Question

Which one is a $C_4$ plant?
A. Papaya
B. Pea
C. Potato
D. Maize

Answer 1

Hint: Two Australian botanists Hatch and Slack discovered that tropical plants are much more efficient in carbon-dioxide utilization. hence, the Hatch-Slack cycle was named. Hatch-Slack cycle also known as the $C_4$ pathway.

Complete answer:
Most of the plants that are adapted to dry tropical regions have the $C_4$ pathway. In $C_4$plants double fixation of carbon-dioxide occurs. The initial or the first product of this pathway is a four-carbon compound-Oxaloacetic acid(OAA) and hence the name. The $C_4$ pathway requires the presence of two types of cells i.e., mesophyll cells and bundle sheath cells. The particularly large cells around the vascular bundles of $C_4$ plants are called bundle sheath cells, these cells may form several layers around the vascular bundles, they are characterized by having a large number of chloroplasts, grana are absent, thick walls previous to gaseous exchange, and no intercellular spaces. This special anatomy of leaves of the $C_4$ plants is called ‘Kranz anatomy’. ‘Kranz’ means wreath and is a reflection of the arrangement of cells. From the above option Maize has a $C_4$ pathway.
So, the correct option is (D).

Note: Importance of $C_4$ plants
1. They can tolerate saline conditions due to the abundant occurrence of organic acids (malic and oxaloacetic acid) in them which lowers their water potential more than that of soil.
2. Can perform photosynthesis even when their stomata are closed due to the presence of strong carbon-dioxide fixing enzyme i.e. PEPcase.
3. The concentric arrangement of cells in the leaf produces smaller areas in relation to volume for better water utilization.