Abstract: The parent strain Trichoderma koningii was implanted with N+ beam for breeding a mutant strain with the ability to produce cellulase. It was found that the curve of survival rate of the parent strain revealed a    saddle    shape and a high positive mutation rate in the treatment dose range of 10× 1014 to 12.5 × 1014 ions/cm2. Under the implantation conditions of 15 keV and 12.5 × 1014 ions/cm2, a high cellulase-producing mutant with excellent genetic stability was obtained and named as HF-6. The activity of cellulase in the screened mutant was roughly 0.217 U/mL, which was 52.82 % higher than the parent strain. Three main factors (fermentation conditions) that affect cellulase production by HF-6 including pH, medium volume in 250-mL shake flask and ammonium sulphate concentration were screened out of 8 factors by Plackett-Burman design. Response surface methodology was employed to optimize the three factors based on a three-factor, three-level Box-Behnken experimental design, which resulted in a mathematical model of filter paper activity (FPA) against the three factors. The optimum fermentation conditions were determined as follows: pH 5.75, 4.23 g/L ammonium sulphate and 63 mL of culture medium in 250-mL shake flask, respectively. Under these conditions, the experimental FPA activity in fermentation broth was 0.233 U/mL (an average of 3 replicates).

Key words: ion beam implantation, response surface methodology, cellulase, Trichoderma koningii, filter paper activity