Answer:
a. they can fix carbon at the lower CO2 concentrations that develop when the stomata are closed
Explanation:
C4 plants are those that have the capacity to fix CO2 even when a tiny concentration of it is available.
Desert condition is characterized by dryness and strong heat with both condition capable of creating water stress in plant as a result of evapotranspiration. In order to reduce evapotranspiration rate, desert plants (most C4 plants) close their stomata during the day.
Stomata closure limit the diffusion of CO2 into desert plants and the small concentration of CO2 that results is utilized by a special enzyme in the plants.
The correct option is a.
Answer:
They can fix carbon at the lower CO2 concentrations that develop when the stomata are closed.
Explanation:
In order to conserve  the substrate Ribulose biphosphate(RuBP)  which  must react with CO2,, and to avoid  wasteful photorespiration reaction  of RuBP  with oxygen. which are  both  catalysed by enzyme Rubisco; C4 plants formed a 4C-compounds -oxaloacatate . While keeping RuBP and enzyme Rubisco  in the bundle sheath away from oxygen.
The C02 is rather absorbed  by mesopyll cell in contact with air and  combines with 3C-Phosphoenopyruvate, a reaction catalysed by Phosphoenocarboxylase, to form 4C-oxaloacetate.
The latter is converted to 4C -Malate, which is shunted from the mesophyll into the bundle shealth where Rubisco and RuBP are concentrated.
Therefore when the stomata is closed,C4 plants can  fixed Carbon as Oxaloacetate and  Malate,  in the bundle sheath,despite the closed Stomata, because of the anatomy which separated the externally contact mesophyll cells from the airtight bundle sheath cells.
