not very realistic but those for the other were in line with what would be expected in practice and tended to favour the hypothesis that larvae able to become female in the roots of resistant plants are double recessives (aa).The computations lead us to suggest that the best policy for potato growers who have fields suitable for resistant varieties is to alternate resistant with susceptible varieties in a crop rotation containing potatoes every 3 or 4 years.