® EXTENSION Know how. Know now. EC1268 Plant Growth Processes: Transpiration, Photosynthesis, and Respiration David R. Holding, Assistant Professor Anne M. Streich, Associate Professor of Practice Extension is a Division of the Institute of Agriculture and Natural Resources at the University of Nebraska–Lincoln cooperating with the Counties and the United States Department of Agriculture. University of Nebraska–Lincoln Extension educational programs abide with the nondiscrimination policies of the University of Nebraska–Lincoln and the United States Department of Agriculture. © 2013, The Board of Regents of the University of Nebraska on behalf of the University of Nebraska–Lincoln Extension. All rights reserved. Plant Growth Processes: Transpiration, Photosynthesis, and Respiration David R. Holding, Assistant Professor Anne M. Streich, Associate Professor of Practice Knowledge of the basic plant The Life Giving Properties (Figure 1). Water has some unique growth processes, including photosyn- properties; it resists temperate changes, thesis, respiration, and transpiration, of Water dissolves molecules of life, and allows is important for gardeners and profes- gas exchange . All of these character- sional landscape managers to under- Water is all around us! Most of istics are essential for life on earth stand how the growing environment the Earth’s surface is covered in water. and they all depend on one chemical and management practices influence Plants and animals are mostly made property of water that few other liq- plant growth and development. Each of water and all the chemical reac- uids share: hydrogen bonding. Water of these plant growth processes relies tions of life take place in aqueous molecules have positive and negative on water to carry out their functions. solution inside plant and animal cells poles that make them bond to each Water beads up into round droplets because of cohesion of molecules (keeps leaf dry) Cell turgor is driven by large Waxy cuticle (prevents water-filled vacuole in all uncontrolled evaporation) plant cells (supports plant structure and cell growth) Upper epidermis cells (flat and transparent with Palisade mesophyll cells no chloroplasts) (channel light to spongy layer) Leaf air space has 100% humidity Vascular cells (bring continuous column of water molecules from roots, held Spongy mesophyll cells together by cohesion) (where most photosynthesis occurs) Lower epidermis cells (flat and transparent with Mesophyll cells covered in a no chloroplasts) microfilm of water molecules (allows gas exchange) Waxy cuticle Chloroplast Two guard cells form one stoma which can open and H2O diffuses out close by changes in cell turgor when stoma is open CO 2 diffuses in when stoma is open Figure 1. Cells are the fundamental unit of all living things. All plant cells contain the same basic makeup of a nucleus, cytoplasm, organelles, cell membrane, and a cell wall. Many water relationships exist within plant leaves. © The Board of Regents of the University of Nebraska. All rights reserved. 3 a b Figure 2. Water will bead up or form thin films depending on the nature of the surface. On hydrophobic surfaces, such as leaf surfaces, water beads up due to its cohesive characteristics (a). On hydrophilic or polar surfaces, such as the inner leaf surface of cells and root hairs, water spreads out to form a thin film. Products can be added to fertilizers and pesticides to lower surface tension of the water on the leaf and in the soil and increase adhesion. This flattens the droplet and allows for better absorption of the fertilizer, pesticide, or water, similar to what is observed on hydrophilic surfaces (b). other temporarily in a process called molecules that plants and animals between the air and the inner sur- hydrogen bonding. The unique physi- need for life are dissolved in water. face of leaf cells. Mesophyll cells cal properties of water allow it to do Small molecules like carbon diox- in leaves are the primary loca- the following functions in plants: ide (CO2) and oxygen (O2) must tion of photosynthesis. In the leaf dissolve in water to enter or leave air spaces, each mesophyll cell is • Regulate Temperature. Water is plant or animal cells; mineral nu- covered in a thin film of moisture resistant to temperature changes trients in the soil must dissolve in allowing water and oxygen to leave and stays in the liquid form over a water to be taken up passively by the cells and carbon dioxide to en- broad range (from 0°C to 100°C plant roots; medium-sized mol- ter the cells. or 32°F to 212°F). Large bodies, ecules needed for plant growth, like oceans, are the most stable such as sugars, amino acids, ATP and are able to resist extreme (adenosine triphosphate) and Water and Transpiration temperature changes; lakes, rivers, hormones, easily dissolve in the streams and puddles are increas- water making up plant and animal Transpiration is the movement of ingly less resistant. The same cells; and large macro-molecules, liquid water into, through, and out of