What is a primary sign of magnesium deficiency in plants?

The correct answer identifies a classic symptom associated with magnesium deficiency in plants, which is the yellowing of the lower leaves while the veins remain green—a condition known as interveinal chlorosis. Magnesium plays a critical role in photosynthesis as it is a central component of chlorophyll. When there is insufficient magnesium, chlorophyll production is impaired, leading to the yellowing effect.

This symptom typically starts in older leaves because magnesium is mobile within the plant; it will be reallocated from older leaves to support new growth if deficiencies arise. Thus, the lower leaves show this discoloration first, while the upper leaves might still appear healthy.

Other symptoms mentioned in the choices, such as stunted growth in new leaves or irregular necrotic lesions, are associated with deficiencies of other nutrients, indicating that while those symptoms can signal various deficiencies or issues, they are not specific to magnesium like the interveinal chlorosis seen in the correct response. Yellow spots may also arise from different stress factors unrelated to magnesium levels. Understanding these symptoms allows for better diagnosis and treatment of nutrient deficiencies in plants.