Debenture bonus is deemed dividend u/s.2(22), since company needs to utilize(debit) its reserves for debenture bonus. It converts free reserves into debt. Debenture bonus is no where mentioned in both income tax act and companies act. So this is just an interpretation that it is deemed dividend as there is no specific provision dealing with it in income tax act.
Market price(nse/bse) of debenture on ex-bonus date can be taken as the value of each debenture bonus to calculate amt for tds u/s.194. Shareholder accordingly pay tax in the year of bonus itself because of sec.8. Such dividend can be included into cost of acquisition for sale of debentures later.
The reason for selecting price on ex-bonus date is that on ex-bonus date, right to share in reserves is unconditionally made available to shareholders i.e., any buy order for equity shares before ex-date entails the shareholder to become member of company as on record date, and thereby eligible for debenture bonus to the extent of holding on record date depending upon on bonus ratio.
Pls note that ex-date is business day immediately preceeding record date.