`star` Light
`star` Carbon dioxide Concentration
`star` Temperature
`star` Water


● We need to distinguish between `color{violet}"light quality, light intensity"` and the `color{violet}"duration of exposure to light"`, while discussing `color{violet}"light as a factor"` that affects photosynthesis.

● There is a `color{violet}"linear relationship"` between incident light and `CO_2` fixation rates at `color{violet}"low light intensities"`.

● At `color{violet}"higher light intensities"`, gradually the rate does not show `color{violet}"further increase"` as other factors become limiting.

● What is interesting to note is that `color{violet}"light saturation"` occurs at `color{violet}"10 percent"` of the full sunlight.

● Hence, `color{violet}"except for plants in shade"` or in `color{violet}"dense forests"`, light is rarely a limiting factor in nature.

● Increase in `color{violet}"incident light"` beyond a point `color{violet}"causes the breakdown"` of chlorophyll and a `color{violet}"decrease in photosynthesis"`.


● `color{Brown}"Carbon dioxide"` is the `color{violet}"major limiting factor"` for photosynthesis.

● The `color{violet}"concentration"` of `CO_2` is very `color{violet}"low in the atmosphere"` (between `color{violet}"0.03 and 0.04"` per cent).

● `color{violet}"Increase in concentration"` upto 0.05 per cent can cause an `color{violet}"increase"` in `CO_2` fixation rates; beyond this the levels can become `color{violet}"damaging over longer periods"`.

● The `C_3` and `C_4` plants `color{violet}"respond differently"` to `CO_2` concentrations.

● At `color{violet}"low light conditions"` neither `color{violet}"group responds"` to high `CO_2` conditions.

● At `color{violet}"high light intensities"`, both `C_3` and `C_4` plants show increase in the `color{violet}"rates of photosynthesis"`.

● What is `color{violet}"important to note"` is that the `C_4` plants show `color{violet}"saturation"` at about `color{violet}"360"` `µlL^(-1)` while C3 `color{violet}"responds to increased"` `CO_2` concentration and saturation is seen only beyond `450"` `µlL^(-1)`.

● Thus, `color{violet}"current availability"` of `CO_2` levels is `color{violet}"limiting"` to the `C_3` plants.

● The fact that `C_3` `color{violet}"plants respond to higher"` `CO_2` concentration by showing `color{violet}"increased rates of photosynthesis"` leading to higher productivity has been used for some `color{violet}"greenhouse crops"` such as tomatoes and bell pepper.

● They are allowed to `color{violet}"grow in carbon dioxide"` enriched atmosphere that l`color{violet}"eads to higher yields"`.


● The `color{violet}"dark reactions"` being enzymatic are `color{violet}"temperature controlled"`.

● Though the `color{violet}"light reactions"` are also `color{violet}"temperature sensitive"` they are affected to a `color{violet}"much lesser extent"`.

● The `C_4` plants respond to `color{violet}"higher temperatures"` and show `color{violet}"higher rate of photosynthesis"` while `C_3` plants have a much `color{violet}"lower temperature optimum"`.

● The temperature `color{violet}"optimum for photosynthesis"` of different plants also `color{violet}"depends on the habitat"` that they are adapted to.

● `color{violet}"Tropical plants"` have a `color{violet}"higher temperature optimum"` than the plants adapted to `color{violet}"temperate climates"`.


● Even though `color{violet}"water is one of the reactants"` in the light reaction, the effect of `color{violet}"water as a factor"` is more through its `color{violet}"effect on the plant"`, rather than `color{violet}"directly on photosynthesis"`.

● `color{violet}"Water stress"` causes the `color{violet}"stomata to close"` hence reducing the `CO_2` availability.

● Besides, `color{violet}"water stress"` also makes `color{violet}"leaves wilt"`, thus, `color{violet}"reducing the surface area"` of the leaves and their `color{violet}"metabolic activity"` as well.