Caress means to touch something gently in a peasant way. So the sun is not doing any harm to the plants, when the rays reach them they only get warm, nothing else happens. That creates the peaceful tone.
In order to create a negative tone the sun needs to harm the plants. The way to do it is by replacing <em>caressed </em>with a negative verb. An option could be the word <em>blaze</em>, which means to "burn fiercely or brightly".
The result would be:
The sun's rays <u>blazed </u>the plants that were drooping from the heavy rains
B, caressed. It's a very peaceful, gentle, calming word. "The sun's rays dried the plants that were drooping from the heavy rains." is a way you could rewrite it by only changing caress.
