What is your substrate in this case?

wheat bran and Avicel...

pretrated lignocellulose (sugar cane bagasse easily preated) as carbon source and urea as nitrogen source.

Preteated in situ available lignocellulose (sugarcane bagasse, wheat/rice straw, etc). The nitrogen source depens on the fungal strain: for Penicillium perhaps NaNO3 or (NH4)2NO3; for Trichoderma (NH4)2SO4 + urea + peptone (or soymeal). Also, for Trichoderma lactose is a good cellulase induicer. Finally, it is also important to know the type of cultivation used (SSF, submerged fermentation). Best

Pretreated Wheat bran and avicel as carbon sources and peptone as organic nitrogen source

Thanks Dr. Marcel. We are using submerged fermentation.

I dont think use of avicel or peptone will be economical for cellulase production at commercial scale moreover, pretreatment of biomass will also add cost. what are your views??

I gree with you. Better is to use wheat bran + soymeal + ammonium sultate + urea and other salts. For Trichoderma you may try whey as carbon source and cellulase inducer; this may be cheap if there is a diary plant nearby.

I`ve obtained good productivity of cellulases using sugar cane bagasse, wheat bran and corn cob as substrate in solid state fermentation. All these substrates are very economical and readily available.

If you want more information I recommend papers of Sevastianos Roussos. He has very experience in this topic. 

Thanks Cruz, 

I know these substrates are good in solid state fermentation but my focus is on submerged fermentation and I need some sort of cheaper, soluble carbon source so as to minimize the use of insoluble carbon sources (avicel, wheat bran/straw, bagasse etc.)

Lactose can be a good option for induction and initial biomass development  but still you should have small amount of insoluble carbon source like wheat bran or bagasse.