The absorbed energy is converted to heat energy. The scenario you mention with the LED is very close to the blackbody problem known as the ultraviolet catastrophe.. There is a Wikipedia article about it here. What happens in the situation you described is that the light proof box gets hotter.
Diagnosing rust on geraniums can be a challenge for growers. Rust begins with yellow spots on the underside of the leaves and can rapidly progress with brown fungal spores called pustules in the center of each spot. The pustules are raised up off the leaf surface and feel rough to the touch.
She states that small pests, including aphids, whiteflies and spider mites, suck the sap from the leaves, which causes curling, yellowing and leaf distortion. Several bacterial and fungal diseases, such as verticillium wilt and symptoms caused by the mosaic virus, may also lead to the yellowing of cucumber plants. Carroll further explains that nitrogen deficiency not only causes the plant to turn yellow, but also leads to stunted growth.