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Chapter 32: 1-11

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Transportation of Nutrients I

WebLecture Topics

Plant Nutrition

When we look at plant transport, we need to consider several different situations.

In order to manage all of these different transport requirements, plants use several mechanisms.

Getting Nutrients into the Plant

Root uptake from soil

Roots both anchor a plant in the ground and act as collectors for water and nutrients, which they absorb using osmosis and diffusion presssure. When concentrations in the plant root cells are less than concentrations in the soil, minerals and ions will diffuse into the plant. When water concentrations in the root cells are less than water concentrations in the soil, water will move into the root cells under osmotic pressure. When the soil contains more nutrients or water than the plant can use, the plant may suffer nutrient "burn" or get rid of excess water through leaf evaporation.

The roots do not simply take in nutrients without discretion. While small molecules and water may pass between cells to the root cortex where phloem and xylym vascular structures can carry them upward, ions and larger molecules enter the plant through the cell membranes and plasmodesmata openings. Regardless of the path, all nutrients must move through the Casparian strip, a waxy barrier that enforces passage across a plasma membrane into epidermal cells that can filter out unneeded nutrients or foreign materials, including bacteria and fungal cells. The Casparian strip not only regulates nutrient intake, it provides some protection from disease and infection as well.

Leaf uptake from air

Leaves have pores or stomata (singular: stoma) which can be controlled by guard cells. When the plant needs to conserve its own internal state, guard cell pairs on each side of a stoma preferentially dump K+ ions and, as a result, water moves by osmosis out of the guard cell so that it goes limp and closes the opening. If the plant needs to release oxygen, carbon dioxide, or water, the guard cells absorb K+ ions, and the increased concentration causes water to move into the cell, swelling and bending it to open the stoma. While plants can expel oxygen or excess water this way, they cannot simultaneously conserve carbon dioxide. Plants may use sensory inputs to time when they open their stomata, for example, opening these only at night to reduce water loss and increase carbon dioxide intake.

Moving Nutrients Around

Once nutrients have crossed the Casaprian barrier, or entered plant cells through the stomata, they can be transported using the vascular system (in ferns, gymnosperms, and angiosperms -- but in mosses, only cell-to-cell transport occurs).

Xylem transports water using root pressure, evaporation, and cohesion of water molecules

Xylem tubes transport water, using a combination of water's own cohesion (water molecules stick to water molecules), water's adhesion to the cell walls, and pressure differences. As roots absorb water from saturated soil, the incoming water molecules push water molecules already in the xylem tubes up. But this pressure is limited, because at the same time, the atmosphere is pushing down on the ground and the plant. Water rising by root pressure can only move up about 32 feet about ground level.

By manipulating the stomata, xylem and leaves can allow water and gas to escape from the xylem tubes at their leaf endpoints. This creates a partial vacuum in the tube that can suck water even higher. Because the lower water molecules are attracted by polar charges to the evaporating water molecules, they move up the xylem tubes, reducing air pressure downward and creating a force pulling upwards. Water can then rise much higher than the 32-foot limit of root pressure, carrying with it essential nutrients dissolved in the water.

Note that xylem flow is always upwards, from roots to stems to leaves.

Phloem tubes transport dissolved sugars

The second type of vascular tissue allows transport of sugars throughout the plant. Since the plant is producing sugars as a product of photosynthesis, or creating starches which it can then break down, it has "source" regions of sugar concentrations. Different parts of the plants may use these sugars as energy sources or store them for future use; the consumers and storers are sugar "sinks". Phloem moves sugars from source to sink locations, regardless of whether this means a flow upwards or downwards through the plant.

Again, the diffusion and osmosis pressures aren't quite enough to move sugars around the phloem system as rapidly as the plant needs to do this, so the xylem and phloem systems work together to provide the forces needed. Xylem continuously moves water upwards, increasing concentrations of nutrients in lower sugar-sink cells by removing water from them. The upward flowing water diffuses into high-concentration source cells in the leaves, and then into even higher concentration xylem cells carrying sugars away. This creates a circular path for some of the water as it moves from root to leaf (carrying nutrients) through xylem tubes, but then from leaf to root (carrying sugars) through phloem tubes.

Soil Fertility

The soil's ability to support plant growth depends on several factors. These include

Soil size and water content

Large soil particles and rocks provide pockets that capture water and air. The latter is important for organisms that live in the soil. Small particle soil like clay provides fewer pockets and can be densely packed, but these fine particles also provide surface layers that can latch onto metal ions of magnesium, calcium, and potassium, preventing them from draining away entirely into the bottom layers of soil.

Gardeners and farmers often supplement soil content with organic materials and fertilizers containing nitrogen, potassium, for nutrition, and lime to balance acidity levels. There is considerable debate over whether artificial methods or organic methods of soil conservation and care work better to produce healthy plants.

Soil layers

Soil usually occurs in layers that contain different levels and mixes of these factors. Topsoil has larger particles, including small rocks, the most living organisms, and humus or partially decayed organic material. This layer is nutrient-rich, but it is also exposed to erosion, and it dries out easily.

The middle layer of soil contains less organic material -- living or dead, but it also has smaller particles and is less subject to weathering. It may be nutrient rich in minerals, as these leach out of the topsoil when water drains downward.

The bottom layer above the bedrock contains mostly rock, and may serve as an aquifer if water is unable to drain further.