Spring sap rises because bud hormone signals trigger root starch conversion to sugar

Evidence

The Maples each year carry out their part of the Original Instructions, to care for the people. But they care for their own survival at the same time. The buds that sensed the incipient turn of the season are hungry. For shoots that are only one millimeter long to become full-fledged leaves, they need food. So when the buds sense spring, they send a hormonal signal down the trunk to the roots, a wake-up call, telegraphed from the light world to the underworld. The hormone triggers the formation of amylase, the enzyme responsible for cleaving large molecules of starch stored in the roots into small molecules of sugar. When the concentration of sugar in the roots begins to grow, it creates an osmotic gradient that draws water in from the soil. Dissolved in this water from the spring-wet earth, the sugar streams upward as rising sap to feed the buds. It takes a lot of sugar to feed people and buds, so the tree uses its sapwood, the xylem, as the conduit. Sugar transport is usually restricted to the thin layer of phloem tissue under the bark. But in spring, before there are leaves to make their own sugar, the need is so great that xylem is called into duty as well. At no other time of year does sugar move this way, only now when it is needed. Sugar flows upstream for a few weeks in the spring. — Robin Wall Kimmerer, Braiding Sweetgrass, 231

Categorization

• QUESTION: Kimmerer frames bud-to-root hormonal messaging as a ‘wake-up call telegraphed from the light world to the underworld’ — worth chasing the precise plant physiology (auxin/gibberellin signaling) to verify the mechanism.
• XREF: The bud-leaves-root reciprocity — roots feed buds, then leaves feed roots — is biological confirmation of the land-as-gift circulation claim from ch6-7 (gift value grows through circulation).