3% glutaraldehyde  in phosphate buffer PH  7.3 is suitable fixative

First of all:

We  (i.e. your external colleagues and eventually friendly helpers....) shall GUESS as long as 26 days and more

whether you need your fixation of potato tissue for

- histological (LM)

- Immunohistochemical (classical enzyme histochem, Immuno, IHC, IF-Microscopy , etc.)

- ultrastructural (EM : SEM, TEM, AFM etc.),

- ??, & & 

examination - morphological - contents evaluation or (nutritional / food) analysis

If you are so kind to inform in this thread about YOUR PROJECT and Methods (to be) used MORE IN DETAIL the chance of  more effective help certainly will be / is increasing. 

I am certain about the fact that there is not ONE preferable fixative wihout knowledge of real task you are working on. Naturally it could be that someone out in the wild (not only eating them...but if that person will read your request is quite doubtful.

Additionally I would propose to read at least : 

https://www.researchgate.net/post/What_is_the_best_chemical_fixative_for_preparing_biological_samples_for_electron_microscopy  and perhaps you'll find similatr questions and answers in RG by using its "Search Function" [SEARCH -pull down menu:  "SEARCH-QUESTIONS", insert e.g.  | fixative biological sample preparation electron microscopy | (resulting in the RG-internal URL: https://www.researchgate.net/search.Search.html?type=question&query=fixative+biological+sample+preparation+electron+microscopy ) where you might find some valuable details from some 100 questions and answers. 

With simple search keywords (e. g.in Google or other search machines, only as ONE example: | potato AND FIXATIVE AND plants |  ) you might find that not much pops up regarding the preferable fixation/fixative for potatoes, but perhaps you'll find at least one hint by reading: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3853006/  = MJ Talbot -  RG White ‎2013, Methanol fixation of plant tissue for Scanning Electron Microscopy improves preservation of tissue morphology and dimensions (interesting detail on shrinkage of potato after methanol vs. glutaraldehyde.....: CITATION:  "In fact, Boyde and Boyde (1980) showed that unfixed potato tuber tissue processed through dehydration and CPD shrank less (15%) than glutaraldehyde fixed tissue (30%; [2])." END of CITATION ).

Also in YY Yu  et al,- ‎2007, cf: http://www.public.iastate.edu/~djh/pdfs/Yu_et_al_2007.pdf : CITATION: "For optimum tissue integrity and quality of RNA from potato stem sections, the best results were achieved using ethanol acetate as the Fixative. "

It is possible that you might find also "oldfashioned" (not OUTDATED) data on fixation and starch treatment (but unfortunately not removal) reading: 

F Olga FLINT- ‎1982, Title: LIGHT MICROSCOPY PREPARATION TECHNIQUES FOR STARCH AND LIPID CONTAINING SNACK FOODS   

(@ digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1037&context=foodmicrostructure ....  

[study using cryosections]  - at least mentioning also Osmium tetroxide vapor as a fixative for potatoes.

Regarding the SECOND POINT OF YOUR QUESTION: removal of starch (non enzymatic) I admit not to have any practical experience....but I can point you to a Google search:

search phrase(s):  

 | Removal of starch |     or also    

| how to remove starch from potatoes |    or perhaps also:  

| how to remove starch from potatoes AND NON-enzymatic  |  .  

It might be that you'll find a solution for your problem out of at least 407,000 results (:-))

Dear Fatemeh Babaei, after your kind personal message via RG I understand now better. If my "extempore" really / in fact was helpful I shall be glad and again should state that there is no "general fixative" for most of the problems in Natural or Life Sciences we have to solve.. (:-)). But, hopefuly, you'd be willing to state some more detail...? Best wishes and regards, Wolfgang