Common table salt melts at 801 degrees Celcius, yet a glass of water will do it at room temperature. So you are quite right when you say that there is likely some other way to "melt" the ice into a form that the plant can use. However, melting ice is an endothermic reaction (takes energy), and that energy has to come from somewhere - thus either the plant must expend energy, or the environment will grow colder and reduce the effectiveness of whatever is melting the ice.
You also have the issue that solid chunks of ice - even very small ones - are not readily transported around within a plant, nor absorbed by the root system.
Hence, I propose:
- The plant exudes from it's roots a natural antifreeze protien
- This soaks into the ground around the roots
- The ice around it melts, dropping the temperature of the ground
- The melted ice is absorbed by the roots
- The ground is reheated by conduction from other ground, or from radiation from the sun, or some other process not dependant on the plant.
These plants are relatively rare, and do not like growing near each other (as the cooling of the ground caused by other similar plants hinders their antifreeze), and prefer areas where the ground is maintained at a constant temperature by some external source (eg near inhabited buildings, near flowing rivers or the sea - both of which are excellent heat(cold) sinks for the ground temperature).
One limitation of this system is that ice-chunks in the soil are not easily replaced. Once the ice near to the roots is gone, so is the source of water for the plant. This means either:
- The plant has a very short life-cycle, pollinating (somehow) and forming seeds within days.
- The plant is 'mobile' (eg sends out runners, and lets the old plant die)
- Some mechanism for replacing the ice exists (ice-fog with earthquakes???)
In the comments, Madlozoz notes that antifreeze protiens can't melt ice. This is because they work by inhibiting crystallization when ice starts to form. Instead he proposes alcohol, which may well make the plant be favoured by local wildlife.... Other than that, some types of sugars may work. At any rate, there are some plausable sounding options for antifreeze. Take your pick.