The 1880 census has a clue:
In Union County, TN, we find a Robert Buckner, age 9 (therefore b about 1871) enumerated as son of the head of household, one Sarah Buckner, 48, widowed, born in TN. Also living with them is another son of Sarah, William E., 18 and supposedly his wife Margaret J., 18.
Using the info about this older brother, we can go back to the 1870 census which is a bit more confusing; he's living with what is likely his mother (the 1870 census did not identify relationships), Sariah, 37, and some additional siblings-- Pauline, Cornelius and Emily Ann. No male head of household is listed there. The next family enumerated is another Buckner family: JJ Buckner, 20 [another son of Sarah's above?] with presumably a wife Sariah, also 20 and a one-month old son William (can't make out the middle initial).
Moving backwards, we find in the 1860 census what might be the same older Sarah Buckner, 29, supposed wife of Thomas J. Buckner, 31, with a number of additional children.
A 12 Aug 1850 - Grainger, TN marriage might be them-- Thomas J. Buckner and Sarah Braden.
This does not confirm that Thomas was father of your Robert-- in fact, it looks like Sarah had Robert long after Thomas was dead.