Sunseed Nurturer

The first ability has an “intervening ‘if’ clause.” That means (1) the ability won’t trigger at all unless you control a creature with power 5 or greater as your end step begins, and (2) the ability will do nothing if you don’t control a creature with power 5 or greater by the time it resolves. (It doesn’t have to be the same creature as the one that allowed the ability to trigger.) Power-boosting effects that last “until end of turn” will still be in effect when this kind of ability triggers and resolves. An ability like this will trigger a maximum of once per turn, no matter how many applicable creatures you control.