57. In which molecule would the radiolabel appear the earliest when wheat and sugarcane leaves are fed with ¹⁴CO₂?
(1) Wheat : malate, sugarcane : 3 phosphoglycerate.
(2) Wheat : aspartate, sugarcane : malate.
(3)  Wheat : 3 phosohoglycerate, sugarcane : 3 phosphoglycerate.
(4) Wheat : 3-phosphoglycerate, sugarcane : malate.

Introduction

When feeding plants with radioactive CO₂ (^14CO₂), the earliest site of radiolabel incorporation differs between C3 plants like wheat and C4 plants like sugarcane due to their distinctive photosynthetic pathways. This difference is crucial for understanding carbon fixation and photorespiration in plants.

Explanation of options and correct answer

(1) Wheat: malate, sugarcane: 3-phosphoglycerate

• Incorrect. Wheat is a C3 plant whose first stable product is 3-phosphoglycerate (3-PGA), not malate. Malate is a C4 intermediate, not prominent in wheat.

• In sugarcane, a C4 plant, malate is indeed the first stable product, but 3-PGA does not appear earliest.

(2) Wheat: aspartate, sugarcane: malate

• Incorrect. Aspartate can be a transport molecule in some C4 plants (especially NADP-ME type), but wheat, a C3 plant, does not produce aspartate as a first product.

• Sugarcane’s earliest labeled product is malate, but wheat’s earliest is not aspartate.

(3) Wheat: 3-phosphoglycerate, sugarcane: 3-phosphoglycerate

• Incorrect. Both plants do not produce 3-PGA first because sugarcane (C4) fixes CO₂ initially into four-carbon organic acids like malate, not 3-PGA.

• This option ignores the fundamental difference between C3 and C4 carbon fixation pathways.

(4) Wheat: 3-phosphoglycerate, sugarcane: malate

• Correct. Wheat, a C3 plant, fixes CO₂ directly via the Calvin cycle producing 3-phosphoglycerate first.

• Sugarcane, a C4 plant, initially fixes CO₂ into oxaloacetate and subsequently malate in mesophyll cells before transporting to bundle sheath cells for the Calvin cycle.

• Therefore, radiolabel appears earliest in 3-PGA in wheat and malate in sugarcane.

This conforms with classic photosynthetic physiology and radiolabeling experiments demonstrating the distinct carbon fixation products and timing between C3 and C4 plants.

Hence, option (4) is correct.