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).

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.

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?).

List the journey of water from the soil to the atmosphere (absorbent hair → root cortex → xylem → leaf mesophyll → stoma), indicating which tissue it crosses at each step.

A dry soil "grips" water more strongly than a humid one. Explain what the tension of water in the soil is (water potential, qualitatively), what the permanent wilting point is, and why watering often with little water is a worse strategy than watering deeply.

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.

A group of students uses a capillary potometer ($1\ \text{mm}$ in diameter) to investigate water loss in a laurel branch (Laurus nobilis). They measure the advance of the bubble during $15$-minute intervals under different experimental conditions:

• Control (laboratory light, $22\ \text{°C}$, calm air): $30\ \text{mm}$.

• Treatment 1 (intense direct light spotlight): $75\ \text{mm}$.

• Treatment 2 (fan at low speed): $65\ \text{mm}$.

• Treatment 3 (very hot and dry air stream for $45$ continuous minutes): $12\ \text{mm}$.

• Treatment 4 (branch covered with a moist transparent polyethylene bag): $4\ \text{mm}$.

1. a) Interpret the results of Treatments 1, 2, and 4, justifying the physiological or physical mechanism acting in each case (photic stomatal opening, reduction of the foliar boundary layer, and vapor pressure gradient).
2. b) Resolve the apparent contradiction of Treatment 3: reason why an extremely desiccating environment caused a drastic drop in transpiration instead of an increase.
3. c) Indicate which quantity the potometer directly measures (as opposed to actual net foliar transpiration) and justify why the basal cut of the stem must necessarily be made submerged underwater when assembling the apparatus.

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.

Fruit growers sometimes ring a branch (they remove a ring of bark) so that its fruits grow fatter. Explain which sap accumulates above the ring, why it improves the fruit, and what risk applying the technique to the whole trunk has (what dies first?).

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).

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.