Resource Acquisition and Transport in Vascular Plants

Why is long-distance transport important for vascular plants?

Some plants can detect increased levels of light reflected from leaves of encroaching neighbors. This detection elicits stem elongation, production of erect leaves, and reduced lateral branching. How do these responses help the plant compete?

If you prune a plant’s shoot tips, what will be the short-term effect on the plant’s branching and leaf area index?

Based on the data, does the initial uptake of water by radish seeds vary with temperature? What is the relationship between temperature and water uptake?

(a) Using the data for 35°C and 25°C, calculate Q₁₀ for water uptake by radish seeds. Repeat the calculation using the data for 25°C and 15°C and the data for 15°C and 5°C. (b) What is the average Q₁₀? (c) Do your results imply that the uptake of water by radish seeds is mainly a physical process or a chemical (metabolic) process? (d) Given that the Q₁₀ for the change in the viscosity of water is 1.2–1.3, could the slight temperature dependence of water uptake by seeds be a reflection of the slight temperature dependence of the viscosity of water? 

Besides temperature, what other independent variables could you alter to test whether radish seed swelling is essentially a physical process or a chemical process?

Would you expect plant growth to have a Q₁₀ closer to 1 or 3? Why?

If a plant cell immersed in distilled water has a Ψ_S of -0.7 MPa and a Ψ of 0 Mpa, what is the cell’s Ψp? If you put it in an open beaker of solution that has a Ψ of -0.4 Mpa, what would be its Ψp at equilibrium?

How would a reduction in the number of aquaporins channels affect a plant cell’s ability to adjust to new osmotic conditions?

How would the long-distance transport of water be affected if tracheids and vessel elements were alive at maturity? Explain.

What would happen if you put plant protoplasts in pure water? Explain.

A horticulturalist notices that when Zinnia flowers are cut at dawn, a small drop of water collects at the surface of the rooted stump. However, when the flowers are cut at noon, no drop is observed. Suggest an explanation.

Suppose an Arabidopsis mutant lacking functional aquaporin proteins has a root mass three times greater than that of wild-type plants. Suggest an explanation.

How are the Casparian strip and tight junctions similar (see Figure 6.30)? 

What are the stimuli that control the opening and closing of stomata?

The pathogenic fungus Fusicoccumamygdali secretes a toxin called fusicoccin that activates the plasma membrane proton pumps of plant cells and leads to uncontrolled water loss. Suggest a mechanism by which the activation of proton pumps could lead to severe wilting.

If you buy cut flowers, why might the florist recommend cutting the stems underwater and then transferring the flowers to a vase while the cut endsare still wet?

Explain why the evaporation of water from leaves lowers their temperature.

Compare and contrast the forces that move phloem sap and xylem sap over long distances.

Identify plant organs that are sugar sources, sugar sinks, and organs that might be either. Explain.

Why can xylem transport water and minerals using dead cells, whereas phloem requires living cells?

WHAT IF? Apple growers in Japan sometimes make a nonlethal spiral slash around the bark of trees that are destined for removal after the growing season. This practice makes the apples sweeter. Why?

How do plasmodesmata differ from gap junctions?

Nerve-like signals in animals are thousands of times faster than their plant counterparts. Suggest a behavioural reason for the difference.

WHAT IF? Suppose plants were genetically modified to be unresponsive to viral movement proteins. Would this be an excellent way to prevent the spread of infection? Explain.

Which of the following is an adaptation that enhances the uptake of water and minerals by roots? 

(A) mycorrhizae

(B) pumping through plasmodesmata

(C) active uptake by vessel elements 

(D) rhythmic contractions by cells in the root cortex

Which structure or compartment is part of the symplast? 

(A) the interior of a vessel element 

(B) the interior of a sieve tube 

(C) the cell wall of a mesophyll cell 

(D) an extracellular air space

Movement of phloem sap from a source to a sink 

(A) occurs through the apoplast of sieve-tube elements. 

(B) depends ultimately on the activity of proton pumps. 

(C) depends on tension, or negative pressure potential. 

(D) results mainly from diffusion

Which of the following would tend to increase transpiration? 

(A) spiny leaves 

(B) sunken stomata 

(C) a thicker cuticle 

(D) higherstomatal density.

Photosynthesis ceases when leaves wilt, mainly because 

(A) the chlorophyll in wilting leaves is degraded.

(B) accumulation of CO₂ in the leaf inhibits enzymes. 

(C) stomata close, preventing CO₂ from entering the leaf. 

(D) photolysis, the water-splitting step of photosynthesis, cannot occur when there is a water deficiency.

What would enhance water uptake by a plant cell?

(A) decreasing the Ψ of the surrounding solution

(B) positive pressure on the surrounding solution

(C) the loss of solutes from the cell

(D) increasing the Ψ of the cytoplasm

A plant cell with a ΨS of -0.65 MPa maintains a constant

volume when bathed in a solution that has a ΨS of -0.30 MPaand is in an open container. The cell has a

(A) ΨP of +0.65 MPa.

(B) Ψ of -0.65 MPa.

(C) ΨP of +0.35 MPa.

(D) ΨP of 0 MPa.

Compared with a cell with few aquaporin proteins in its membrane, a cell containing many aquaporin proteins will 

(A) have a faster rate of osmosis. 

(B) have a lower water potential. 

(C) have a higher water potential. 

(D) accumulate water by active transport.

A Minnesota gardener notes that the plants immediately bordering a walkway are stunted compared with those farther away. Suspecting that the soil near the walkway may be contaminated from salt added to the walkway in winter, the gardener tests the soil. The composition of the soil near the walkway is identical to that farther away except that it contains an additional 50 mM NaCl. Assuming that the NaCl is completely ionized, calculate how much it will lower the solute potential of the soil at 20°C using the solute potential equation: 

Ψ_S = -iCRT

where i is the ionization constant (2 for NaCl), C is the molar concentration (in mol/L), R is the pressure constant [R = 0.00831 (L · MPa)/(mol · K)], and T is the temperature in Kelvin (273 + °C). How would this change in the solute potential of the soil affect the water potential of the soil? In what way would the change in the water potential of the soil affect the movement of water in or out of the roots?

EVOLUTION CONNECTION Large brown algae called kelps can grow as tall as 25 m. Kelps consist of a holdfast anchored to the ocean floor, blades that float at the surface and collect light, and a long stalk connecting the blades to the holdfast (see Figure 28.12). Specialized cells in the stalk, although nonvascular, can transport sugar. Suggest a reason why these structures analogous to sieve-tube elements might have evolved in kelps.

SCIENTIFIC INQUIRY Cotton plants wilt within a few hours of flooding their roots. The flooding leads to low-oxygen conditions, increases in cytosolic Ca²⁺ concentration, and decreases in cytosolic pH. Suggest a hypothesis to explain how flooding leads to wilting.

Imagine yourself as a water molecule in the soil solution of a forest. In a short essay (100–150 words), explain what pathways and what forces would be necessary to carry you to the leaves of these trees.

WRITE ABOUT A THEME: ORGANIZATION Natural selection has led to changes in the architecture of plants that enable them to photosynthesize more efficiently in the ecological niches they occupy. In a short essay (100–150 words), explain how shoot architecture enhances photosynthesis.