How does coloured light and light intensity affect the rate of transpiration in plants?
This is a complicated topic. It is usually difficult to separate light from heat, and you have to be careful that it is only LIGHT that is changing and not HEAT as well. Of course, a bright, sunny day produces a higher transpiration rate and that will be because of the HEAT from the sun. However, light does also affect the opening and closing of stomata. Blue light (wavelengths between 430 and 460nm) is nearly 10 times as effective as red light (between 630 and 680nm) in producing a given amount of stomatal opening. There is only a slight response to green light. The wavelengths that are effective in the red part of the spectrum are the same wavelengths that are effective in photosynthesis and inhibitors of photosynthesis knock out the response to red light. The blue light response, however, is independent of photosynthesis – so there are at least two different systems involved. We have covered a similar topic in relation to phototropism here where you will find details of experiments
