Leafy plants don’t need a lot of sun to stay alive but it depends on the leaf size, plant size, temperature, pressure and other nutrients. Plants will change the amount their pores are open, which regulates how much oxygen leaves, if the outside temp is too high or when their is no sun, so if the plant is losing too much water through its pores it will slow down gas exchange which helps conserve water loss though the pores and so plants may not be as efficient in their processes at certain conditions. It’s similar to saying when would it ve best for an athlete to drink water throughout the day?. It depends on a lot of circumstances.
The suns light is more intense at certain times of the day Different tines of day will change the intensity of the light based off the angle it is hitting the earth so when the sun is directly overhead it will be most intense. It may be too intense for some plants at certain times of day and trigger the previous mentioned response.
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