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How would plants be affected by a 48 hour day, instead of 24 hour?
Would plants be larger to store more energy for photosynthesis?
Also, how about the temperature of the earth be different on both day/night?
How would the different temperature affect plants?
(Note: this is a more precise question, based off of a much more broad question I asked that I've since deleted.)
You can actually try this with regular plants by keeping them in a room in which they are given a full day of light and a full day of darkness in turns. I haven't done this long term, but in the short term (weeks) they do not seem to suffer at all. It would not be unreasonable to assume somewhat greater energy storage as a long term evolutionary outcome though.
It really depends on the extremity of the temperature change, but on Earth we see plants that have to survive in extremely cold climates being much smaller than those in more temperate zones. If the temperature extremes are reasonable, then there might not be much change at all. Consider plants in one of Earth's hot deserts - they experience excessive temperature swings most days due to their environment but do not have any particular adaptations to those temperature swings. (To be clear, they do have a great many adaptations for heat both on a macroscopic and microscopic scale, but I cannot find anything related to daily temperature changes.)
The answer to this question depends very much on the environment. In the American Southwest there are regions where the day/night temperature difference for a 24 hour day is 40°F (22°C) and in Hawaii it's only 10-15°F (6°C.) On Venus, which has a 2802 hour day, the temperature is believed to be nearly uniform from equator to pole, day or night. In general though, for a temperate climate, you would expect the temperature swings to be somewhat more extreme in both directions.
Plants can grow with no problems beyond polar circle, where sun does not set during summer, and no sun during winter.
Plants do not store energy for photosynthesis - they use it if available, and live off reserves (sugar they produced by photosynthesis) during night (exhale CO2).
With longer nights, temperature differences would be bigger, but plants would adapt.
Plants survive the night by producing excess energy during the day. This excess they then convert to starch and store. Research shows that chemical reactions in the plant's cells then calculate, based on their energy store and the rough length of the night, how to proportion their starch stores to last them the night. In the experiment linked above, scientists shut off the light early, giving the plants a shorter day and much longer night (8/16 hrs instead of ~12/12). Still, the plants managed to have enough starch left over in the morning.
Given that plants can adapt to a change in day/night cycle like this, they can easily adapt to a simple double. The day/night proportions remain the same, it's just the time that has changed. Since the daytime has also doubled, the starch the plants store in the day also doubles, so they have twice as much to use at night - meaning they can use the same amount per unit time.
The temperature differences wouldn't have too much effect. A longer night would mean a greater drop in temperature and a greater rise in the morning, but given that we have desert plants that survive +40oC to -20oC, our current plants would either just survive or adapt to the change.
An aspect of this not yet covered by other answers: Some plants measure the length of night-time darkness and use this to decide what time of year it is i.e. is it time to flower, is it time to drop leaves. etc.
The behaviour is called photoperiodism and a switch to a 48 hour day/night cycle could be bad news for the long term prospects of those plants which rely on it.
BTW the link, and others I looked at, suggests the mechanism involves measuring actual elapsed time rather than just day/night proportions, and the plants are using an innate 24 hour circadian rhythm that is tuned to but not controlled by the actual day length.
