The transport of sap

Differentiate between raw sap (xylem sap) and processed 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, plant hormones, and alarm signals. Explain what it means that phloem 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.

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

What is root pressure? Explain in which conditions it shows itself (nights of high humidity, sap oozing when a stem is cut in spring) and why it is not enough to lift water to the crown of a tall tree.

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.

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.

During droughts, air bubbles can form inside the xylem (cavitation and embolism). Explain why an embolism breaks the water column and puts that vessel out of service, and name two plant strategies to avoid or limit it (narrower vessels, closing stomata, spreading risk among many vessels).

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.

In a Mediterranean pine forest subjected to several consecutive years of extreme drought, massive mortality due to hydraulic failure and a devastating attack by bark-boring beetles (Tomicus sp.), which excavate galleries in the phloem, are observed.

1. a) Explain the cause-and-effect relationship between the continuous drop in soil water potential, the tension in xylem vessels, and the occurrence of embolisms. Reason why preventive long-term stomatal closure does not prevent the tree's death (carbon starvation vs. catastrophic cavitation).
2. b) Healthy pine trees defend themselves against these insects by pumping pressurized resin through their resin ducts to expel them from the bark. Physiologically justify why a dehydrated tree with closed stomata loses this mechanical and chemical defense.
3. c) Propose two ecological forest management measures aimed at reducing the vulnerability of these pine forests to future droughts and justify their biological basis (intraspecific competition for soil water and host diversification).

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?).

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.

A dry soil "grips" water more strongly than a moist 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.

In a semi-arid region, a mature forest plot is compared with an adjacent plot recently deforested for dryland farming with soil devoid of permanent vegetative cover. It is noted that, in summer, the air temperature above the forest canopy is up to $4\ \text{°C}$ lower than that of the treeless area, and local fogs or drizzles are noticeably more frequent.

1. a) Justify the thermoregulatory effect of the forest by explaining the mechanism of foliar transpiration and the biophysical property of water responsible for cooling the environment.
2. b) Explain the role of evapotranspiration in continental moisture recycling ("flying rivers") and predict how large-scale deforestation can alter the rainfall regime in areas hundreds of kilometers away.
3. c) Following a torrential autumn storm, stream flow from both plots is recorded. Reasonably predict how the hydrographs will differ (peak flow and response time) and analyze the risk of soil erosion based on the presence or absence of roots and leaf litter.

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 (light-induced stomatal opening, reduction of the leaf 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 leaf transpiration) and justify why the basal cut of the stem must necessarily be made submerged underwater when assembling the apparatus.

Unit challenge. How does a local crop manage water and light stress? Imagine you are advising an agricultural cooperative in your region.

1. Data collection: Check the National Weather Service website for the maximum temperature (e.g., $24.5\text{ °C}$), relative humidity, and hours of sunshine in your locality over the last $3$ days.

2. Physiological analysis: Evaluate how these variables have conditioned leaf transpiration and the ascent of raw sap according to the cohesion-tension theory. Deduce whether there was a risk of stomatal closure and its impact on $\text{CO}_2$ assimilation during photosynthesis.

3. Sap dynamics: Explain how the estimated photoassimilate production affected sucrose loading and phloem sap transport by pressure flow towards sink organs.

4. Agronomic proposal: Prepare a brief technical report including a time graph and a reasoned irrigation schedule to prevent xylem cavitation without reducing photosynthesis.

Assessment criteria:

• Rigor in the use of real data and its graphical representation.

• Understanding of the interdependence between photosynthesis and xylem and phloem transport.

• Biological justification and feasibility of the measures proposed in the report.

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 bigger. 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.

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.

In an ecophysiological study on plant parasitic relationships, two species are analyzed: mistletoe (Viscum album), which has green leaves and lives on pine branches, and dodder (Cuscuta sp.), a filamentous and yellowish parasitic plant that colonizes alfalfa stems.

1. a) At midday, the water potential in the leaves of the host pine is $-1.6\ \text{MPa}$, while in the leaves of the mistletoe it is $-2.4\ \text{MPa}$. Interpret this biophysical gradient, explaining which host tissue the mistletoe's haustoria connect to and why it needs to maintain a more active stomatal conductance than that of its host tree.
2. b) Dodder lacks chlorophyll and functional roots in the adult stage. Deduce which vascular tissue of the host it must direct its haustoria toward in order to survive and classify it functionally compared to mistletoe (holoparasite vs. hemiparasite).
3. c) A farmer proposes combating a dodder infestation in their field by applying a herbicide whose active ingredient inhibits the photosynthetic enzyme Rubisco. Predict the effectiveness of this treatment on the parasite and on the crop, and reason a more selective agronomic control method consistent with the parasite's physiology.