# DBT JRF 2020 — Question 150

Exam: DBT JRF 2020 · Section: B · Topic: Plant Biology · Difficulty: Medium

## Question

Deficiency of this macronutrient causes older leaves to turn dark green or reddish purple

## Options

- **A.** calcium
- **B.** magnesium
- **C.** phosphorous
- **D.** nitrogen

## Answer

Option C

## Explanation

Phosphorus (P) is a mobile macronutrient in plants — it is readily remobilised from older (source) leaves to younger, metabolically active (sink) tissues during deficiency. Therefore, phosphorus deficiency symptoms appear FIRST and most severely in OLDER LEAVES. Characteristic P-deficiency symptoms: (1) Purple/reddish coloration — accumulation of anthocyanins (flavonoid pigments, synthesised from phenylalanine); relative N excess relative to P leads to excess carbohydrates and anthocyanin synthesis; (2) Dark green colour — due to relative retention of chlorophyll and increased chloroplast number per cell compensating for reduced growth; (3) Stunted root growth; (4) Delayed maturity. Compare: Nitrogen deficiency — older leaves turn pale yellow (chlorosis, N is mobile); Calcium deficiency — young leaves affected (Ca is IMMOBILE); Magnesium deficiency — interveinal chlorosis in older leaves (Mg is mobile, cofactor of chlorophyll).

Source: https://mypyqbuddy.com/questions/deficiency-of-this-macronutrient-causes-older-leaves-to-6a05a8a02adafec4702932ae