is son responsible for mothers promissary notes?

Ifs and buts.

If the notes are genuine and you inherit her property, you are responsible. Else. Forget it.

If you inherit property from her, it is your duty to pay of the debts incurred by her.

it is like marriage, property come with troubles

Legal heir is no precondition for paying the debts of one's parents. The basic thing is whether the heirs have inherited their property.

my mother made a large amount of debts without my knowledge.
she signed promissary notes for all of them for about 15-20 lakhs
but none of the notes have my sign.
Ans :  Since the promissary notes do not have your  "sign",   you can always claim  "non-existance" of such forged promissory notes belonging to your mother.

she is a government employee and she still is working for the same ..,she dosent have any asserts on her name
Ans :  "IF  &  SINCE"  the Mother is still alive & working for the Govt.  there is  "NO QUESTION"  OF  "YOU"  presently being respnisble for her promissary notes.
Ans :   "IF" the mother does not own  "ANY"   assets on her name,  there is  "NO QUESTION"  OF  "YOU"   inheirting  anything from her AFTER her death.

LEGALLY,   "IF"  you are  "not inheirting anyhting"  from your mother,  THEN there is  "NO QUESTION"  of  you inheirting  any  "debt or liability" of your mother,  which includes her promissory notes.

i am a private job holder and i have a house on my name and paying loan for the same
am i responsible for her promissary notes?
will i be liable for them after her death?
Ans :  Since you already  "OWN" a   "self-acquired"    on-loan-house,  with your  "OWN FUNDS",  there is no question of any  "responsibility to pay her promissary notes" after her death.

IT IS THE NORM & DEMAND OF  PRESENT TIMES  (KALYUG)  that   IF YOU ARE ABLE to keep away the  "sentiments and emotions"  attached to your mother,  THEN it is best to  totally  "disown"  all the  "debts and liabilities"  that may have incurred during your mothers life-time.   Presently,  refrain from signing any and all documents, that pertain to your mothers name, IF you wish not to settle your mothers  "debts & liabilities".

LEGALLY,  since you do not stand to  "inheirit any assets"  of your mother,  you are not bound legally to or reponsible for payment of her promissory notes,  before or after her death.

Keep Smiling .... Hemant Agarwal

thank you for correcting me mr Anil Agarwal... 

"as legal hier if he inherit property the onus comes to pay dues"

You need not worry so long your mother is alive. You will be held responsible, if you inherit her property