EFB530 Plant Physiology

Sugar maple sap flow

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Pressure in the xylem causes sap to flow in some woody plant species - notably birch, grape, maple

• mainly in the spring, before the leaves have developed (before there is much transpiration)
• sap flow in the xylem, often with relatively high concentration of sucrose

• measure the pressure in roots and stems in late spring, find positive pressure in roots and stems, the levels parallel each other in birch (similar in grape)
• in the late spring, warmer temps induce the breakdown of starch reserves in the roots, generating more negative water potential in the roots, drives the influx of water, hydrostatic pressure forces water up the stem
• not closely dependent on temperature

Sugar maple (Acer saccharum) sap flow is different (also sycamore, butternut)

• rarely positive pressure in the roots, even when there is positive pressure in the stems

• requires freezing temperatures (early spring), then thawing to induce sap flow

With most trees, segments of stems will exude sap when the stem is frozen (as the sap expands during freezing) and absorb sap upon thawing

• maple does the opposite-it absorbs sap when frozen, exudes sap when thawed

Milburn and O'Malley model for freeze-induced sap absorption (1984)

• in most trees, ice forms in intercellular spaces (displacing the air in those spaces)
• these intercellular air spaces are rare or absent in maple, thus ice probably forms in the gas-filled fiber cells, lining the walls
• this process of freezing generates tension that pulls water in from the cell walls
• also traps compressed gas inside the fiber cells
• when temps warm, ice thaws, compressed gas builds pressure, sap is exuded

Johnson and Tyree have shown that sap flow is also dependent on sucrose concentration

• sucrose concentration increases in mid-winter (see Johnson & Tyree, 1992, Fig. 2) after solid period of sub-freezing temps, due to water loss in the stems (see Johnson & Tyree, 1992, Fig. 5)
• replacing the xylem sap with increasing concentration sucrose solutions leads to greater rates of exudation after a freeze - thaw (see Johnson et al., 1987, Fig. 8)

Finally, both gas pressure and high sucrose concentration seem to be necessary

• simply raising the sucrose concentration by dehydration at cold, but not freezing temps does not induce exudation - must have freezing & dehydration

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