Plant Transport and Physiology

Whiptail disease in cauliflower and other brassicas is specifically caused by a deficiency of molybdenum.

Transpiration is the evaporation of water from leaves. High humidity reduces the water vapor concentration gradient between the leaf interior and the atmosphere, thereby decreasing the rate of transpiration.

Transpiration occurs primarily through stomata, but also to a lesser extent through the cuticle (cuticular transpiration) and lenticels (lenticular transpiration).

Girdling experiments involve removing a ring of bark (phloem) from a tree trunk. This causes food to accumulate above the cut, demonstrating that phloem is responsible for organic nutrient transport.

The pressure flow hypothesis (also called mass flow hypothesis) is the accepted mechanism for phloem translocation. It explains how osmotic pressure gradients drive sugar movement from source to sink.

While phloem sap consists primarily of water and sucrose, it also transports amino acids, hormones, signaling molecules, and other organic compounds. D is correct as phloem carries all these substances.

Xylem transport is driven by transpiration pull, which moves water and minerals from the roots upward to the leaves, making it a unidirectional process.

Phloem transport is bidirectional because the source and sink relationship can change depending on the plant's developmental stage or seasonal needs.

In early spring, stored sugars in roots or stems are mobilized to act as a source for developing buds, which function as sinks.

Phloem transports sucrose from photosynthetic sources (mature leaves) to various sinks including growing tissues, storage organs, and developing fruits. This is the fundamental translocation pathway.

While xylem is primarily for water and minerals, it can transport some organic nitrogen and hormones; similarly, phloem can transport some inorganic ions.

Plants recycle essential minerals by moving them from senescing (dying) leaves to developing tissues, which is a key survival strategy for nutrient conservation.

The endodermis contains the Casparian strip (suberin) that blocks apoplastic flow, forcing all solutes through selectively permeable transport proteins. This creates a one-way valve allowing controlled entry into the stele toward xylem.

The endodermis acts as a checkpoint for water and solutes entering the vascular cylinder (xylem) from the root cortex, ensuring selective uptake.

C4 plants lose less water because they concentrate CO2 in bundle sheath cells, reducing the need to open stomata widely. The CO2-concentrating mechanism allows higher water-use efficiency - approximately half the water loss per CO2 fixed compared to C3 plants.