The transport of sap

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.

Besides sugars, the phloem transports amino acids, phytohormones and alarm signals. Explain what it means that elaborated sap travels "from source to sink" and name two different sinks depending on the time of year (roots, fruits, new buds).

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 metres without a pump.

The cohesion-tension theory explains the ascent of water without a pump. Explain which three properties of water and of the xylem make it possible (evaporation at the stoma, cohesion between molecules, adhesion to the vessel walls) and discuss why the water column does not break in such narrow vessels.

Distinguish between guttation and transpiration: when each occurs (humid nights versus dry days), through which structure the water exits in each case, and what guttation demonstrates about the pressure pushing the sap from the root.

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 what plant transpiration is and discuss what percentage of the water absorbed by the root is actually used in photosynthesis (what happens to the remaining $98\ \%$ and what is the point of losing it?).

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.

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.

Aphids drive their stylet into the phloem and the sap flows in by itself under pressure. Explain what this demonstrates about the pressure inside the phloem, what the honeydew aphids excrete is, and how ants exploit it (mutualism).

To study the biophysical functioning of conducting tissues in a mature herbaceous plant, an experiment is designed with two treatments in the middle third of the stem:

• Treatment A: the stem is locally cooled to $0\ \text{°C}$ or infiltrated with a respiratory inhibitor that abolishes ATP production, keeping the leaves intact at room temperature and well illuminated.

• Treatment B: the bark is sectioned peripherally down to the wood and the plant is exposed to high evaporation.

Tritiated water ($^3\text{H}_2\text{O}$) is supplied to the roots and labeled $\text{CO}_2$ ($^{14}\text{CO}_2$) to the upper leaves.

1. a) Predict what will happen to the transport of raw sap and elaborated sap in Treatment A, justifying your answer based on the metabolic requirements of the xylem versus the phloem (Münch pressure-flow hypothesis).
2. b) Explain the relationship between cellular structure and function in both tissues: why xylem elements achieve their functionality following cellular apoptosis, whereas sieve cells must remain metabolically alive and associated with companion cells.
3. c) If an adult tree can transpire $300\ \text{L}$ of water per day and transport $0{,}5\ \text{kg}$ of sugars, reason from an evolutionary and thermodynamic standpoint why transport through the xylem is not based on cellular active transport pumps.