Photosynthesis requires water to happen. The water is photolysed (split apart) using the energy gained from sunlight, which will draw more up through the xylem from the roots. However more significantly for photosynthesis to take place there needs to be gas exchange through the stomatal pores predominantly found on the lower sides of the leaf.
If these pores are closed very little water can evapourate (transpire). So if there are low levels of photosynthesis (say, at night) then there will also be relatively low levels of transpiration. The water just cannot get out. However if the pores are left open a much greater volume of water can be lose this way.
You can therefore assume factors that increase the rate of photosynthesis (higher temperature, light intensity etc.) will also increase the rate of transpiration.
JackBean, if you’re replying to me then it could be the other way round but in this circumstance it wasn’t. It was a question on a mock exam paper which I have now found the answer to, it was basically this:
More CO2 absorbed
Which therefore increases the stomatal opening.
Try to consider the light and dark reaction. A plant it active during the day and photosynthsise and in the process release excess H2O contents to the atmosphere (transpiration). So the phenomenom are complimentary.