Exudation of liquid (“bleeding”) from injured trees is caused by pressures within trees that may be localized in different organs depending on species. Most bleeding is caused by root pressure (as in birch and grape), but some is caused by pressure localized in stems (as in maples and palms).
Root pressure develops commonly in trees of tropical rain forests but is uncommon in trees of the temperate zone. The most satisfactory explanation of root pressure is that it is caused by accumulation of solutes in the xylem sap. There is a correlation between salt accumulation in the roots and the occurrence of root pressure. It develops only if root systems are healthy, well-aerated, provided with a dilute solution of minerals, and kept at a moderate temperature. In in-tact herbaceous plants evidence of root pressure is the exudation of droplets of water from the margins of leaves and leaf scars. Such water loss (guttation) occurs from pores called hydathodes that are located on leaf margins. Guttation occurs only from plants growing in moist warm soil, when the air is humid, and transpiration is very low. When root pressure develops water is forced out of the hydathodes. Guttation occurs commonly in trees of tropical rain forests but is relatively uncommon in temperate-zone trees because the appropriate combinations of soil and atmospheric conditions do not occur often. However, guttation from leaves and leaf scars has been observed in some temperate-zone trees. An example is hackberry. The composition of the exuded liquid varies from almost pure water to a dilute solution of organic and inorganic salts.
Exudation of liquid from sugar maple, palms, and a few other plants is caused by stem pressures.
Perhaps the best example of the exudation from injured stems and branches is the flow of sap from sugar maple trees. The flow of maple sap is caused by a pressure localized in the stern, not in the roots. Sap flow will occur from isolated portions of stems. In intact maple trees root pressure cannot be detected at the time that sap is flowing. In contrast, root pressure is al-ways observed when sap flow occurs in grape.
Sap flow of sugar maple can occur from late autumn to early spring, any time that freezing nights are followed by warm days with temperatures above freezing, but the best flow occurs before noon. Sap flow often stops in the after-noon. The yield of sap varies widely from about 35 to 70 liters per tree during a season, but in individual trees the yield may be appreciably greater.
In the tropics large amounts of sap are obtained from palms and used as a source of sugar and wine. Sap is obtained by cutting out the inflorescence.
Another example of exudation by wounding is the flow of oleoresins from pines. Oleoresins come from specialized resin ducts and the flow is not related to the sap flow in deciduous trees. The flow of latex from wounded stems of rubber trees is another example of exudation. The amount of pressure and rate of latex flow are correlated with environmental factors that control turgor (pressure) in the latex vessel system. The rate of flow usually is greater in the morning, when turgor is high, than in the afternoon, and it is reduced during dry weather.
To simplify information, trade names of products have been used. No endorsement of named products is intended, nor is criticism implied of similar products which are not mentioned.
AUTHOR: Theodore T. Kozlowski, Visiting Scholar, Department of Environmental Science, Policy and Management, UC Berkeley
EDITOR: Pavel Svihra, Horticulture Advisor UCCE 1682 Novato Blvd., Suite 150E Novato, CA 94947
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