Answer: This can be caused by a variety of reasons. Often it is due to watering problems. It might be due to overwatering if it is at the bottom portion of the plant. It might be due to underwatering, if at the top of the plant. The soil should be evenly moist like a wrung out sponge(not saturated) and never dried out. As the plants grow they will need proportionately more water because they are getting bigger and use more of it, they may also need repotting to provide ample root space and enough moisture. They may be too warm. Most seedlings grow on well at a temperature in the mid sixties or so. If your room is warmer than 70 degrees or so this may be causing them to dry out quickly, this can also happen if they are in a draft from a heating source or in a very dry room such as one heated by a wood stove. Touching the bulbs can cause the foliage to burn, keep them a few inches below the fluorescent lights. Overfertilizing can cause foliage symptoms as well. Seedlings are sensitive to this. If you fertilize, use a water soluble product at a very weak strength and apply it only to the soil, not to the foliage. Although they should not be in a draft, they do need some air circulation. Sometimes you will see a fungal infection if they are overly damp and the air is stagnant, or if they are too crowded. Lastly if the leaves that are drying are the original or seed leaves that formed first before the true leaves appeared, it may be normal that they are fading once they are replaced. I hope this helps you troubleshoot.
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