whitish or yellowish leaf discoloration caused by lack of chlorophyll. Often caused by nutrient deficiency.

Merriam-Webster Online Dictionary
chlorosis (noun)
an iron-deficiency anemia especially of adolescent girls that may impart a greenish tint to the skin - called also greensickness
a diseased condition in green plants marked by yellowing or blanching
chlorosis (Wikipedia)
A corn plant with severe chlorosis (left) beside a normal plant (right)

In botany, chlorosis is a condition in which leaves produce insufficient chlorophyll. As chlorophyll is responsible for the green color of leaves, chlorotic leaves are pale, yellow, or yellow-white. The affected plant has little or no ability to manufacture carbohydrates through photosynthesis and may die unless the cause of its chlorophyll insufficiency is treated, although some chlorotic plants, such as the albino Arabidopsis thaliana mutant ppi2, are viable if supplied with exogenous sucrose.

Chlorosis is derived from the Greek khloros meaning 'greenish-yellow', 'pale green', 'pale', 'pallid', or 'fresh'.

In viticulture, the most common symptom of poor nutrition in grapevines is the yellowing of grape leaves caused by chlorosis and the subsequent loss of chlorophyll. This is often seen in vineyard soils that are high in limestone such as the Italian wine region of Barolo in the Piedmont, the Spanish wine region of Rioja and the French wine regions of Champagne and Burgundy. In these soils the grapevine often struggles to pull sufficient levels of iron which is a needed component in the production of chlorophyll.

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