Chlorosis

In botany, chlorosis is a condition where leaves do not produce enough chlorophyll. Because chlorophyll gives leaves their green color, chlorotic leaves appear pale, yellow, or yellow-white. When a plant cannot make enough chlorophyll, it struggles to create carbohydrates through photosynthesis, which can eventually harm or kill the plant if not treated. Some chlorotic plants, like a special mutant of Arabidopsis thaliana, can survive if given extra sucrose from outside sources called exogenous.

The term chlorosis comes from the Greek word khloros, meaning "greenish-yellow" or "pale green."

In viticulture, chlorosis often shows up in grapevines as yellowing leaves, especially in certain soils. This is common in places with lots of limestone, such as the Italian wine area of Barolo in Piedmont, the Spanish wine region of Rioja, and the French wine areas of Champagne and Burgundy. In these soils, grapevines sometimes cannot absorb enough iron, an important part of making chlorophyll.

Causes

Chlorosis happens when leaves can't make enough chlorophyll, often because they lack important nutrients like iron, magnesium, or zinc. This can also occur if the soil is too acidic or basic, making minerals hard for plants to absorb, or if the soil is waterlogged and damages the roots.

Some other causes include certain pesticides or herbicides, exposure to harmful gases like sulphur dioxide, and infections from bacteria or fungi. Different plants react differently; for example, azaleas need acidic soil, while rice can handle waterlogged conditions.

In grape vines

Grape vines can suffer from chlorosis, especially when grown in soil rich in limestone. This problem became more noticeable after The Great French Wine Blight, when European grapevines were grafted onto American rootstocks to resist Phylloxera. However, these rootstocks sometimes struggled in lime-rich soils, leading to iron deficiency in the vines. This specific type of chlorosis in France was called chlorose calcaire. The issue was mostly solved by choosing lime-resistant American vines for hybrid rootstocks. Still, growers must balance chlorosis resistance with other important needs for the best wine production. One common rootstock, 41 B, is a mix of the Chasselas grapevine and Vitis berlandieri, offering good but not perfect resistance to both chlorosis and Phylloxera.

Main article: The Great French Wine Blight

Treatments

If a plant's leaves turn pale due to missing nutrients, we can help it by giving it extra food. Adding iron, magnesium, or nitrogen to the soil can fix these problems. Sometimes, the soil might be too basic or too acidic, so we can use lime to make it less acidic or sulfur to make it less basic, which helps the plant grow better.