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.
However, manganese is less mobile in a plant than magnesium so that the symptoms of deficiency appear first on young leaves. It is always best to get a sample to determine the exact cause of the symptoms. Other problems such as iron deficiency, nematodes, and herbicide injury may also cause leaves to yellow.y Susan Patterson, Master Gardener. The role of manganese in plants is important for healthy growth. Keep reading to learn more about how to fix manganese deficiencies to ensure the continual health of your plants.