A plant cell has an osmotic (or solute) potential of -2.1 MPa. Neglecting the changes in volume due to turgor pressure, what will its internal pressure be when allowed to come into equilibrium with each of the following solutions:

a. Water (Answer: cells internal pressure after incubation in water is Ψp = 2.1)b. -2.1 MPa mannitol (Answer: Ψp = 0)c. -2.5 MPa mannitol (Answer: Ψp = -0.4; plasmolysis will likely occur)d. -1.0 MPa triethyl citrate to which the cell is highly permeable (Answer: Ψp = 2.1)

A wind-borne pollen grain with an osmotic (solute) potential of -1.5 MPa has beendrying out while aloft. Thus, it has lost all turgor pressure. It lands on a stigma, whose cells have an osmotic potential of -2.0 MPa and an internal pressure of 0.8 MPa. In answering the following questions, assume that the stigma is massivecompared with the pollen grain:

a. In which direction will water flow? Why?b. What will be the internal pressure of the pollen be after it has been on the stigma for quite some time?c. A fresh pollen grain with the same osmotic potential but an internal pressure of -1.3 MPa is placed on the same stigma. In which direction will water flow? Why?

Answers:a. Ψw of stigma = Ψs + Ψp (Ψw = -2.0 MPa + 0.8 MPa; Ψw = -1.2 MPa Ψw of pollen in air = Ψs + Ψp (Ψw = -1.5 MPa + 0 MPa; Ψw = -1.5 MPa Water will flow from stigma (Ψw = -1.2 MPa ) to pollen (Ψw = -1.5 MPa) because the water in the stigma has more energy (more free water in stigma) than in the pollen. b. At equilibrium, Ψw of pollen will be equal to that of the stigma (Ψw = -1.2 MPa). Ψw of pollen on stigma = Ψs + Ψp (-1.2 MPa = -1.5 MPa + ? MPa); Ψp = 0.3 MPac. Ψw of fresh pollen = Ψs + Ψp (Ψw = -1.5 MPa + 1.3 MPa; Ψw = -0.2 MPa Water will flow from pollen (Ψw = -0.2 MPa) to stigma (Ψw = -1.2 MPa ) because the water in the pollen has more energy (more free water in fresh pollen) than in the stigma.

A plant 47 meters tall is transpiring at a rate of 14 liters per hour. Suddenly, the rate drops to 7 liters per hour. What factors could have causedthe rate to decrease? Give as many factors as you can.

Answers could include: cloud cover increased (reduced temperature-lower driving force); wind speed decreased (increased boundary layer and increased resistance); reduced photosynthetic rate in guard cells- to affect solute production and water potentialIn guard cell, which affects size of stomatal opening and decreases conductivity;Increase in humidity (rain), which reduces driving force