Transport in Plants & Mineral Nutrition

Water and mineral transport in plants operates through physical and physiological mechanisms governed by water potential, root pressure, and transpiration pull. Understanding these concepts alongside mineral nutrition and nitrogen fixation forms a high-scoring foundation for NEET.

Water potential (Ψw) determines the direction of water movement and is expressed as Ψw = Ψs + Ψp, where Ψs is the solute potential (osmotic potential, always negative) and Ψp is the pressure potential (positive in turgid cells, zero at incipient plasmolysis). Pure water has a Ψw of zero — adding solutes lowers Ψw. Osmosis is the movement of water from a region of higher Ψw to lower Ψw across a semipermeable membrane. When a plant cell is placed in a hypertonic solution, plasmolysis occurs — the protoplast shrinks away from the cell wall as water exits. Imbibition is the adsorption of water by hydrophilic colloids (dry seeds swelling is a classic example, generating enough force to crack seed coats).

Water absorption by roots follows two pathways. The apoplast pathway moves water through cell walls and intercellular spaces — it is faster but non-selective. The symplast pathway moves water through the cytoplasm of adjacent cells via plasmodesmata — it is slower but selective. Critically, the apoplast pathway is blocked at the endodermis by the Casparian strip, a band of suberin that forces all water into the symplast, ensuring selective mineral uptake.

Root pressure generates positive hydrostatic pressure in the xylem of roots, pushing water upward. Evidence for root pressure includes guttation — the exudation of water droplets from leaf tips and margins through hydathodes (specialised pores), typically observed at night or early morning when transpiration is minimal but root pressure continues. However, root pressure alone cannot account for water ascent in tall trees.

The cohesion-tension theory (Dixon and Joly) explains the primary mechanism for long-distance water transport. Transpiration from leaf mesophyll cells creates a negative pressure (tension) in xylem vessels. Cohesion (hydrogen bonding between water molecules) and adhesion (attraction between water and xylem walls) maintain a continuous water column from roots to leaves. This transpiration pull is the dominant force driving water ascent. Transpiration occurs through three routes: stomatal (90-95%, through stomatal pores regulated by guard cells), cuticular (5-10%, through the waxy cuticle), and lenticular (negligible, through lenticels in bark). Factors affecting transpiration include light intensity (opens stomata), temperature, humidity (high humidity reduces transpiration), and wind speed.

Mineral nutrition requires 17 essential elements classified as macronutrients (9 elements needed in large amounts: C, H, O, N, P, K, Ca, Mg, S) and micronutrients (8 elements needed in trace amounts: Fe, Mn, Zn, Cu, Mo, B, Cl, Ni). Arnon and Stout established three criteria of essentiality: the element must be absolutely required for normal growth, its deficiency cannot be compensated by another element, and it must be directly involved in plant metabolism. Deficiency symptoms depend on element mobility: mobile elements (N, P, K, Mg) show deficiency first in older leaves (elements translocated to younger growing tissues), while immobile elements (Ca, Fe, S, Mn, B) show deficiency first in younger leaves. Common symptoms include chlorosis (yellowing due to deficiency of N, K, Mg, S, Fe, Mn, Zn, Mo), necrosis (cell death — Ca, Mg, Cu, K), and stunted growth (N, S, Mo).

Biological nitrogen fixation converts atmospheric N₂ into ammonia (NH₃) by the enzyme nitrogenase, a molybdenum-iron (Mo-Fe) protein. The reaction requires 16 ATP: N₂ + 8H⁺ + 8e⁻ + 16ATP → 2NH₃ + H₂ + 16ADP + 16Pi. Nitrogenase is irreversibly inhibited by oxygen, so anaerobic conditions are essential. In legume root nodules, leghemoglobin (an O₂-scavenging protein with a red pigment) creates the microaerobic environment required. Symbiotic nitrogen fixers include Rhizobium (in root nodules of legumes) and Frankia (in Alnus, Casuarina). Free-living aerobic fixers include Azotobacter; free-living anaerobic fixers include Clostridium. Cyanobacteria such as Anabaena and Nostoc fix nitrogen in specialised thick-walled cells called heterocysts.

The nitrogen cycle operates through: fixation (N₂ → NH₃), nitrification by chemoautotrophic bacteria — Nitrosomonas converts NH₃ to NO₂⁻ (nitrite) and Nitrobacter converts NO₂⁻ to NO₃⁻ (nitrate) — assimilation by plants (NO₃⁻ uptake), and denitrification by Pseudomonas and Thiobacillus (converting NO₃⁻ back to N₂, completing the cycle).

The key testable concept is the distinction between apoplast and symplast pathways (with the Casparian strip as the critical barrier), the mechanism of guttation (root pressure, not transpiration), and the roles of specific bacteria in nitrification (Nitrosomonas for NH₃ to NO₂⁻; Nitrobacter for NO₂⁻ to NO₃⁻).