The gemara says that if we assume: 1. kinyan of peiros doesn't qualify as kinyan haguf. 2. we don't hold of breira (at least on a d'oraysa). 3. brothers who split are "buyers" rather than inheritors, it would be impossible for one to bring bikurim and read the parsha (which includes the declaration "the land that you have given to me") except for an only son who has an only son e.t.c. all the back to yehoshua bin nun. The rationale is that any yerusha which requires brothers to split are considered to be receiving a portion of their brothers share, and it is not entirely theirs. Tosafos (47b d.h. tevel) asks, even if we do not accept breira as a determination that each one received their full portion, nonetheless each one still has some portion of the land and therefore have some portion of the fruits they receive, so they should be able to make the declaration "the land that you have given to me" (although Tosafos implies that the question is in regard to the bringing of the bikurim rather than the reading of the passage, the maharam shif explains that the only issue that exists is the ability to read the passage)? Based on this Tosafos concludes that we don't consider each person to necessarily have any portion in the fruits they are bringing, rather it is possible that the entire portion that reuven received technically belongs to shimon his brother.
The Maharam Shif answers for Rashi - It is true that in each and every kernel of wheat both reuven and shimon have a portion. The problem is that according to the approach of ein breira, since when yovel arrives the entire field goes back to the "pot" and has to be redivided, it is not sufficient that the fruit grew in a portion that was partially reuven's. Reuven cannot declare "the land that you have given me" unless he would have the land forever, but since he has to return it when yovel arrives, the land doesn't qualify as his.