Plants require a variety of mineral elements from the soil or from another medium in which they grow. If a plant's growth medium lacks an essential mineral element, the plant will display a characteristic set of symptoms before it dies. Nitrogen and iron are essential mineral elements for plants. Plants deficient in these elements are stunted in growth and show yellowing symptoms in their leaves. In the accompanying animation, we describe the yellowing symptoms of nitrogen and iron deficiencies.
Chlorophyll molecules are embedded in the thylakoid membranes of chloroplasts. Each chlorophyll molecule contains four nitrogen atoms. When nitrogen is no longer available to the plant, chlorophyll breaks down and is not remade. The mature leaves undergo yellowing, or chlorosis, as the green pigment disappears.
Note that as the mature leaves become yellow, the new leaves grow in green. The nitrogen that was in the mature leaves is transported through the vascular system and taken up by the developing leaves. These new leaves use the nitrogen to make a variety of molecules, including chlorophyll.
These two plants are healthy and green and have been grown on a medium containing iron, but the one on the right has just been transferred to an iron-deficient medium. Iron is an essential element. Although it is not a component of chlorophyll, it is required during the synthesis of chlorophyll molecules.
In the iron-deficient medium, mature leaves remain green. During their earlier development, they received iron, which they retain in their tissues. The iron that remains in the mature leaves is unable to be transported throughout the plant. Without iron, new leaves cannot make chlorophyll and, therefore, grow in yellow.
Nitrogen and iron are essential elements for the growth of a plant. The molecule chlorophyll contains nitrogen, and thus the plant requires nitrogen to produce this green pigment. Without nitrogen, the plant becomes yellow, or chlorotic, as the yellow pigments that are present in the plant become unmasked by the disappearing chlorophyll.
If a plant has been grown with adequate nitrogen, its leaves will grow in deep green. If such a plant is transferred to a nitrogen-deficient medium, as in the accompanying animation, the plant will begin to display deficiency symptoms. In this case, the nitrogen in the mature leaves is transported out of these leaves and donated to the developing leaves of the plant. These new leaves grow in green, but the older leaves, which donated the nitrogen, undergo chlorosis.
In the iron deficiency, there is a different symptom. Iron remains sequestered in the mature leaves and is not transported to new leaves as they develop. Therefore, if a plant is transferred to an iron-deficient medium, its mature leaves remain green, but its new leaves grow in yellow. Although iron is not a component of the chlorophyll molecule, it is required in the production of chlorophyll. Therefore, without iron, the new plant tissue cannot make chlorophyll and undergoes chlorosis.
As depicted in the iron and nitrogen deficiencies, a plant's symptoms depend on how the plant normally uses an essential element, as well as whether the plant can transfer that element through its vascular system from mature tissue to developing tissue.