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

The appearance of deficiency symptoms in younger parts of the plant versus mature organs can be attributed to several factors related to nutrient mobility, allocation, and plant physiology. Here are some reasons why this difference occurs:

1. Nutrient Mobility: Some nutrients are mobile within the plant, meaning they can be remobilized from older tissues to younger, actively growing tissues when there is a deficiency. For example, nitrogen (N), potassium (K), and magnesium (Mg) are mobile nutrients. When these nutrients become limited, the plant prioritizes their allocation to younger tissues, resulting in deficiency symptoms appearing first in older, mature organs.

2. Nutrient Immobility: Conversely, certain nutrients are immobile within the plant, meaning they cannot be easily translocated from older to younger tissues. Examples of immobile nutrients include calcium (Ca), boron (B), and iron (Fe). When these nutrients are deficient, the plant cannot redistribute them efficiently, leading to deficiency symptoms appearing first in younger, actively growing tissues.

3. Nutrient Uptake and Allocation: The uptake and allocation of nutrients within the plant can vary depending on factors such as nutrient availability, plant species, and developmental stage. In some plants, nutrients may be preferentially allocated to younger tissues to support growth and development, resulting in deficiency symptoms appearing first in mature organs. In other plants, nutrients may be primarily allocated to mature organs for storage or structural support, leading to deficiency symptoms appearing first in younger tissues.

4. Physiological Differences: Variations in plant physiology, including nutrient uptake mechanisms, nutrient storage capacity, and metabolic processes, can also influence the pattern of deficiency symptom development. Different plant species or cultivars may exhibit unique physiological responses to nutrient deficiencies, resulting in variability in the timing and location of symptom appearance.

5. Environmental Factors: Environmental conditions, such as soil pH, soil moisture, temperature, and light intensity, can affect nutrient availability and uptake by plant roots. Changes in environmental conditions may influence nutrient uptake rates and nutrient distribution within the plant, impacting the expression of deficiency symptoms in different plant parts.

Overall, the appearance of deficiency symptoms in younger or mature plant parts is influenced by a combination of factors related to nutrient mobility, allocation, plant physiology, and environmental conditions. Understanding these factors can help growers diagnose and address nutrient deficiencies effectively to support healthy plant growth and development.