How can I tell the difference between etiolation and a sunburn?

Question

I'm attempting to grow plants from seed during the winter, by using a desk lamp and a timer to simulate 12 hour days. Since I know sprouts require a lot of light, I placed the desk lamp such that the bulb is about 2 inches (5cm) away from the soil. The bulb is a 60W incandescent.

Unfortunately, there's something wrong with the one surviving sprout. The leaves are yellowed, with tinges of brown at the edges. The plant still engages in heliotropic behavior - that is, a few hours after moving the lamp, the leaves have moved to face the bulb. The plant is a Sweet Basil.

I've never tried this before, so it's hard to tell if a) the bulb is too hot too close, and is burning the plant or b) the bulb is too dim, and the plant cannot complete sufficient photosynthesis. A Google image search shows leaves with this coloration for both etiolated and sunburned plants.

How can I tell if the plant is etiolated or sunburned? Alternatively, in this specific case, is it burned or etiolated?

Answer 1

You can tell the difference by a closer examination of the plant. Etiolation causes stretched, thin, pale stems, and thin, pale leaves. Your seedling is only showing the chlorotic look in the areas that point directly at the light source, but the plant isn't showing signs of excess light, so I believe it's heat from the lamp that's burning the leaves.

Looks like the plant is getting scorched by heat from the bulb. This can happen even with fluorescent bulbs at close range. It is possible to burn a plant (especially a seedling) with the heat of a lightbulb, even if the light from that bulb is inadequate.

The solution is to use direct sunlight if possible, use higher wattage at a greater distance, or switch to a high wattage led grow light.

The plant doesn't look very etiolated, look at the leaf undersides, and the plant stem. In etiolation, they would be paler than the upper leaf surface, because of the lack of light. In this case, only the upper leaf surface is pale, because the cause is damage to the tissue, by excessive heat from the lamp. This will affect photosynthesis, because the damaged tissue will no longer function. This may set back your plant quite a bit.

Another thing - once the seedling uses up the stores from the seed leaves, it will rely on photosynthesis to provide energy. It cannot perform photosynthesis without some basic building blocks, which is why you fertilize. If your plant lasts more than a week with no apparent attempt at putting out some true leaves, I would begin a half-strength all-purpose fertilization with the watering, every seven days until it puts out some new leaves. Once it has three or more true leaves, I'd start fertilizing full-strength. Basil loves nitrogen.