Final answer:
CAM photosynthesis' primary disadvantage compared to C3 and C4 photosynthesis is its slower growth rate due to nocturnal CO2 fixation, which is less efficient under normal light conditions.
Step-by-step explanation:
The major disadvantage of CAM photosynthesis relative to C3 or C4 photosynthesis is that it is not optimized for carbon fixation under normal light conditions. When compared to C3 and C4, CAM plants fix CO2 at night, storing it as malic acid in vacuoles. This strategy allows them to minimize water loss by keeping their stomata closed during the day, but it limits their carbon fixation capacity to the accumulated malic acid, reducing their growth rates when compared to plants following the C3 and C4 pathways. The water-saving ability of the CAM pathway does present an advantage in hot, arid environments, but this comes at the cost of growth potential. Hence, the primary disadvantage is the slower growth rate due to the nocturnal CO2 fixation strategy.