12: Transport
- Page ID
- 123962
The structure of plant roots, stems, and leaves facilitates the transport of water, nutrients, and photosynthates throughout the plant. The phloem and xylem are the main tissues responsible for this movement. Water and nutrients are absorbed from the soil by roots ands transported through the xylem. Much water is lost through the stomata in the leaves, and plants have a variety of adaptations to reduce water loss (Figure \(\PageIndex{1}\)). The products of photosynthesis move through the phloem from sources to the tissues and organs that need them. These mechanisms of transport allow plant organs to specialize because they can export excess substances and import what they do not produce or collect locally.
Attribution
Curated and authored by Melissa Ha using 30.5 Transport of Water and Solutes in Plants from Biology 2e by OpenStax (licensed CC-BY). Access for free at openstax.org.
- 12.1: Water Transport
- Most plants secure the water and minerals they need from their roots. The path taken is: soil→roots→stems→leaves. The minerals travel dissolved in the water (often accompanied by various organic molecules supplied by root cells), but less than 1% of the water reaching the leaves is used in photosynthesis and plant growth. Most of it is lost in transpiration, which serve two useful functions: providing the force for lifting the water up the stems and cools the leaves.
- 12.2: Water Potential
- Water potential is a measure of the potential energy in water. Water moves from high water potential to low water potential, which drives the flow of water in the plant.
- 12.3: Transpiration
- Transpiration refers to the loss of water vapor through plant stomata, mainly in the leaves. Hot, dry, and windy conditions increase transpiration rate.
- 12.4: Adaptations to Reduce Transpiration
- Adaptations to reduce transpiration rate in dry environments include a thick cuticle, trichomes, and succulence. Summer deciduous plants loose their leaves when hot, dry conditions would otherwise cause too much water loss. C4 and CAM photosynthetic pathways also help conserve water while limiting photorespiration.
- 12.5: Stomatal Opening and Closure
- Stomata are pores in the epidermis of the plant, mostly found in leaves. Blue light triggers stomatal opening, and water stress triggers stomata closure. Whether a stoma is open or closed is determined by the turgor pressure of the two guard cells that surround it.
- 12.6: Cohesion-Tension Theory
- According to the cohesion-tension theory, the main force that drives water up a plant is transpiration and cohesion and adhesion of water in the xylem.
- 12.7: Water Absorption
- Root hairs increase the surface area of roots to increase water absorption. Before entering the root vascular tissue, water and minerals pass through endodermal cells. The endodermis is reinforced by the Casparian strip, which prevents substances from moving between the cells. Water may move through an apoplast, symplast, or transmembrane pathway.
- 12.8: Translocation (Assimilate Transport)
- Translocation is the movement of the substance in the phloem (assimilate). Phloem loading may be apoplastic or symplastic. The pressure-flow hypothesis explains how translocation is driven by phloem loading and unloading and the resultant osmosis of water.
Thumbnail image: A potometer is used to measure transpiration rate, the loss of water through the stomata. Image by Theresa Knott and Rachel Knott (CC-BY-SA).