Sunburn is not something we associate with plants, but it's actually quite common among them. It's because some trees have very thin bark, which dessicates in the heat and causes stress to the tree. In severe cases, sun damage can be fatal for plants.
Think of an oak tree with its deep, ridged bark—there is no way that UV rays can penetrate it. Now think of a citrus tree—the bark is paper thin. Many trees can suffer from sunburn, but citrus, with bark that is about one millimeter thick, is arguably the most susceptible. Just like us, citrus 'skin' needs protection. Fortunately, citrus sunscreen is easy to find and easy to apply—ordinary white latex paint works perfectly.
When Paint Is Needed
If left to their own devices, citrus plants grow as a shrubby thicket where the foliage naturally shades the bark (the reason they get by in nature without sun protection). However, most nurseries offer citrus that has been trained to a single trunk with the lower branches removed, a form that most gardeners preserve with annual pruning. If you have a shrubby citrus with no exposed limbs, there is no need to paint it. But any bark that is exposed to the direct rays of the sun for more than a few hours each day is better off painted.
Lemons are the most susceptible type of citrus, but all types of citrus are susceptible when young or recently pruned. Climate also makes a difference—in desert regions, it's almost guaranteed that your citrus will end up sunburned without protection, but in cool coastal areas you can generally get by without painting the trunk.
How to Do It
Painting citrus bark is easy. Special tree paint is available, but any flat white latex paint will work. Do not use enamel or oil-based paints as they will harm the tree. The quantity of paint and the size of your brush will depend on the size of the branches you're painting, but a quart can of paint and a 2-inch brush will do for most trees.
Dilute the paint at a 1:1 ratio with water and apply it over all the exposed bark. Make sure to paint all the way to the base of the trunk, but don't worry about painting up into the canopy where the bark is green—it will have enough shade from the leaves. If the paint looks translucent after the first coat, apply another to provide sufficient protection.
White paint isn't the only way to protect citrus bark. You can also wrap it with burlap, shade cloth, or one of the commercially-produced tree wraps that are available. White is the traditional color, but other light tones are also effective. Just avoid black, navy blue, and other dark colors that absorb the heat of the sun rather than reflect it. Citrus trunks painted pink or sky blue will certainly make a statement in the garden, but many gardeners choose to use something earth-toned that matches the natural color of the bark and blends in with the surroundings.