Plant Physiology Biochemistry

(A) Presence of more number of stomata on the grass leaves.

• While grass leaves do have stomata, the presence of more stomata alone does not explain why grass leaves roll inward. Stomata are involved in gas exchange and transpiration, but the rolling mechanism is specifically due to bulliform cells, not just the number of stomata.
• This is not the correct explanation.

(B) Undifferentiated mesophyll in grass leaves.

• Mesophyll refers to the tissue between the upper and lower epidermis of a leaf, where photosynthesis occurs. Whether the mesophyll is differentiated or undifferentiated does not directly relate to the rolling of grass leaves.
• This is not relevant to the phenomenon observed.

(C) Presence of Bulliform cells in the grass leaves.

• Bulliform cells are specialized cells found in grass leaves. These cells are responsible for the rolling mechanism. When water is lost from these cells, they shrink, causing the leaf to curl inward. This is a well-documented adaptation in grasses to reduce water loss under stressful conditions like high temperatures or low soil moisture.
• This is the correct explanation.

(D) Due to higher rate of transpiration.

• While a higher rate of transpiration can contribute to water stress in grass, it does not directly explain why grass leaves roll inward. Transpiration is a general process affecting all plants, but the specific rolling mechanism is driven by bulliform cells in grass leaves.
• This is a contributing factor but not the primary reason for the rolling.

Thus, the correct option is (C): presence of Bulliform cells in the grass leaves.

C₃ plants are those that utilize the C₃ pathway (Calvin cycle) as their sole mechanism for carbon fixation during photosynthesis. This pathway occurs in the mesophyll cells.

Let's analyze the enzymes listed:

• ATP synthase: This enzyme is essential for producing ATP during photophosphorylation in the light-dependent reactions of photosynthesis, which occur in all photosynthetic plants, including C₃ plants. It is also found in mitochondria for ATP production during respiration. So, ATP synthase is found in C₃ plants.
• RUBP carboxylase (Ribulose-1,5-bisphosphate carboxylase/oxygenase, or RuBisCO): This is the key enzyme of the Calvin cycle (C₃ pathway). It catalyzes the initial fixation of atmospheric CO₂ by combining it with Ribulose-1,5-bisphosphate (RuBP). This enzyme is characteristic and highly abundant in C₃ plants. So, RuBP carboxylase is found in C₃ plants.
• NADP reductase (Ferredoxin-NADP⁺ reductase): This enzyme catalyzes the final step in the electron transport chain of the light-dependent reactions, reducing NADP⁺ to NADPH. NADPH is required for the reduction phase of the Calvin cycle. Therefore, NADP reductase is found in C₃ plants.
• PEP carboxylase (Phosphoenolpyruvate carboxylase): This enzyme catalyzes the reaction between phosphoenolpyruvate (PEP) and CO₂ (as bicarbonate) to form oxaloacetate. While C₃ plants do possess PEP carboxylase in their cytoplasm for various metabolic functions (like replenishing TCA cycle intermediates), it does not play the primary role in photosynthetic carbon fixation as it does in C₄ and CAM plants. In C₄ and CAM plants, PEP carboxylase is responsible for the initial fixation of atmospheric CO₂, often in different cells (C₄) or at different times (CAM) than RuBisCO activity. The primary photosynthetic carbon fixation in C₃ plants is solely mediated by RuBisCO.

Therefore, considering the context of primary photosynthetic carbon fixation pathways, PEP carboxylase is the enzyme listed that is characteristic of C₄/CAM pathways but not the primary photosynthetic carboxylase in C₃ plants.