Why is it better to water plants in the morning? Why, because water is needed by plants in the light-dependent phase of photosynthesis.
Plants and other photosynthetic organisms start the light-dependent phase of photosynthesis as soon as there is light. During this process, light energy trapped by chlorophyll splits water into an oxygen molecule and 4 protons. The oxygen diffuses out of the plants (and this is what we breath in) while the protons (or hydrogen ions) are used to generate energy molecules.
What happens then if we water our plants late in the afternoon? Well, I think you know the answer to that - this does not give the plants enough time to generate more energy as well as to release more oxygen.
The energy molecules generated here are used for the carbon fixation or the light-independent phase of photosynthesis.
By the way, since plants release oxygen during this process, this is another reason why we have to thank a green plant. Don’t you agree?