[Campbell Biology P.859] A plant grown in a nutrient-deficient medium begins to show chloros... | Practice Question

A plant grown in a nutrient-deficient medium begins to show chlorosis yellowing of leaves . If the chlorosis first appears in the older leaves, while the new growth remains green, which of the following minerals is most likely deficient and why?

  • A: Iron (Fe), because it is an immobile nutrient, and older tissues have adequate reserves.
  • B: Magnesium (Mg), because it is a mobile nutrient, and young tissues act as a sink, drawing Mg from older leaves.
  • C: Nitrogen (N), because it is required in large amounts, and its deficiency always affects older leaves first.
  • D: Boron (B), because it is a micronutrient, and deficiencies of micronutrients always manifest in older leaves.

Explanation

The text explains that if a nutrient is mobile within the plant, its deficiency symptoms will appear first in older organs because younger, actively growing tissues will draw the mobile nutrient from the older parts, acting as a 'sink'. Magnesium is explicitly given as an example of a mobile ion, and its deficiency is described as first showing signs of chlorosis in its older leaves.