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Winter Cold Inury to Grapevine TrunksAnatomy in Relation to Injury |
Cane and trunk anatomy in relation to cold injury
A grape cane is a shoot that has matured (has formed visible periderm or "bark") and has lost its leaves. Buds on canes produce shoots, leaves and clusters during the subsequent crop year. Canes retained beyond the second season become trunks. Older wood can be distinguished from canes by bark which shreds. Sequential, annual production cork cambium in older wood results in bark which does not adhere tightly and which shreds. Shoots have green bark, canes brownish bark which adheres tightly, and trunks have brown bark which shreds. Horizontal trunks are called cordons, and short trunk extensions on which canes or spurs are retained are called arms. In terms of cold injury, arms and cordons behave like trunks.
Above is a diagrammatic cross section of a cane. Moving from the outside toward the cane center one finds in order: dead cortical and epidermal cells of the first year shoot, primary phloem separated from the balance of the phloem by cork cells, the perimderm (cork cells), the cork cambium, phloem, the vascular cambium, xylem and pith.
In reality the visible exterior is the remnant of the epidermis, cortex and primary phloem of the shoot which becomes cut off from the rest of the shoot when the cork cambium formed in late summer. Cells of the cork cambium divide and some differentiate into cork cells which are heavily suberized creating bark and other daughter cells remain undifferentiated so they can continue to divide. Tissues external to the cork cambium form the periderm. At first the external tissues remain green, but then they dry to a straw color. Sunlight is needed to oxidize phenolic compounds which color the periderm brown. Thus depth of brown color is one indication of the sun exposure status of the developing shoot and cane. About the time that cork cambium is formed, xylem and phloem cells in the cane begin to store starch, which is used to fuel spring growth. Terminal portions of shoots often do not form periderm and will not survive winter cold.
Grape phloem is made up of alternate bands of fiber cells and bands of so-called soft phloem. Cells of the soft phloem conduct sugars and other nutrients through the plant. Functional grape phloem is living and is active for more than one season. The last band of phloem created by the vascular cambium in a season is always soft phloem. In the fall the conducting cells of the phloem produce a carbohydrate called callose which plugs the conducting cells, ending movement of sugars for the year. In the spring the callose disappears and the phloem cells become reactivated. I believe plant hormones moving from the growing shoots down the reactivated phloem cause the vascular cambium to become reactivated.
The vascular cambium is made up of undifferentiated cells which retain the ability to divide. It is quite distinct when shoots, canes or trunks are actively growing, but becomes indistinct when growth slows. In the dormant season I can only take the word of plant anatomists who claim that they can still see the vascular cambium. It must be very thin, and perhaps isn't complete. Regardless, the residual cells in the vascular cambium do resume division early in the season and form additional bands of phloem and xylem. Yearly bands of xylem are retained and buried within the trunk as it increases in circumference, but the new bands of phloem are cut off by successive cork cambia. If you closely inspect the loose bark that falls from vines, you can see these annual phloem bands.
The xylem is a complex tissue made up of dead water conducting cells, living companion cells (parenchyma) and living wood fibers, which have great strength. The large conducting cells in the xylem are connected end to end to create conduits for water and nutrient movement. At maturity these vascular elements are not living and yet still function. Sometimes in response to stress and often in the fall, companion cells generate matter which balloons into and otherwise plug the conduction cells. Older bands of xylem are encased in a matrix and form visually distinct heartwood.
In the one-year-old cane the pith is distinct, but it is usually more difficult to distinguish in older trunks. Canes which develop properly will have a small diameter pith and relatively large diameter xylem. Thus relative pith diameter is also an indication of cane quality.
All the vascular tissues inside the cork cambium are arranged in pie-shaped segments alternating with vascular rays made up of living, relatively undifferentiated parenchyma cells. Ray cells live for more than one year, and are thought to be important for radial flow of nutrients..