Deficiency of this macronutrient causes older leaves to turn dark green or reddish purple
Deficiency of this macronutrient causes older leaves to turn dark green or reddish purple
Select an option to attempt
Correct 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).