الفهرس 
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Abstract
A hundred samples were collected from rice fields located at Kafr EI-Sheikh, El-Gharbia and El-Beheira Governorates to isolate the dominant cyanobacteria. Three out of sixty isolates of cyanobacteria were purified as bacterial free unialgal cultures and identified as strains of Anabaena cylindrica , Aulosira fertilissima and Nostoe muscorum. Those strains variably responded to increased concentrations of ammonical N (0,
2. 4. 6, 8, 10, 12 and 14 ppm N) or the recommended concentrations of the herbicides, Londax (0.03 ppm), Sindax (0.08 ppm) and Londax + Ordram (0.03 + 10 ppm) when added to the culture medium. While the growth rate of Anabaena cylindrica GH 11 was decreased with increased N-concentration, Nostoc muscorum KS 51 was relatively more tolerant. However, herbicide supplemented media appeared to support higher level
of growth of the 3 strains than the untreated controls. ln this respect, •the dry weight of Aulosira fertilissima KS 35 and Nostoc muscorum KS 51 were greater than that of Anabaena cylindrica GH 11. The interacting effects of mixed cyanobacterization with the abovementioned strains, N fertJ!ization and herbicides application on the development of the inoculated cyanobacteria and rice performance gave interesting results. Densities of the colony-forming unit (CFU) of cyanobacteria sharp!) increased with cyanobacterization especially when combined with the application of 72 rather than 144 kg N/ha. Moreover. application of
(Londax + ordram) stimulated field colonization with cyanobacteria
compared with the 2 other herbicides which showed a retarding effect. Microscopic examination revealed that Aulosira fertilissuna KS 35 failed to propagate in the rice field under experimentation. Generally. Anabaena cylindrica GH 11 was more dominant compared with Nostoc muscorum at different stages of rice growth and under the influence of the two studied factors. However, a balanced state between the proportional existence of the two strains was reported after harvest. The effect of cyanobacterization on rice perfonnance was found to vary according to the pattern of weed control and the amount of N dose. This finding was observed in the records of growth, yield and N uptake by rice plants. For example, increase in grain yield were accounted for 2.0 and 13.8% with cyanobacterization and handweeding plus fertilization with 72 and 144 kg N/ha, respectively. The corresponding increases due to the application of Sindax and (Londax + Ordra!TI) were 29.6 and 43.0% under the same
abovementioned conditions, respectively.