For years, the ability to grow and eat his own gooseberries has symbolized the lifestyle Nikolai dreamed of, so the fact that he now has them represents his achievement of that dream. The fact that Nikolai finds the berries sweet, while Ivan finds them “tough and sour” thus represents the brothers’ different opinions about Nikolai’s success: while Nikolai seems happy and fulfilled on the surface, Ivan thinks that his brother is only deluding himself into happiness, just as he’s deluding himself into thinking that the berries are sweet. Nikolai was, after all, only able to purchase Himalayskoe by depriving himself and his late wife for years. And now that he has the estate, the land isn’t what he imagined (it’s covered in brush and backs up against a polluted river), and Nikolai has become lazy, pompous, and cruel. For those reasons, Ivan thinks that Nikolai’s success is tainted by the means he used to achieve it, as well as the kind of person that success has turned him into. But it’s impossible for readers to know what the gooseberries
actually taste like, so it’s unclear which of the brothers is really deluding themselves: Nikolai does seem genuinely happy, so it’s possible that Ivan is the one imagining the “tough and sour” taste out of resentment.