Q.The element which is not remobilized?

Calcium is an immobile essential element — once it is incorporated into plant structures (like the middle lamella of cell walls), it cannot be withdrawn and redistributed to younger tissues.

Mineral elements absorbed by plants can be broadly grouped as mobile or immobile, based on whether they can be remobilized (withdrawn from older/mature tissues and transported to actively growing regions) when demand rises or supply falls short. Nitrogen, phosphorus, and potassium are highly mobile — they are readily translocated from older leaves to young leaves, seeds, and other growing parts, which is why their deficiency symptoms appear first in older leaves. Calcium, in contrast, is deposited permanently as calcium pectate in the middle lamella of the cell wall and is also required for membrane stability; it cannot be remobilized once laid down. As a result, calcium deficiency symptoms appear first in the youngest, growing tissues (e.g., death of shoot and root tips), not the older leaves.

Mineral elements are classed as mobile or immobile based on whether the plant can withdraw them from older tissues and resupply them to younger, growing tissues; calcium is the classic immobile element because it becomes permanently incorporated into cell wall structures (calcium pectate) and membranes, so it cannot be remobilized.

Mobility of a mineral nutrient within the plant depends on whether it can be re-translocated (usually via the phloem) from mature/senescing tissues to actively growing regions once it is no longer immediately needed there:

1. Phosphorus — highly mobile; readily remobilized from older leaves to younger growing tissues and developing seeds.
2. Potassium — highly mobile; exists largely as a free ion and moves easily between tissues.
3. Calcium — immobile. Once incorporated into structures like the middle lamella (as calcium pectate) and cell membranes, or precipitated as calcium oxalate crystals, it cannot be withdrawn and redistributed. This is exactly why calcium deficiency symptoms always appear in NEW, actively growing tissues first (e.g. shoot tips, young leaves) — the plant cannot draw calcium out of older tissue to supply them.
4. Nitrogen — highly mobile; extensively remobilized from senescing leaves (broken down and transported as amino acids/amides) to developing organs.