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ven2doc7_(h2orelations)

ven2doc7_(h2orelations) - VEN 2 Grapevine Water Relations...

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VEN 2 Grapevine Water Relations and Water Use Larry E. Williams, Professor Introduction Water is important to all living organisms. It is an essential constituent of cells with 80 to 90% of the fresh weight of living cells being comprised of water. It is a solvent in which gases, salts and other solutes are able to move in and out of cells and from organ to organ. Water is a reagent in photosynthesis and in a number of other biochemical or biophysical processes. Lastly, water is essential for the maintenance of turgidity (a certain degree of turgidity is required for cell enlargement or growth, stomatal opening, and the maintenance of the form of non-lignified structures). Water movement occurs along gradients of decreasing free energy or molecular activity, often expressed as differences in water potential. If the difference in water potential is produced by some external agent the movement of water is termed mass flow . An example of mass flow in a plant would be the upward movement of water in the vine (in the xylem tissue) caused by evaporation from the leaves. If the movement of water results from random motion of molecules, as in evaporation, the process is called diffusion . Osmosis is an example of diffusion caused by a difference in potential of water on two sides of a membrane, usually caused by differences in the concentration of solutes across the membrane. Water movement in the soil-vine-atmosphere continuum Stomata are the microscopic pores located on the lower surface of grape leaves where water vapor is lost by and CO 2 is taken up into the leaf. Stomata are closed in the darkness, therefore, little water is lost from the vine once the sun sets in the evening. When the sun comes up in the morning, stomata will open gradually and are fully open at approximately one third of full sunlight. Stomatal opening will result in the loss of water vapor from the leaf due to the large gradient in water potential between the atmosphere and inside the leaf (which is considered to be at 100% relative humidity). The loss of water from the plant is called transpiration . As water vapor is lost from the leaf water will move from the cells surrounding the substomatal cavity into that cavity and the process continues. The movement of water out of a cell causes its water potential to become more negative (a decrease in free energy), therefore, water will move from a cell with a higher water potential to one with a lower water potential. Water loss from the cells will continue until water is lost from cells located next to the vascular tissue in the leaf. The vascular tissue contains specialized cells, called vessels that transport water from the roots to the leaves. As water is lost from these cells in the leaf a tension is created within the xylem which is ultimately transmitted down the length of the vascular tissue in the vine. Thus water is pulled up through the vine due to the strong cohesive properties of water molecules in these small water conducting cells (analogous to microscopic water pipes).
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