Crassulacean acid metabolism (CAM) pathway

Crassulacean acid metabolism (CAM) is the adaptation of the succulents and some epiphytic plants to arid conditions in order to prevent water loss. CAM pathway involves the temporal separation of the biochemical reactions that helps in reducing the water loss through transpiration. Plants such as Orchids, Cacti, Aloe, Pineapple, Agave, Moringa show CAM pathway.

Two phases are involved in the CAM pathway:

Night time: The stomata opens during the night time (for capturing Co2) when the humidity is high and the temperature is cooler. The Co2 combines with phosphoenol pyruvate (PEP) in the presence of the enzyme PEPcarboxylase and forms a four-carbon compound, Oxaloacetic acid (OAA). Oxaloacetic acid is further converted into malic acid and stored in vacuoles of the chloroplast during the night time. Day time. The stomata remain close and there is no gaseous exchange. The malic acid is transported from the vacuoles to the stroma where it is decarboxylated and Co2 is released.
Finally, this Co2 enters the Calvin cycle thereby completing the carbon fixation. The concentration of the Co2 around the enzyme RuBisCo ensures efficient carbon fixation. Factors affecting photosynthesis Both Internal and external factors influence the rate of photosynthesis. A lot of factors affect the photosynthesis process but the rate of the process is usually determined by the only one major factor that is present at a sub-optimal level.

Blackman’s (1905) Law of Limiting Factors:

If a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value: it is the factor which directly affects the process if its quantity is changed. For example, the plant may not photosynthesise even after it is provided with all the optimal conditions except one (such as low temperature). This leaf, if given the optimal temperature, will start photosynthesising.

**Internal factors/Plant factors: **

These factors are dependent on the genetic predisposition and the growth of the plant. The plant factors include the number, size, age and orientation of leaves, mesophyll cells and chloroplasts, internal CO2 concentration and the amount of chlorophyll.

External factors

1. Light: Three parameters are considered within light, Light quality, intensity and duration of exposure. At lower intensity of light, a linear relationship exists between the incident light and CO2 fixation rates. At higher intensity of light, there is breakdown of chlorophyll which leads to the reduction of photosynthesis. Light is rarely a limiting factor because light saturation occurs at 10 per cent of the full sunlight.