size ratio applies to living things; like DNA, protein and complex the plant (Figure 3). Water lost through elephants and giant sequoia trees sugars, are covered in positive (+) transpiration enters the plant through are very good at resisting tem- and negative (–) charges and can the roots, moves up through the stem perature change, while mice and be surrounded and dissolved by in the xylem, and exits through open- small plants have to work harder charged water molecules. ings in the leaf called stomata. Cohe- to keep a stable temperature. As sion and adhesion create the property water evaporates from the leaves • Allow Gas Exchange. Cohesion, of capillarity, which allows water mol- of plants, heat energy is lost and or sticking of water molecules to ecules to rise up against the forces of the plant cools down. each other, combined with adhe- gravity. This works only as long as the sion, sticking of water molecules water is constrained in tubes with a • Dissolve Molecules of Life. Water to polar surfaces, allows water to large surface area. Surface area is what is one of the most versatile sol- form very thin films (Figure 2). the xylem tissue of plants provides, lots vents for dissolving the molecules This is essential for gas exchange of life. Most of the small and large of very narrow interconnected tubes. 4 © The Board of Regents of the University of Nebraska. All rights reserved. Environmental Change Transpiration Response Reason b Light Transpiration Light causes most stomata to open Temperature Transpiration Warm air hold more moisture Soil water Transpiration Less water enters the plant roots Wind Transpiration Reduces humid boundary layer around the leaf Humidity Transpiration Air moisture gradient is not as steep Figure 3. The rate of transpiration is affected by several environmental conditions. The xylem tissue of vegetative plants their tissues do not overheat. On because less CO2 enters the leaves. or the lignified (woody) tissue of trees which surface would you rather is made of the cell wall remnants of play soccer on a 100°F day — grass • Maintaining turgor. Since 90 elongated cells that the plant sacrificed or artificial turf? percent of plant tissues constitute through a process called programmed water, the structure of plant tissues cell death. Transpiration is essential • CO2 acquisition. All the carbon depends on cell turgidity and since for: incorporated into carbohydrate plant cells are leaky, water needs through photosynthesis comes to be continually taken up (think • Evaporative cooling. Plants are from atmospheric CO2 entering of a plant cell like an inflated tire able to keep cool when they are in through pores in the leaves called with a puncture). Cell expansion, direct sunlight through the evapo- stomata. Water loss through the a driving force of growth, is also ration of water that occurs in tran- stomata is a continuous process driven by cellular water pressure. spiration. As water changes from that occurs as long as stomata the liquid to gas phase, heat energy are open. Plants are able to close • Mineral nutrient uptake. In addi- is lost and the plant is cooled. their stomata to restrict water tion to carbon assimilation from Plants rely on transpiration for loss during times of drought or the air, plants incorporate mineral evaporative cooling so that despite high temperature, but this directly nutrients dissolved in water taken being exposed to direct sunlight, reduces photosynthetic output up from the soil. These are distrib- uted throughout the plant by way of the transpiration process. The Role of Photosynthesis Carbon Dioxide and Respiration in Energy Generation in Plants Oxygen All life on earth depends on plants. Plants are autotrophic, meaning they can convert simple molecules like CO2 from the atmosphere and minerals from the soil into the complex carbohydrates, proteins, and fats, forming the basis of living organisms. The most important set of chemical reactions in plants harness Nutrients the energy of sunlight in the process and Water of photosynthesis which generates sugar, oxygen, and a molecule called Figure 4. Photosynthesis uses water and mineral nutrients from ATP (Figure 4). ATP is energy in the soil, CO2 from the air, and light energy from the sun to create its simplest form and powers the photosynthates (sucrose and starch) used in respiration or are chemical reactions that support life in stored. Oxygen is a byproduct of the light reactions in photosynthesis. both plant and animal cells. Animals © The Board of Regents of the University of Nebraska. All rights reserved. 5 Photosynthesis Respiration Occurs in chloroplasts Occurs in mitochondria and cytoplasm Uses light energy (sun) Uses chemical energy (ATP, NADPH) Uses low energy, unreactive CO2 Uses high energy, reactive carbohydrate Building process Breaking-down process Uses H2O Produces H2O O2 is released from splitting of water Uses O2 Produces carbohydrate (sugar) Produces CO2 Occurs only under light (sun or artificial) Occurs with and without light Figure 5.