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Bronzing is a physiological disease caused by ferrous ion (Fe⁺⁺) toxicity and anaerobic conditions in the soil. Occurrence of this disease has been frequently reported from the wet zone low country of Sri Lanka.
Bronzing can be induced in liquid culture grown plants using excess Fe⁺⁺ and low pH values in the culture medium. Disease resistant and disease susceptible varieties could be distinguished when grown in a medium with 425 ppm Fe⁺⁺ at pH 5.0. The varieties H9, BG11-11, BG90-2, LD125 and IR8 are susceptible to the disease and the varieties H4 , Kuatic Putih, IR2061-464-2-4-4-6, IR2070-189-4-1-3 and Mashuri are resistant to the disease. Experiments using varying levels of Fe⁺⁺ and hydrogen sulphide (H₂S) in the culture medium, showed that H₂S can enhance the rate of bronzing development in susceptible varieties.
Fresh leaf sections showed accumulation of minute reddish brown globules in chlorophyllous tissue of disease leaves. The oily nature of these globules was confirmed as had been previously reported. Leaf lipids were analyzed by gas-liquid chromatography. Using four rice varieties R9, BG 11-11, BG 90-2,and IR 2061-214-2-7-6-3, the content of fatty acids was found to be more in diseased condition. Furthermore, arachidic acid (C-2C) was found only in the diseased leaf samples.
A remarkable difference of rates of respiration of roots was observed between bronzing resistant and susceptible varieties. However, no influence of Fe⁺⁺ toxicity on root respiration could be detected. Six reciprocal genetic crosses were made between susceptible varieties H9 and BG11-11 and resistant varieties H4, Mashuri, Kuatic Putih and IR2061-464-2-4-4-6. The parental plants, F₁, F₂, F₃ generations were screened for bronzing resistance using liquid culture conditions. It appeared that the resistance to bronzing is a dominant character and has no maternal influence. The increased resistance observed in hybrid plants and their subsequent generations, indicated a strong heterotic effect in all the crosses conducted. Polygenic nature of the resistance to bronzing was assumed and was confirmed from the results obtained from backcrosses.
The heritability of the resistance to bronzing was high or moderately high in most of the genetic crosses. Study of correlation between grade of bronzing other metric characters namely, plant height, number of tillers per plant, leaf length, leaf breadth, diameter of stem, angle of leaf with stem, panicle length, panicle weight, grain length, grain width, grain thickness, grain weight, harvest and straw weight did not help to establish a definite relationship between resistance to bronzing and any other metric characters tested.
