2.1 Water Relations of the Cell

Water potential (Ψw) = solute potential (Ψs) + pressure potential (Ψp). Ψs is always negative; Ψp is positive in turgid cells, zero at incipient plasmolysis, and can be negative (tension) in xylem during transpiration. Pure water Ψw = 0. Water moves from high Ψw to low Ψw. Osmosis requires a semipermeable membrane. Plasmolysis (hypertonic solution) and deplasmolysis (hypotonic solution) are reversible. Imbibition is adsorption of water by colloidal systems — no membrane required.

2.2 Absorption Pathways

Two pathways exist for radial transport of water across the root cortex:

• Apoplast: cell walls and intercellular spaces; fast; non-selective; no membrane crossing; blocked by Casparian strip at endodermis.
• Symplast: cytoplasm connected via plasmodesmata; slow; selective; crosses plasma membranes; continuous through endodermis.

The Casparian strip (suberin bands in endodermal cell walls) is the mandatory filter point for all mineral absorption.

2.3 Root Pressure and Guttation

Root pressure is generated osmotically in root xylem when transpiration is low. Guttation = liquid water lost through hydathodes at leaf margins, driven by root pressure, occurring at night or early morning. Contrast with transpiration = water vapour lost through stomata, driven by cohesion-tension, occurring mainly during the day.

2.4 Long-Distance Water Transport

Cohesion-tension theory (Dixon and Joly): transpiration creates tension in xylem → cohesion between water molecules (H-bonds) + adhesion to xylem walls maintains continuous column. This is the main ascent-of-sap mechanism. Transpiration routes: stomatal (90–95%), cuticular (5–10%), lenticular (negligible). Guard cells control stomatal aperture via $K^{+}$ ion flux and ABA signalling.

2.5 Essential Elements and Criteria

17 essential elements total: 9 macro (C, H, O, N, P, K, Ca, Mg, S) + 8 micro (Fe, Mn, Zn, Cu, Mo, B, Cl, Ni). Criteria (Arnon & Stout): absolutely necessary, non-substitutable, directly involved in metabolism. Mobility determines symptom location: mobile elements (N, P, K, Mg) — older leaf symptoms; immobile elements (Ca, Fe, S, Mn, B) — younger leaf symptoms.

2.6 Deficiency Diseases

2.7 Nitrogen Fixation

Nitrogenase (Mo-Fe protein) catalyses $N_{2}$ → $NH_{3}$ using 16 ATP. Irreversibly inactivated by $O_{2}$. Leghemoglobin in legume nodules scavenges $O_{2}$. Key organisms: Rhizobium (symbiotic, legumes), Frankia (symbiotic, Alnus), Azotobacter (free-living aerobic), Clostridium (free-living anaerobic), Anabaena/Nostoc (cyanobacteria, in heterocysts).

2.8 Nitrogen Cycle

Fixation → Nitrification (Nitrosomonas: $NH_{3}$→$NO_{2}^{-}$; Nitrobacter: $NO_{2}^{-}$→$NO_{3}^{-}$) → Assimilation → Denitrification (Pseudomonas: $NO_{3}^{-}$→$N_{2}$).