Light is needed by plants to perform photosynthesis. This can be seen in the graph where the light intensity increases and the rate of photosynthesis also increases. At around 40% is the light intensity where the rate of photosynthesis reaches a maximum. From 40% onwards, the rate of photosynthesis plateaus to show that increasing the light intensity any further will not have an effect on the rate. Hence, light intensity is not a limiting factor here (Choice A). Instead, the concentration of carbon dioxide or temperature could be the limiting factors.
In a carbon dioxide concentration graph, when the rate of photosynthesis plateaus, it means that the rubisco enzyme is saturated with the carbon dioxide substrate. Hence, carbon fixation has reached the maximum level. Therefore, Choice C is only applicable for a graph that shows the effect of carbon dioxide concentration on the rate of photosynthesis.
In a temperature graph, the rate of photosynthesis does not plateau but instead decreases beyond the optimum temperature. This means that the enzymes are denatured due to high temperatures. Therefore, Choice D is applicable for a graph that shows the effect of temperature on the rate of photosynthesis.
