it depends on what season of the year, but for the most part, energy is transported from the leaves down to the roots. Roots and most lignified stems are parasitic energy-hoarding ‘animals’. And these really are like animals, as the living parts of roots and stems ‘breathe in’ oxygen, and breathe out CO2, as they cannot utilize the energy in carbohydrates without oxygen. Just as CO2 and CO are toxic to animals, the same gases are toxic to roots, being waste products of respiration.
leaves also utilize O2 to utilize the carbs they make, but being the solar panels that they are, they produce more chemical energy from CO2 and sunlight in the form of sugars(which includes wood), hence the carbon in CO2 is locked in, and this makes foliage a net producer of oxygen, and a net absorber of CO2.
as for the season of the year mentioned, roots and stems become the ‘givers’ during spring when leaves have yet to sprout and develop. Think maple sap --or that sweet potato one left on the kitchen counter which started sprouting–it is sugar energy amassed from the foliage from the previous year’s production. When leafing out, the buds must rely on the previous year’s storage to develop, but once the foliage are up and running, the stems/roots reverse roles with the leaves, with the latter again resuming the major role of ‘giver’, and the former resuming the major role as ‘taker’.
only time when roots will reverse roles in late spring, summer, or early fall is if you intentionally remove all green parts of the plant. Or, say, you cut down a tree into a leafless stump. Developing buds will have to tap the energy from stems and roots anew.