Transport of Material in Plants

Internal Transport in Plants Small plants rely on simple diffusion or branching tubules to transport material throughout their bodies Larger plants rely on a system of vascular tissue (xylem and phloem) to perform this function

Movement of material is relatively slow Xylem tissue carries water and dissolved minerals up from the roots to the top of the plant Water is lost through transpiration and evaporation from the leaves Phloem tissue distributes substances in all directions within the plant

Movement of Water Tall trees can reach more than 100 m in height and can lose thousands of litres of water per day through transpiration (the loss of water through the surfaces of a plant – mostly through the stomata of leaves) and evaporation Scientists have a developed a number of theories to explain how plants can move water such large distances and in such large amounts

Bulk/Mass Flow Capillary action plus adhesion and cohesion theories can only partially explain the movement of water through plants Capillary action is the ability of a substance to draw a substance up against gravity. Adhesion is when water molecules are attracted and stick to other substances (such as glass tubes or the inside of tracheids or vessel members) Cohesion is the result of water molecules that like to stay closely together (due to hydrogen bond attraction)

Capillary Action

Capillary Tubes and Movement of water The narrower the tube the greater the capillary action and the greater the column of water

Several processes are involved in the movement of water in a plant besides capillary action Water evaporates from the inner leaf cell walls into the air spaces and out the stomata when they are open (transpiration) This water loss creates a reduced water pressure in the leaves and water flow upwards from the roots where relative pressure is higher

Water loss in the leaves is replaced by bulk flow from the roots When water moves upwards from the roots, the cell membranes of the root cells allow more water and dissolved minerals to enter by osmosis, diffusion and/or active transport

Translocation Movement of the products of photosynthesis throughout the plant is the result of a process called translocation The most accepted explanation of translocation is the pressure-flow hypothesis Fluids flow from an area of higher pressure toward an area of lower pressure Carbohydrates will flow from their source (where they are made) to their sink (where they are stored or used). The driving force is a positive pressure gradient from source to sink

Factors Affecting Plant Growth Internal Factors – Hormones Plants use hormones to coordinate their growth in response to external factors such as light, gravity, nutrients, and competition, or in response to internal factors such as the need for repair or reproduction

Plant Hormones There are three groups of plant hormones: auxins, gibberellins and cytokinins Two hormones are not classified in these groups: abscisc acid and ethylene

Auxins Causes elongation of cells When auxins in the tip of shoots are exposed to light auxins move down the stem causing the cells of growing stem to elongate. When stems are exposed to light auxins move to the shaded side of the stem resulting in uneven cell elongation causing the stem to bend

In root cells, auxins cause roots to grow away from sunlight

Gibberellins Like auxins promote cell division and elongation in plant shoots Can cause stem to elongate just before flowering (called bolting) to produce a long stem raising flower up to pollinators Used commercially to stimulate seed germination Control fruit development

Bolting of Stem from Gibberellins

Cytokinins Stimulate cell division and mesophyll growth Mostly concentrated in endosperm tissue and young fruit Used in tissue cultures to allow the production of many new plants genetically identical

Ethylene Has a significant role in fruit ripening Affects the colour, texture and sugar content or fruit Ethylene can be applied externally to cause fruit to ripen

Abscisic Acid A growth regulator that usually acts an inhibitor Promotes closure of stomata, induces seed and bud dormancy Provides resistance to water stress in order to survive harsh conditions

Tissue Cultures using Cytokinins

Tropisms Plant growth in response to external stimuli are called tropisms Positive tropisms are growths toward the stimulus Negative tropisms are growths away from the stimulus Phototropism: the bending and growth of a stem toward a light source

Gravitropism (geotropism): growth response to gravity (stems show negative and roots show positive gravitropism) Thigmotropism – respond to barriers Chemotropism – responds to certain chemicals in environment Turgor rapid movement in response to touch through rapid changes in turgor pressure

Thigmotropism: respond to touch Chemotropism: response to certain chemicals in the environment Turgor Responses: rapid movement in response to touch stimuli through rapid changes is turgor pressure