Here's why:

* Chlorophyll: Chlorophyll is the green pigment found in plants that is essential for photosynthesis.

* Chlorosis: When a plant experiences chlorosis, it means it is not producing enough chlorophyll. This can be due to various factors like nutrient deficiencies (especially iron, magnesium, or nitrogen), poor drainage, root damage, or diseases.

* Yellowing: Without enough chlorophyll, the green color of the leaves fades, giving them a yellowish appearance.

So, the answer is yellowing.