-20 is NOT cold enough to store tissues for RNA.  You need to use -80*C. 

 thank you Ms Katie,  I mean in case if I store it under -20 say it far overnight. 

Thank you Mr Ali,  you answered the question. And now, can lysing by mortar and pestle  in presence of LN breaks the cell wall and membrane? 

I'm usually allocating 3-5 min lysing 50mg leaf samples 

 Thanks a lot, mr Ali, how can I have your relevant updated protocol ?

If you use freeze-dryer, your sample will be OK for RNA extraction up to 5 years in -20C. I did it and RNA quality was still fantastic for sequencing.

Wow Young Cho sir, I was wondering that I kept the samples at first under -80 ,and  while extraction I transferred them under -20 assuming that the -80 deactivated all RNAses and -20 will be enough for short period until I extract the rna. But, I noticed some reduction in rna quantity after that in my blueberry leaf disc samples, repeated the extraction 3 times but the quantity was lower again, than I came to believe that it is because of that -20 storage.

Thank you very much Mr Ali 

Thank you very much, Mr Ali, I fully got the answer.