lecture 04

lecture 04 - 1/18/11 Plant Development and Transloca2on...

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Unformatted text preview: 1/18/11 Plant Development and Transloca2on BTNY 304 Some photographs were taken off various websites and are intended for instruc8onal purposes only Plant Development Stages Seedling – the first stage of development [email protected] seeds germinate. The cotyledons are present and usually ac2vely providing food for the growing plant. Vegeta2ve – the stage of rapid growth [email protected] the plant is established Reproduc2ve – the stage of floral and seed development and in the case of perennials, the enlargement and matura2on of storage and reproduc2ve structures. Maturity – the stage of slow growth. For annuals this is the final stage before death. For biennials and perennials, it is the preparatory stage to dormancy. For weed control purposes, it is important to understand the basic differences in seedling development between broadleaf plants and grasses. In both types, the root 2p (radical) emerges from the seed first followed by the shoot (stem) 2p. In broadleaf plants, the growing point of the shoot is completely above ground in a few days to a week. In grasses the growing point of the shoot does not emerge above ground level un2l several weeks [email protected] germina2on. Thus, broadleaf plant seedlings have growing points above ground early and can be exposed to herbicide applica2ons; whereas, grass seedlings have growing points at the base of the plant and protected from herbicide contact. Transport Pathways in Plants Materials essen2al for normal growth and development of plants must be transported from the point of entry (root or foliar uptake) to the site of u2liza2on. Two major transport systems Apoplast – major pathway for water and minerals , usually accounts for upward movement from the roots to leaves via the xylem 2ssue Symplast – major pathway for sugars, usually accounts for movement from the site of produc2on (leaves) to the site of u2liza2on (developing 2ssue) or site of storage (fruits or storage structures) via the phloem 2ssue 1 1/18/11 These two transport systems play an important role in the movement of externally applied materials such as herbicides or other pes2cides and plant nutrients. Common Barriers to Herbicide Movement Once a herbicide has penetrated the leaf or stem cu2cle or the root epidermis, there are s2ll many barriers to its movement to the site of ac2on. Some common barriers to prevent herbicide movement are: 1.  Compartmentaliza2on – the herbicide moves into a por2on of the cell not containing a site of ac2on (i.e. vacuoles) 2. Conjuga2on – chemically bonded (adsorbed) to cell components 3. Deac2vated – chemicals are broken down to non ­ac2ve compounds by metabolism. If the herbicide is not immobilized in the leaf or root, it is available for long distance movement in the plant by u2lizing the Xylem (apoplast) and/or the phloem (symplast) transport systems. In both systems herbicides dissolved in water move along with the mass flow of water throughout the plant. Some selec2vity (selec2ve mechanisms) between weeds and crops are a result of differen2al compartmentaliza2on or inac2va2on by metabolism. Long Distance Movement Xylem (apoplast) – Herbicides that enter the roots may move upward in the xylem with the water transport stream. The majority of preemergence herbicides enter the root at the root hair zone and moves to the xylem. Water moves in the xylem by two mechanisms: 1. Root Pressure – a minor mechanism. Must have high soil moisture and high rela2ve humidity for this to work. There must be an ac2ve transport of ions into the cells which builds up an osmo2c poten2al (gradient) across the plasma membrane that pulls the herbicides with the inflow of water. Once inside the endodermis, an ac2ve transport of ions is needed to deliver the herbicides back into the apoplast again suppor2ng the water flow in the xylem upward to the above ground plant parts. 2. Transpira2onal Pull – this is the most common type of xylem transport of water and dissolved substances. Transpira2on, the driving force for the movement of water and minerals, is purely a physical process. Transpira2on begins with evapora2on of water on the leaf surface. As water evaporate, more water moves to the leaf surface to take its place. This water is pulled up through the plant from the soil water. Water moves into the root hairs and then into the cortex to the center core of the vascular conduc2ng 2ssue by a passive process; no energy is needed. 2 1/18/11 Long Distance Movement Charged par2cles (minerals, N, P, K and some herbicides) can move through cell walls and intercellular spaces. But at some point they must be pumped across the plasma membrane using ATP pumps to get them into the cytoplasm of the endodermis. Once in, they are again pumped (using ATP) back across the plasma membrane of the living cells into the xylem system and up the plant into the leaves and other ac2vely growing points. Long Distance Movement Phloem (symplast) – The driving force for phloem movement is the mass flow of solutes (sugars) dissolved in water to move from regions of high solute concentra2on to regions of low solute concentra2on. This gradient is created by the synthesis and loading of sugars at the source and the u2liza2on of sugars at the sink, and is referred to as the source to sink concept. The symplast is the pathway by which substances (herbicides) applied to the foliage of plants move to the growing points in the above and underground structures of a plant. Fate of Herbicides in the Plant: To get into the leaf (unlike the root) the herbicide must overcome many leaf barriers on the leaf surface. Once through the leaf cu2cle, substances enter the living system by penetra2ng the plasma membrane and passing into the cytoplasm of cells, then from cell to cell to the phloem and then to the sink 2ssue. 1.  Inactivation: Oxidation, hydrolysis, reduction, deamination, demethylation and ring cleavage. (potentially by the same processes – e.g. 2,4DB activated to 2,4-D). 2.  Activation: 3.  Conjugation: With sugars (glucose) or glutathione. 4.  Compartmentalization: Moved to inactive sites such as the vacuole or the cell wall. 5.  Exudation: Moved outside the plant. 3 1/18/11 4 ...
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This note was uploaded on 04/03/2011 for the course BTNY 304 taught by Professor Staff during the Spring '08 term at Purdue.

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