Exercises: Sap transport

Differentiate between raw sap (xylem sap) and elaborated sap (phloem sap): what each transports, which vessel it travels through, and in which direction it moves.

Indicate which cellular elements form the xylem (tracheids and vessel elements) and the phloem (sieve tubes and companion cells), and state a structural difference between them related to their function.

Where does soil water enter the plant, and through which structures is most of it lost to the atmosphere? Name the process of water loss and explain its importance.

Explain the cohesion-tension theory of xylem sap ascent: what role leaf transpiration, cohesion between water molecules, and adhesion to xylem walls play, and why water can rise tens of meters without a pump.

Describe the pressure flow mechanism (Münch hypothesis) of phloem sap transport, explaining what happens at the 'source' (leaf) and at the 'sink' (root, fruit) and why this transport requires energy.

The diagram shows the two vascular tissues in the stem of a vascular plant. a) Indicate which number corresponds to xylem and which to phloem, justifying your answer by the direction of the arrows. b) Name the sap transported by each vessel and its composition. c) Explain which force drives the ascent through vessel 1.

In horticulture, girdling (removal of a ring of bark) is sometimes practiced on a specific branch to obtain larger fruits, but if it is performed at the base of the main trunk, the tree eventually dies.

1. a) Explain, according to the pressure-flow model (source-sink), why the fruits on the girdled branch increase in size and sugar content.
2. b) A worker states: “The tree girdled at the trunk should not die, since the xylem remains intact and the leaves can still receive water and continue photosynthesizing normally.” Refute this argument by predicting the temporal sequence of physiological failures in roots and canopy that lead to the tree's death.

During a very hot and dry summer day (maximum temperature of $38\ \text{°C}$ at $14{:}00\ \text{h}$), the following parameters are monitored in a well-illuminated crop plant:

• From $08{:}00$ to $11{:}00\ \text{h}$: transpiration and net $\text{CO}_2$ assimilation increase in parallel.

• From $12{:}00$ to $16{:}00\ \text{h}$: transpiration drops drastically by $70\ \%$ and net assimilation is reduced almost to zero, despite light intensity being at its peak.

• From $17{:}30\ \text{h}$ onwards: a slight recovery of both parameters is observed before dusk.

1. a) Interpret the data by explaining what osmotic and ionic change guard cells undergo at midday and what hormonal signal triggers this process.
2. b) Justify why stomatal closure, although vital to avoid desiccation, causes glucose synthesis to collapse despite abundant solar radiation, and predict what will happen to leaf temperature during those central hours of the day when transpiration is interrupted.

Imagine that an air microbubble occurs in the xylem of a tree in summer and, at the same time, a gaseous microembolism occurs in an artery of a mammal.

1. a) Explain why the biophysical mechanism of crude sap transport (cohesion-tension theory) makes the xylem vulnerable to cavitation, and predict the immediate hydraulic consequences of the rupture of the water column in that vessel.
2. b) Compare both transport systems by contrasting: the origin of the driving energy (solar/water potential gradient vs. metabolic/myocardial ATP), the nature of the conduits (lignified dead cells vs. living endothelium), and why a plant could not maintain its energy balance if it had to actively pump crude sap via active cellular transport.