Plant Water Relation

Plants are mostly water. A lettuce is 95% water. A tree can pull hundreds of liters of water from the soil and lift it to its highest leaves every day — all without a heart, without muscles, without any moving parts. How? That's the question this chapter answers. It's the story of water's journey through a plant: from the soil into root hairs, through the root cortex to the xylem, up the stem against gravity, and finally out of the leaves as vapor. And every step is driven by physics — water potential, osmosis, cohesion, and transpiration. Let's start with water potential — the driving force for water movement. Water always moves from areas of high water potential to areas of low water potential. Water potential is measured in pressure units (megapascals or bars). Pure water at standard conditions has a water potential of zero. Adding solutes lowers it (makes it more negative). Applying pressure increases it. Plant cells have negative water potential because they contain dissolved solutes. Soil water typically has higher water potential than roots, so water moves from soil into roots. The key components are solute potential (always negative) and pressure potential (turgor pressure, usually positive). Water enters root hairs through osmosis. Root hairs are single-celled extensions of epidermal cells that massively increase surface area for absorption. They grow into spaces between soil particles and contact water films. The root hair cell sap has a higher solute concentration than soil water, so water moves in by osmosis. From the root hair, water travels through the root cortex to the xylem. There are two pathways: the apoplast pathway — water moves through cell walls and intercellular spaces (fast, no membrane crossing); and the symplast pathway — water moves through cytoplasm via plasmodesmata (slower but regulated). At the endodermis, the Casparian strip (a band of suberin) blocks the apoplast pathway, forcing all water to enter the symplast. This is a critical checkpoint ensuring the plant controls which solutes enter the xylem. Once in the xylem, water must travel upward — sometimes 100 meters or more. The driving force comes from the top. As water evaporates from leaves (transpiration), the water column in the xylem is pulled upward. This is the cohesion-tension theory. Water molecules are cohesive (they stick together via hydrogen bonds) and adhesive (they stick to xylem vessel walls). Transpiration creates tension at the top, and because the water column is continuous and cohesive, this tension pulls water up from the roots. Transpiration is the loss of water vapor from plant surfaces, primarily through stomata. It's a necessary evil — necessary because it drives water and mineral transport and cools the plant, but evil because the plant loses precious water. Over 90% of water absorbed by roots is lost through transpiration. Stomata regulate this balance. Each stoma is flanked by two guard cells. When guard cells take up water and become turgid, the stoma opens; when they lose water, it closes. Potassium ion movement controls this — K+ enters guard cells, water follows by osmosis, guard cells swell, stoma opens. Mineral nutrition goes hand in hand with water transport. Essential elements are divided into macronutrients (needed in large amounts: C, H, O, N, P, K, Ca, Mg, S) and micronutrients (needed in trace amounts: Fe, Mn, Cu, Zn, B, Mo, Cl, Ni). Nitrogen fixation converts N2 to ammonia — done by Rhizobium bacteria in legume root nodules or free-living bacteria like Azotobacter.