Turkey is in negotiations with Russia and Ukraine to open a corridor via the Bosphorus for grain exports from Ukraine, a senior Turkish official told Reuters on Thursday. Ukraine’s Black Sea ports have been blocked since Russia invaded in February and more than 20 million tonnes of grain are stuck in silos there. Russia and Ukraine account for nearly a third of global wheat supplies and the lack of exports from Ukraine is contributing to a growing global food crisis. “Turkey is negotiating with both Russia and Ukraine for the export of grains from Ukraine,” the official said, requesting anonym…