Let's set up this whole passage as a formal argument. We can do this through logic, or key words like it follows that and therefore. I've put numbers on repeating ideas that we should watch for:

Given:
1. [Rain-soaked soil^5] contains [less oxygen^1] than does drier soil.
2. The roots of melon plants perform [less efficiently^2] under the [low-oxygen conditions^1] present in [rain soaked soil^5].
3. When the [efficiency of melon roots is impaired^2], the roots do not supply sufficient amounts of the proper nutrients for the plants to perform [photosynthesis at their usual levels^3].

Therefore:
1. Melon plants have a [lower-than-usual rate of photosynthesis^3] when their roots are in [rain-soaked soil^5].

And given that:
1. When the [photosynthesis of the plants slows^3], [sugar stored in the fruits^4] is drawn off to supply the plants with energy.

Therefore:
1. Ripe melons harvested after a prolonged period of [heavy rain^5] should be [less sweet^4] than other ripe melons.

This, of course, is overkill. Really we could just rely on key words, and assume the rest based on the order of the paragraph (first bold is in facts before "it follows that", second bold is after "it follows that". "It follows that" is before "therefore").

But, it's good practice.