9/22/2023 0 Comments Sugar maple leaf cross section![]() ![]() Measuring volume production by sugar maple trees, Proctor research station, Underhill, VT. Maple species other than sugar maple also apparently make delicious maple syrup but have lower sap sugar contents, so one needs more sap to make syrup. Throughout the tropics sap is collected from the crowns of palms for miel de palma (“palm honey”) and palm sugar. I suppose one might be able to extract sap from conifers during periods of root pressure sap flow, but this might be practically unfeasible because the collected sap might be inevitably contaminated with resin, the sticky pitch that visibly oozes from conifer wounds (which is not sap, even though it is sometimes called that). In North America sap is also sometimes collected from species in the Juglandaceae family (hickories and walnuts) and basswoods ( Tilia spp: Tiliaceae). Often, however, the birch sap is drunk or bottled as is, as a refreshing tonic. Sometimes the sap is boiled down for syrup, and there’s a growing birch syrup industry in Alaska, Russia and Scandinavia. Many species of birch trees ( Betula spp., Fagaceae) are tapped for sap throughout the northern hemisphere. Sugar maple is not the only tree species from which xylem sap is collected for consumption. Riverside yellow birch ( Betula alleghaniensis) in the sun are good candidates for sap collection That is why maple syrup tastes differently from syrup from other trees, such as birch, or from syrup made from other plants, like sugarcane or sorghum (molasses is made from the combined xylem and phloem sap from both species). The distinctive maple flavor is still something of a mystery but is variously attributable to the organic constituents of the sap and is amplified by heating. Maple syrup is mostly sucrose, but the maple xylem sap stream also contains glucose, inorganic salts, protein precursors (peptides and amino acids), some enzymes, and a few mystery organic compounds. If they tap too early, the tap site might dry out, and they will collect mostly lower sugar content sap. ![]() Producers should tap right before the peak sugar content sap flows. Therefore, there is quite an art to the timing of the tree tap. Sap sugar content also varies within a sap flow season, peaking in the middle. Most of the carbohydrates in the rays are actually stored as starch grains, and an enzyme released into the xylem throughout late winter converts the starch to sucrose and mobilizes it in the xylem. The sugar in the sap stream is from carbohydrates stored in the living ray parenchyma and fiber cells adjacent to the vessels in the xylem and is mobilized in late winter and exuded into the xylem sapstream to fuel flowering and leaf expansion (maples flower before they leaf out). Walnut (Juglans) has maple-like late winter sap flow Some tree species (see below), most notably the birches ( Betula), will exude sap from a spile in the later spring in response to positive root pressure. Most other species in the northern woods (including willow ( Salix), aspen ( Populus), elm ( Ulmus), ash ( Fraxinus), and oak ( Quercus)) do not exude sap and spend the winter with gas-filled vessels and water-filled adjacent fibers. The process occurs in all maples and sycamore (genera Acer and Platanus) and some other species, including butternut and walnut ( Juglans spp., Juglandaceae) that spend the winter with water-filled vessels with air-filled fiber cells adjacent to the vessels. In any given season, a maple producer may have the opportunity to collect sap on one or ten or more daily “runs” of sap. Tubes run from the vacuum device to the tapped sugar maples and carry sap to the collection location, Proctor research station, Underhill, VT.īecause it depends on weather (and water availability) instead of spring leaf flush, maple winter sap flow is intermittent and highly variable from year to year. When photosynthesis is done for the day, tension relaxes on the column of water in the xylem, and vessel pressure approximately equilibriates with that of soil water at the root surface. Transpiration during photosynthesis therefore pulls water up through the plant from the soil. Water moves up through the vessels to replace water lost through transpiration, in response to this pressure gradient. This water loss is called transpiration and creates tension, or negative pressure, in the water in the xylem. In doing so, however, the leaf must inevitably lose water by evaporation out of the stomata. In order to turn carbon dioxide into sugar through photosynthesis, the leaf must open its stomata (small pores on the leaf surface) so that carbon dioxide can diffuse into the leaf. Three primary processes can cause xylem sap to flow: transpiration, root pressure and stem pressure. Sap in the xylem moves passively, in response to physical forces, pressure and osmotic gradients. Tracheary elements sculptures, Kew Botanic Garden, UK ![]()
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