• Volume 81, Issue 6

      June 1975,   pages  229-261

    • Breeding for disease resistance in plants

      T S Sadasivan

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      In breeding for resistance, specific resistance is not the ideal to aim at. Generalized resistance to biotypes of the pathogen is the ultimate goal but difficult to attain. The concept of evolving multiline hybrids to combat the ever increasing threat from bioraces, especially in the cereal rusts, now engages the attention of plant breeders. Locating gene centers as the repositories of greatest wealth of hereditary variants, displaying naturally inbred characteristics, are indispensible as gene pool for ongoing breeding programmes and development of “Population resistance” or “Synthetic horizontal resistance”.

      Two concepts of breeding for Vertical Resistance (VR) or for Horizontal Resistance (HR), the former conditioned by oligogenes and the latter by polygenes, are invaluable in evolving resistant varieties that can slow down epidemics and offer protection. VR confers complete but impermanent protection, whereas HR confers incomplete but permanent protection.

      A point of interest is that differentially, pathogen races, differ in virulence and other races differ in aggressiveness. A race with unnecessary virulence is less fit to survive on a strong resistance gene host than a weakly virulent one. With the discovery of ‘Phytoalexins’ producedin vivo in host under pathogenesis, which are essentially phenolics, with a possible rôle in initiating defence mechanism in plants, the search is on for similar substances in resistant cultivars. Enhanced synthesis of lignins are also considered as a means of building up resistance in plants to pathogens.

      Plant pathologists have been looking for genetic markers in the many end-products of biosynthesis that could resist pathogenic invasion of tissues. Alterations in respiratory rates and oxidase activity, in general under pathogenesis, have received much attention. More recently, germination of a fungus spore has been shown to be accompanied by mRNA synthesis with subsequent changes in ribosomal activity. It has been suggested that host-parasite interaction could be between host genes and infective RNA. One of the major roles of activated oxidative processes could be that of detoxificationin vivo of the pathogen’s toxin, a characteristic of resistant cultivars so far examined. The present trend may lead on to a better understanding of the much debated question as to whether natural immunity exists in plants against specific pathogens.

    • Effects of temperature-salinity combinations on the digestion rates ofGambusia affinis

      Katre Shakuntala

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      The presence of a ‘physiological stomach’ (pH 2–4) inGambusia affinis is proposed based on measurements ofpH in the gut and food intake of fish starved for 5 days, as a function of temperature and salinity of the environment. Digestion was fastest in 3‰ salinity at 30° C and slowest at 20° C in freshwater.

    • Physiology of digestion inPuntius sophore (Ham.)

      Krishan A Goel

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      Distribution of digestive enzymes in the digestive system ofPuntius sophore (Ham.) and thepH of the system under fed and starved conditions have been determined. The sites of secretion of carbohydrases, proteases and esterases in the alimentary canal have been established.

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