Question

Which does not show HSK pathway
A. Maize
B. Jowar
C. Sugarcane
D. Sunflower

Answer 1

HSK pathway refers to the Hatch and Slack pathway of photosynthesis. It is one of the adaptations present in the plants of hot and dry climate. The pathway occurs in the photosynthetic mesophyll cells and the closely associated bundle sheath cells.

Complete answer:
Photosynthesis is the process by which green plants fix atmospheric carbon dioxide into organic compounds. It includes two phases: the light-dependent phase and the light-independent phase.
The light-independent phase of photosynthesis uses ATP and NADPH formed during the light-dependent phase and converts carbon dioxide into glucose by the pathway called the Calvin cycle. The raw material for the Calvin cycle, carbon dioxide, is obtained by the opening of the stomata.
However, plants found in hot and dry regions close the stomata during the daytime to reduce the water loss through transpiration. These plants exhibit a specific mechanism called the Hatch and Slack pathway (${C_4}$ pathway) to ensure the continued supply of carbon dioxide for the Calvin cycle.
The HSK pathway includes the reaction of carbon dioxide with three-carbon compound phosphoenolpyruvate (PEP) to form oxaloacetate (a compound with 4-carbon atoms). The reaction is catalyzed by the enzyme PEP carboxylase and occurs in the mesophyll cells. It is followed by the conversion of oxaloacetate into another four-carbon compound, usually malate. The malate is transported to the chloroplasts of the bundle sheath cells where it is decarboxylated into pyruvate (a 3-carbon compound) and $C{O_2}$. The released $C{O_2}$ enters into the Calvin cycle while the pyruvate is transported back to the mesophyll cells where it is converted into PEP to complete the cycle.
HSK is exhibited by food crops such as maize, jowar, sugarcane, and grasses while sunflower is a ${C_3}$ plant and does not exhibit HSK.

So, the answer is option D. Sunflower.

Note: Plants exhibiting HSK pathway have special anatomy of leaves. The mesophyll cells are closely associated with the well developed and chloroplast containing bundle sheath cells. This specific arrangement of two different photosynthesizing cell types is called Kranz anatomy. Mesophyll cells have PEP carboxylase while bundle sheath cells have RuBisCO to fix the carbon dioxide. PEP carboxylase has a much higher affinity for carbon dioxide than RuBisCO.