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Unformatted text preview: 2-17 When water is moving out of the cell: 1. Water potential is higher inside the cell than outside (e.g., water evaporating from mesophyll cell surfaces). 2. Water moves out of the cell, reducing the water pressure on the wall, so the wall pressure goes down, as well. 3. The lowering of the pressure and the loss of water from the cell lowers its water potential, which will continue until the water potentials on both sides of the cell membrane become equal. Water moving in the xylem from roots to leaves 1. During the day, water moves into a plant in response to a lowering of the water potential in the plant below that of the soil water, which is usually rather high. a. This development occurs in the morning on most days. 2. Stomata are opened by guard cells in response to light shining on the leaf. 3. Since the water potential of the water vapor in the air outside of the leaf is less than that of the water in the leaf air, water moves out of the leaf air via stomata. a. The water potential of water vapor is dependent on the RELATIVE HIMIDITY of the air (amount of water in the air relative to what it could hold at a given temp.). 100% R.H. = air saturated with water b. R.H. of the leaf air is usually maintained at close to 100%. The atmospheric R.H. is usually less than 100%. 4. The water lost from the leaf air is replaced by water evaporating from the leaf mesophyll cells into the leaf air, lowering their water potential below that of the water in the xylem of the leaf vein. Environmental Factors that Increase Water Loss: 1. Wind (reduces layer of still air over the leaf). 2. Dry air (low relative humid.). 3. High Temp. 4. Bright Light. Plants Can Reduce Water Loss, as will be discussed in a later lecture. Over-fertilizing a plant increases the salt content of the water in the soil. Thus, the water potential of the soil drops potentially below that needed for the plant. ...
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- Spring '08