What causes a normally green plant to turn yellow? There is only one universally correct answer to this question...it has lost its chlorophyll. The yellow pigment is always there but the green of the chlorophyll molecule dominates and masks the yellow. Once the chlorophyll is gone, the yellow expresses itself.
Now, the next logical question is, "Why did the plant lose its chlorophyll?" That is not always such an easy question to answer but, again, it has to do with something missing from the requirements of the plant to maintain its chlorophyll. Some of the major suspects would include:
1. Nutrient Deficiency - Like everything in the plant, chlorophyll is made up of certain elements. Nitrogen is one of the keys and, if the soil is deficient in nitrogen, one of the first signs is a yellowing of the leaves. So, sometimes, all it takes is a little fertilizer or some form of nutrient that contains N to green up the plant.
2. Light - In order to maintain the chlorophyll molecule, the plant needs to be exposed to a certain intensity and duration of light. Why do tree leaves turn yellow in the fall? No, it is not because of the cooler temperatures but rather, because of the shortening of the days. Once the length of daylight dips below the minimum needed to maintain the chlorophyll, the leaves begin to yellow.
3. Temperature - Cool temperatures may slow down the transpiration from the leaves and thus slows down the movement of water through the plant. When this happens, the nutrients needed to build chlorophyll are not present in the leaf and they may revert to yellow.
4. Water - Too much or too little water in the soil around the roots will also cause the slow down of the movement of water and nutrients through the plant. Again, the chlorophyll is not replenished and the yellowing occurs.
5. Insects - Sucking insects such as aphids, leaf hoppers and thrips as well as spider mites draw sap from the plant tissue. Included in the juices they remove are chlorophyll molecules. This results in the typical "mottled" appearance of the leaves caused by streaking patterns of green chlorophyll and yellow areas on the leaf.
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