Entry Five: Resource Acquisition and Transport in Plants

Land plants typically inhabit two worlds – above ground, where their shoot systems acquire sunlight and carbon dioxide, and below ground, where their root systems acquire water and minerals. Without adaptations that allow acquisition of these resources, plants could not have colonized land. As land plants evolved and increased in number, however, competition for light, water, and nutrients intensified. Natural selection favors plants capable of efficient long-distance transport of water, minerals, and products of photosynthesis. We shall examine in this entry how the basic architectural design of shoots and roots helps plants move water and other nutrients throughout the organism.

Shoot Architecture and Light Capture:

In shoot systems, stems serve as supporting structures for leaves and as conductors in the transport of water and nutrients. Stems also contribute in the amount of light capture needed for photosynthesis. The arrangement of leaves on a stem, known as phyllotaxy, and leaf orientation is an architectural feature of significant importance in light capture. The height of shoots and their branching patterns are two other architectural features affecting light capture. Branching generally enables plants to harvest sunlight for photosynthesis more effectively.

Root Architecture and Acquisition of Water and Minerals:

Soil contains resources mined by the root system. The evolution of root branching enabled land plants to more effectively acquire water and nutrients from the soil. Plants can rapidly adjust the architecture and physiology of their roots to exploit patches of available nutrients in the soil. The roots of many plants, for example, may respond to pockets of low nitrate availability in soils by extending straight through the pockets instead of branching within them.

The Plant Tissue and the Movement of Substances:

Plant tissues have two major compartments: the apoplast and symplast. The apoplast consists of everything external to the plasma membranes of living cells and include cell ways, extracellular spaces, and the interior of dead cells such as vessel elements and tracheids. The symplast consists of the entire mass of cytosol of all the living cells in a plant, as well as the plasmodesmata, the cytoplasmic channels that interconnect them. The compartmental structure of plants provides three routes for transport within a plant tissue or organ: the apoplastic, symplastic, and transmembrane routes. In the apoplastic route, water and solutes move along the continuum of cell walls and extracellular spaces. In the symplastic route, water and solutes move along the continuum of the cytosol. Substances must cross a plasma membrane before moving from cell to cell via the plasmodesmata. In the transmembrane route, water and solutes move out of one cell, across the cell wall, and into the neighboring cell. This route requires repeated crossings of plasma membranes.

Transpiration:

Although all living plant cells absorb nutrients across their plasma membranes, the cells near the tips of roots are particularly important because most of the absorption of water and minerals occurs there. Water and minerals that pass from the soil into the root cortex cannot be transported to the rest of the plant until they enter the xylem of the vascular cylinder, or stele. After passing into the vascular cylinder, the xylem sap, the water and dissolved minerals in the xylem, gets transported long distance by bulk flow (movement of liquid in response to a pressure gradient that is faster than diffusion or active transport) through the tracheids and vessel elements and into the veins that branch throughout each of the leaf. The process of transporting xylem sap, however, involves the loss of a significant amount of water by transpiration, the loss of water vapor from leaves and other aerial parts of plants. The question remains: Was the xylem sap pushed upwards from the roots, or is it mainly pulled upward? Well, water that flows in from the root cortex generates root pressure, a push of xylem gap; however, root pressure is a minor mechanism driving the ascent of xylem sap. According to the cohesion-tension hypothesis, transpiration provides the pull for the ascent of xylem sap, and the cohesion of water molecules transmits this pull along the entire length of the xylem from shoots to roots. Hence, xylem sap is normally under negative pressure.

Translocation: 

Transpiration cannot meet all the long-distance transport needs of the plant. The flow of water and minerals from the soil to roots to leaves is largely in a direction opposite to the direction necessary for transporting sugars from mature leaves to lower parts of the plants, such as root tips that require large amounts of sugars for energy and growth. The transport of the products of photosynthesis, or translocation, is carried out by the phloem. In angiosperms, the specialized cells, in the phloem, that conduct translocation are the sieve-tube elements. Arranged end to end, they form long sieve tubes. Between these cells are sieve plates, structures that allow the flow of sap along the sieve tube. Phloem sap, the aqueous solution that flows through sieve tubes, differs from the xylem sap that is transported by tracheids and vessel elements. Phloem sap contains primarily sugar, as well as amino acids, hormones, and minerals. Unlike the transport of xylem sap from roots to leaves, phloem sap moves from sites of sugar production to sites of sugar use or storage. A sugar source is a plant organ that is a net producer of sugar, by photosynthesis or by breakdown of starch. A sugar sink is an organ that is a net consumer of depository of sugar. Growing roots, buds, stems, and fruits are sugar sinks. Sinks usually receive sugar from the nearest sugar sources. So, what is the mechanism for translocation? Researchers have concluded that phloem sap moves through the sieve tubes of angiosperms by bulk flow driven by positive pressure, known as pressure flow.