Question

A plant in your garden avoids photorespiratory losses, has improved water use efficiency, shows a high rate of photosynthesis at high temperature and has improved efficiency of nitrogen utilization. In which of the following physiological groups would you assign this plant?
A) $C_3$
B) $C_4$
C) CAM
D) None of the above

Answer 1

Hint: Some plants may avoid the worst consequences of photorespiration, unlike wheat and soybeans. The $C_4$ and CAM pathways are two mechanisms that enable certain organisms to reduce photorespiration, advantageous features resulting from natural selection.

Complete answer:
Photorespiration is an inefficient pathway arising when RubisCo, the Calvin cycle enzyme, works on oxygen instead of carbon dioxide. RubisCo, the carbon-fixing enzyme of the Calvin cycle, continues this inefficient metabolic pathway with oxygen instead of carbon dioxide. It uses fixed fuel, consumes energy, and appears to occur in order to minimize water loss when trees close their stomata (leaf pores). It is made even worse by high temperatures.

Now, let us find the solution from the options-
- A "natural" plant is called a $C_3$ plant, one that does not have photosynthetic adaptations to suppress photorespiration. The first phase of the Calvin cycle is the carbon dioxide fixation by RubisCo, and for the three-carbon compound (3-PGA) that the reaction generates, plants which use only this 'ordinary' carbon fixation process are named $C_3$ plants. $C_3$ plants, like soyabean, wheat, rice and all plants, comprise about 85 percent of the planet's plant species. Thus, option A is not the correct option.

- This plant is a plant of $C_4$. As they have a specific type of leaf anatomy called Kranz Anatomy, $C_4$ plants are specific. They can withstand higher temperatures, demonstrate a response to high temperatures, skip a wasteful photorespiration mechanism, and have increased nitrogen utilization performance. The $C_4$ pathway has several of the plants that are suited to dry tropical regions and are known as $C_4$ plants. Double fixation of carbon dioxide happens in these plants. The light-dependent reactions and the Calvin cycle are actually segregated in $C_4$ plants, with the light-dependent reactions happening in the mesophyll cells and the Calvin cycle arising in special cells near the leaf veins. These cells are named as bundle-sheath cells.

- CAM plants isolate these processes in time rather than splitting the light-dependent reactions but the use of carbon dioxide throughout the Calvin cycle in time. CAM plants expand their stomata at night, enabling the absorption of carbon dioxide into the leaves. PEP carboxylase fixes this carbon dioxide into oxaloacetate and then converts it into malate or another form of organic acid. Thus, option C is not the correct option.

Thus, the correct answer is option (B) $C_4$.

Note: The Calvin cycle is used by $C_3$, $C_4$ and CAM plants to produce sugar from carbon dioxide. Such carbon dioxide fixation pathways have various benefits and drawbacks and make plants suitable for various environments. In cool conditions, the $C_3$ system functions well, while $C_4$ and CAM plants are suited to humid, dry regions.