Chlorosis - Yellow Plant Leaves May Be Heat Damaged

Exceptionally high temperatures or drought are a few of the unfavorable conditions that have increased the amount of yellowing that we could see in plants. Chlorophyll is the green pigment in plants that functions in trapping sunlight so plants are able to produce food and grow. The pigment is constantly being produced and destroyed with the plant leaves. Under stress, it may be broken down faster than it can be produced, or under some conditions, a plant may not be able to produce it. Since the production of chlorophyll is so very sensitive, many conditions can lead to chlorosis.

Almost everyone has seen plants or seeds, grown in the dark, with pale yellow leaves. When light is adequate, however, either high temperature, low moisture, oxygen starvation, low nitrogen, or mineral element imbalances can also lead to chlorosis. Although plants vary greatly in their response, chlorophyll is produced best between 60 and 85 degrees. Temperatures in the upper 90's and 100's may lead to pal green color and chlorosis, particularly on new growth.

Very bright sunlight can also lead to chlorosis. This is especially noticeable on shadeloving plants placed in full sun. Often the upper or outer leaves show the most drastic yellowing white those under other leaves may maintain a better green color. Magnesium or iron problems may also lead to chlorosis. Magnesium deficiency appears as a mottle chlorosis on the older leaves of many plants. It may be corrected by the use of dolomitic limestone or the addition of magnesium sulfate (also known as epsom salts).

The most frequent mineral element cause of chlorosis is inadequate iron in a plant. Iron does not readily move through the plant once it has been utilized. Therefore, old leaves may remain green while young leaves develop a chlorotic iron deficiency pattern that appears as a yellow leaf with green veins. Although iron chlorosis may occur on almost any green plant, some are more likely to exhibit the problem than others. Pin oak, poplar, spirea, rose, azalea, rhododendron, apple, and cherry are a few common plants that may quickly develop the problem when the conditions are right.

While iron chlorosis may be due to an actual deficiency of iron in the soil, most often it is the result of unavailability of the iron already there. This may result from root damage, overwatering, excess drying, disease, or excess root damage during transplanting. Persistent chlorosis may be caused by unavailability of iron to plants that require acid soil, but are planted in alkaline soil. While a quick temporary green can be produced by treating the leaves, the most lasting result is given by soil treatment with iron sulfate to not only supply iron but make the soil more acid in alkaline situations.

Iron chelates may be used if a pH change is not needed. When applying iron sulfate to trees and shrubs do not apply closer than one foot from the base of the plants. Add about one pound of iron sulfate per gallon of water. Dig a trench four to six inches deep around the plant, and pour about a gallon of solution into it. For small plants or during active growth, it is best to apply only about half this amount at one time.

http://en.wikipedia.org/wiki/Chlorosis