Question

Use the pressure-flow model to explain what is likely to happen to water, pressure and solute potential in the following scenarios and how that is likely to affect sugar transport in the phloem:

a. Potatoes rapidly growing and accumulating starch.

b. An Arabidopsis thaliana mutant homozygous for a loss-of-function mutation in the sucrose-H+ transporter SUC2.

c. A plant watered with salty water accumulating higher than normal levels of Na+ and Cl- ions in the root xylem.

d. A plant in which all emerging fruit are removed.

e. Potential adaptation(s) in Sea Asparagus (Salicornia virginica) growing on BC ocean beaches to get sugar to move from photosynthetic leaves to non-photo-synthetic leaves. These plants transport Na and Cl ions in the xylem and secrete it from leaves.

Answer 1

a. Transport through phloem is pressure flow and it moves from source to sink. The water along with the sugars flows like a river flows and carries along with it the sucrose also to growing parts of the plant part. When the starch content in potato is increasing, its water potential will be low. This will draw more water in to the potato. Along with water more sucrose will also reach the potato. With increased starch , solute potential will also increase. With more water absorption, pressure potential will increase too. Water potential is the sum of pressure potential and solute potential. Because starch is accumulating raidly, there will be less water potential and hence more movement of sucrose and water in to the potato.

b. Loss of function mutation, does not allow sucrose to be transported in to the seive elements. So the water can't get absorbed in to the phloem resulting in no pressure flow of sucrose and water in the phloem. No sucrose in phloem , water potential high, pressure potential low and solute potential also low.

c. More accumulation of Na and Cl- in the xylem will lower the water potentail as its solute potential will be high. Xylem will absorb more water in to it from the cells around increasing the water potetial. Water will fail to leave the xylem in to the phloem. Sucrose transport in phloem will be affected.

d. Emerging fruits are removed, there will be effect on the movement of sucrose through phloem. Since sucrose moves from source to sink, if the sink is removed, there will not be pressure flow in the phloem.