WATER TRANSPORT IN PLANTS

An Overview of Transport in Plants

Transport Occurs on 3 Levels 1.Uptake of water and loss of water and solutes by cells. Aquaporins assist water transport. 2.Short-distance transport from cell to cell at the level of tissues and organs: loading sieve tube by companion cells. 3.Long-distance transport of sap within xylem and phloem at the level of the whole plant. 4.Selective channels aid in transport of K + or Na +. 5.Some channels are gated: Respond to specific stimuli. 6.Proton pump is the most important “active” transporter.

Differences in Water Potential Drive Water Transport Net uptake or loss of water by a cell occurs by osmosis. The Water Potential (  ) or psi of pure water in an open container is zero.  = 0 Water moves across a membrane from higher to lower water potential. Adding solute to water decreases . Physical pressure,  p, can push or pull water. Solute potential,  s, is proportional to solute concentration and also known as osmotic potential.  =  p +  s (  is usually a negative value)

Transport of Water Water and minerals enter root through root hairs osmosis: Two pathways toward center of root exist. 1. Apoplast pathway. (nonliving path): Water moves through cell walls never entering cells: 2. Symplast pathway (living path): Water moves from cell to cell via plasmodesmata: Water crosses endodermis to vascular cylinder (stele) of root via symplast pathway. Apoplast blocked by suberin in Casparian strip. Endodermal cells are selective: ex. Hold back Na +. Once through endodermis, water and minerals continue by apoplast to the xylem.

Three Mechanisms for Water Movement 1.Osmosis: Concentration gradient must exist: Two ways – Xylem removing H 2 O from root, and active concentration of minerals in the stele. At night, positive root pressure may develop forcing xylem sap upward. Guttation is the exudate of water droplets due to upward root pressure and reduced transpiration. 2.Capillary action due to adhesion – Minimal effect. 3.Cohesion-Tension Mechanism Transpiration – Evaporation of water from the leaves causes a negative pressure or tension in the leaves.

Cohesion: Hydrogen bonding of water molecules produces a single, polymer-like column of water in xylem, extending from roots to leaves. Bulk Flow: Occurs as water molecules evaporate from leaves. As a molecule of water evaporates by transpiration, the entire water column is pulled upward. The Sun causes transpiration to occur and is the driving force behind the ascent of sap through plants. Also termed transpiration pull. Bulk flow is mainly due to a low  p in the leaves.

Loss of Turgor and Wilting When Water Loss Exceeds Supply

Control of Transpiration The Photosynthesis – Transpiration Compromise Opening and closing of stomata influence gas exchange, transpiration, ascent of sap, and photosynthesis. Water loss is a trade-off of allowing CO 2 into the cell. C 4 plants can assimilate CO 2 faster than C 3 plants when stomata are partially closed and lose much less water. Transpiration delivers minerals to leaves and cools leaves ºC. When water enters guard cells, they expand unevenly, curving like a bananas. This opens the stoma. Stoma close when temperature are high, but open when CO 2 is low inside the leaf. Stomata are open during the day but close at night: CO 2 response.

Guard cells have thicker cell walls on the side facing the stoma. Microfibrils are radially arranged to help the cell bend one way when vacuoles are turgid.

Active transport of K+ into the guard cells and tonoplasts causes water to enter the guard cells. They swell and curve, opening the stoma.

Transport of Sugars Sugar is produced by the leaves and actively transported to the sieve tube members. Water enters sieve tube members and creates a positive pressure. Pressure moves sugar solution from source to sink. Sugars are removed from the sieve tubes at the sink maintaining the gradient. Creating starch removes sugar.

Aphids tap into phloem. “Honey dew” droplet is phloem sap minus absorbed nutrients. Drops can be continuously collected for study by scientists.