2023-08-25-BIOL254-W6-Notes (1).docx

LECTURE 12: PRINCIPLES OF PLANT PHYSIOLOGY Practice Exercise If two solutions: A and B separated by a membrane. Water potential of solution A = -0.248 Water potential of solution B = -0.745 Which solution has a higher water potential? The solution with less negative value has high water potential,...

LECTURE 12: PRINCIPLES OF PLANT PHYSIOLOGY Practice Exercise If two solutions: A and B separated by a membrane. Water potential of solution A = -0.248 Water potential of solution B = -0.745 Which solution has a higher water potential? The solution with less negative value has high water potential, while the one with more negative have lower water potential. The Principle Water moves from the region of higher water potential to a region of lower water potential. The answer to the practice question is then, water moves from A to B. Now in Real Life Situation The roots of plants are surrounded by water, sand particle, air spaces, clay particle. The root must take up water from the soil solution. It is necessary to know the water potential of a soil solution and the water potential in root cells/ root hairs. Then to answer the question, which one has a high-water potential? Then we look at the numbers, the one with the less negative value has the higher water potential. Remembering our principle, then the solution then moves to the root cells or the water potential will allow the root cells to be able to absorb water from the soil solution. Note: Potential examination questions might come from the above section Pulling Force of Water from the Root It is believed that there is a force pulling the water in the roots from the root hair up the plant through the xylem. Looking at the diagram (B), if the entire surface is equally permeable, the lower part of the root will dry up or it will be dissipated. Pathway of Water System in the Root Water molecule enters the root hair, then thicken up and then cross the membrane and cell wall of the epidermal cells and enter the cytoplasm of the cortical cells and all the wall from one cell to another through the cell wall of neighbouring cell. This pathway is called symplastic and transmembrane pathways. The water molecule comes to the last cortical cell – the endodermis then into the pericycle cell and continues and travel trough into the cells to the xylem. Another way is through the apoplast pathway, here water enters the root hairs and enter the cell wall space of the root hairs and then move through the cell wall of the epidermal cell, touching the cell wall of the cortical cell and through the cell wall of the cortical cells into the next layer and next layer of the cortical cells. Here water molecules do not go into the cytoplasm at all. When the water molecule come into the cell wall of the cortical cell which in contact with the cell wall of the endodermis, it enters the cytoplasm of the endodermis, through other cells and to the xylem. Why is endodermis the point for water molecules to enter into its cytoplasm For water to cross membrane, it diffuses though membrane. But endodermis has some channels that help facilitate high speed movement of water into the centre of the xylem vessels. These water selecting channels or pores are constructed with transmembrane proteins known as Aquaporins. The number of aquaporins control how big the water selecting channels is and it is done by the expression of genes that code for it. The endodermis is some sort of control for the speed of water movement in the root cells. Soil Water deficit When there is soil water deficit, there is lowering water potential. This lower water potential leads to water stress to plants. Plants do not give up easily or stop physiological process when it starts to undergo water stress. When plant notice that the water potential of the soil solution is decreasing outside the root, they will try to synthesis some small molecules inside the cells such as proline, sorbitol, glycine betaine and others. These molecules accumulate inside the cells trying to maintain more negative water potential inside the root. Effects of Water Stress on Photosynthesis and Leaf Expansion of Sunflower Here as soon as the cells in the leaves sense water shortage, leaf expansion shuts down completely. When this has happened, photosynthesis in leaf keeps on going without affecting anything. Comparison of Photosynthesis and Translocation Photosynthesis starts to decline when the water potential keeps decreasing but the sugar/translocation keep working. Water potential of plants under various growing conditions When plant is well watered everything goes on well. But then when its less water, cell expansion shuts down, then wall synthesis. Then protein synthesis, then stomatal conductance and photosynthesis. The plant tries to accumulate more solutes inside the vacuoles and abscisic acid accumulation. This is the evidence that there is a programmed process in plant when there is water stress.