Plant Transport and Physiology

The petiole becomes flattened and performs photosynthesis.

Transpiration is the process by which plants lose water through their leaves, primarily through small openings called stomata. This water loss occurs as water vapor, not liquid water or gas. It helps plants cool down and transport nutrients.

Stomatal transpiration occurs through stomata (tiny pores on leaf surfaces) and accounts for about 90% of total water loss in plants. Cuticular transpiration through the waxy leaf cuticle is minimal, while lenticular (through stems) and foliar transpiration are even less significant.

Wilting occurs when water transport through xylem is blocked, preventing water from reaching leaves. Without water, turgor pressure in cells drops and leaves droop. Blocking phloem would affect sugar transport, not water delivery.

Transpiration is a physiological process because it occurs through living stomatal guard cells that actively open and close. Evaporation is purely physical - water molecules escape from any exposed surface without biological control. The key difference is living tissue involvement.

Transpiration cools leaves through evaporative cooling - as liquid water evaporates from leaf surfaces, it absorbs heat energy (latent heat of vaporization). This is similar to how sweating cools human skin. Respiration and photosynthesis are metabolic processes that may produce heat, not cooling.

Levitt (1974) proposed the K+ ion exchange hypothesis for stomatal movement - K+ ions actively move into guard cells to open stomata (increasing turgor) and exit to close them. Darwin studied phototropism, Scarth worked on stomatal physiology earlier, and Khorana is known for genetic code research.

A potometer measures transpiration rate by recording water uptake by a plant cut shoot - as water is lost through transpiration, an equal amount is absorbed, moving the air bubble in the capillary tube. Auxanometer measures growth, porometer measures stomatal opening, and potentiometer measures electrical potential.

Temporary wilting occurs when plants lose turgor during the day due to high transpiration rates exceeding water uptake, but recover at night when transpiration slows and roots can catch up. Permanent wilting occurs when soil water is permanently unavailable. Guttation is water droplet excretion from leaf margins, while exudation is general oozing of fluids.

About 90% of water absorbed by herbaceous plants is lost through transpiration. This high loss is the trade-off for opening stomata for CO2 uptake during photosynthesis. The other percentages (80%, 60%, 40%) underestimate actual transpiration losses in most herbaceous plants.

Root pressure builds up in roots when water absorption exceeds water loss. This occurs when transpiration is low (minimal water loss from leaves) and absorption is high (active water uptake by roots). Option A correctly identifies this condition. Option B (high transpiration) would reduce root pressure, and Option C (both low) would not generate significant pressure. Option D describes conditions that would deplete water pressure.

Guard cells contain chloroplasts and photosynthesize during daylight. They produce sugars like glucose, which lowers the guard cell's water potential. Water enters by osmosis, increasing turgor pressure and causing stomata to open. Option A correctly explains the mechanism. Options B and C describe structural features but not the cause of opening. Option D states the purpose, not the cause.

When CO2 concentration rises around a leaf, it triggers a feedback mechanism - stomata partially close to reduce CO2 intake and conserve water. This decreases transpiration. Option D correctly describes this response. Options A and B are incorrect because high CO2 causes closure, not opening. Option C is incorrect because transpiration is affected through stomatal behavior.

Wilting occurs when water loss exceeds water uptake, causing loss of turgor pressure. High temperature (Option A) increases transpiration rate significantly, which can lead to wilting if water absorption cannot keep up. Options B and C (high humidity, rainfall) would reduce water stress. Option D is incomplete - absorption itself is not a condition; rather, insufficient absorption causes wilting.