In what sense does the sun conspire with autumn?

Autumn and the sun conspire together to produce the beauty in nature that is found in early autumn. This is the "mature" sun, which has spent two seasons already ripening the earth for a bountiful harvest. Through its work, vines become loaded with fruit. Trees bend under the weight of perfectly ripened apples. Fruit ripens to a decadent taste. Gourds, which could include pumpkins (often a symbolic representation of autumn itself), swell, and hazel nuts are sweetened. Bees are swept into believing that the warmth will never cease as autumn and the sun conspire together to produce blooms found only during this season.
The effect is a magnificent display of imagery that is resplendent in the colors and sights of autumn. This natural beauty is only possible, according to Keats, thanks to the way a mature sun works in conjunction with this season, the two being partners in the artwork of autumn.