Q6:

Why is that in certain plants deficiency symptoms appear first in younger parts of the plant while in others they do so in mature organs?

Step 1: Theoretical Framework of Nutrient Mobility

The manifestation of deficiency symptoms in plants is primarily governed by the mobility of mineral elements within the phloem. Elements are categorized based on their ability to be translocated from older, mature tissues to younger, developing tissues when external supply is insufficient.

Step 2: Mechanism of Symptoms in Older (Mature) Organs

When an element is highly mobile, the plant preferentially transports it from older, senescing leaves to the younger, actively growing apical buds and young leaves [Per the Sink-Source Relationship].

• Physiological Process: As a leaf matures, it becomes a source for nutrients. When a deficiency occurs, these mobile ions ($N, P, K, Mg$) are mobilized and exported via the phloem to satisfy the requirements of metabolic sinks (younger tissues).
• Consequence: The older leaves exhibit symptoms first, as they are being depleted of these essential structural and enzymatic components.
• Representative Elements: Nitrogen ($N$), Phosphorus ($P$), Potassium ($K$), and Magnesium ($Mg$).

Step 3: Mechanism of Symptoms in Younger (Apical) Organs

In contrast, certain minerals are structurally integrated into plant tissues and lack the biochemical capacity for re-translocation through the phloem. These are termed immobile elements.

• Physiological Process: Since these minerals cannot be moved from older tissues, the young, developing parts of the plant are entirely dependent on the continuous uptake of these minerals from the soil via the xylem.
• Consequence: If the uptake is interrupted or deficient, the actively dividing cells in younger parts—which have a high demand for these structural components—cannot form new tissues correctly, leading to immediate chlorosis or necrosis in these regions.
• Representative Elements: Calcium ($Ca$) and Sulfur ($S$).

Step 4: Comparative Data Analysis

Step 5: Illustrative Schematic of Nutrient Translocation

Final Solution: The disparity in the site of deficiency symptoms is determined by the mobility of the element within the plant. Mobile elements are translocated from older tissues to younger ones, causing symptoms to appear in mature organs. Conversely, immobile elements (such as Calcium and Sulfur) are structurally fixed, rendering the plant unable to redistribute them, thereby causing symptoms to manifest first in younger, actively growing parts.