As the sun sets each night, most terrestrial plants close their stomata. It is reasoned that plants open their stomata to acquire CO2. At night, with no photosynthesis, there is no need to acquire CO2, and so the stomata can close.
Mike Cramer and coauthors just published a review in Oecologia that challenges some basic assumptions of the benefits of closing stomata at night. The authors state that mass flow of water to roots carries nutrients with it. For non-limiting nutrients, this flux alone can meet a plant's demand, but also could benefit the plant for a limiting nutrient that has low concentration in the soil solution. They cite many instances where NO3- can regulate water inflow into a root as additional evidence of the role of mass flow in plant nutrient acquisition.
In their summary, the authors state that "some plants [might be] designed not to conserve water, but rather to maximise the flux of water when it is abundant." This is a gauntlet-throwing statement.
The calculations of the role of mass flow in nutrient acquisition are 30+ years old. This doesn't make them wrong, but the models did not always ask the most pertinent questions. Would a plant competing against another plant be benefitted from a higher transpiration rate? Would a plant that keeps its stomata open at night acquire more nutrients than one that keeps them closed? And even if not, what are the negative consequences to a plant that left them open?
It's good that the authors raise such a basic question about how plants acquire resources from the soil. It's a good review that lays bare some fundamental questions about the constraints faced by terrestrial plants and ultimately their evolution.
Cramer, M. D., H. J. Hawkins, and G. A. Verboom. 2009. The importance of nutritional regulation of plant water flux. Oecologia.