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The second version is fine if you change "tomatoes growers" to "tomato growers." (Here, "tomato" is serving as an adjective modifying "growers," and in English the adjective does not need to agree in number with the noun it is modifying.) However, the original is a more standard way to express the idea. This is a good example of how almost every grammar rule has exceptions! There isn't really any way to misread the sentence as is--"who" can't refer to tomatoes--so there's no need to rearrange anything.

Besides, any time we have more than one modifier, some interpretation is needed. If we read the phrase "heirloom tomato growers," what is the intended meaning? Are we talking about "(heirloom tomato) growers"--people who grow heirloom tomatoes--or "heirloom (tomato growers)"--tomato growers who are heirlooms? Clearly the first version makes much more sense, so we'll go with that, but notice that we can avoid that ambiguity by saying "growers of heirloom tomatoes." That's why it's so important to use process of elimination in SC. Start by getting rid of the answers that absolutely cannot be correct, and then narrow down more carefully from the remaining choices. Sometimes you'll have to select an answer that seems to break a rule because it is the only one that fixes some other problem, or that conveys a clear meaning.