Transpiration, Photosynthesis, and Respiration

® 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 CO2 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.

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. Although the chemical processes of photosynthesis and respiration are very different and involve different parts of the cell, they can be thought of as essentially opposite reactions. are heterotrophic, meaning that they must consume macronutrients, carbohydrates, proteins, and fats in their diets. Ultimately all these compounds are derived from plants. The ATP that animal cells use for energy comes from the process of respiration powered by the chemical energy of sugar, also derived from plants. Plant cells also use respiration to make ATP. This occurs all the time, day and night, even when the sun is not shining. Thylakoid membranes Although the chemical processes Contain chlorophyll and of photosynthesis and respiration are create large surface area for Chloroplast stroma very different and involve different light absorption Contains enzymes like parts of the cell, they can be thought of rubisco which fix CO2 as essentially opposite reactions (Figure into carbohydrate 5). Photosynthesis uses sunlight to Figure 6. Photosynthesis occurs in the chloroplast; the light reactions drive chemical reactions that are in the thylakoid membranes and the carbon reactions in the stroma. thermo dynamically unfavorable (requires energy to occur), while respiration is a thermodynamically favorable set of reactions (releases arranged in stacks (Figure 6). These energy ). The carbohydrate is ‘burned’ The Basics stacks are called thylakoid membranes in a controlled way to release the of Photosynthesis and are solar panels with a large sur- energy as ATP instead of just heat face area that organize chlorophyll and energy as would happen if it were just Photosynthesis literally means pigments called carotenoids that can ignited in air. The classic chemical “to put together with light.” All of the collectively absorb and utilize light reaction (CO2 + H2O  C6H12O6 + O2) reactions of photosynthesis happen energy (Figure 7). Photosynthesis has commonly written for this reaction inside chloroplasts. Chloroplasts are two distinct parts: is just a summary of many different small organelles that are green because chemical reactions. they contain chlorophyll. Mesophyll • Light reactions. The light absorp- cells in the leaves and stem contain tion part of photosynthesis is many chloroplasts, each having a referred to as the light reactions. It highly ordered array of membranes relies on energy from the sun, so it

Plants are classified based on how they complete photosynthesis. The dark reactions described above are found in more than 95 percent of the plants on Earth. They are called C3 plants because the first organic mol-

ecule that CO2 is incorporated into is a three-carbon molecule. Two variations of the carbon reactions have evolved in angiosperm plants as ways to get around the problem of photorespiration. They are:

• C4 photosynthesis. Plants with Figure 7. Chlorophyll (green) and other carotenoid pigments such as C4 photosynthesis include corn, lycopene (red), carotene (orange), and zeaxanthin (yellow) are found buffalograss, and many weedy in plants, and are used in the light reactions. The colors of the other grasses including crabgrass (Figure pigments are seen only in plant tissues without chlorophyll, such as 8). It is called C4 photosynthesis fruits or fall leaves after chlorophyll production has slowed or stopped because the first organic molecule and the colors of the other pigments are unmasked. that CO2 is incorporated into is a four-carbon malate molecule. These C4 plants minimize photo- occurs only during the day in the respiration and water loss through thylakoid membrane of the chloro- Photorespiration a specialized cellular architecture plasts. In the light reaction, water is in the leaves: light reactions occur in one cell type and the carbon re- split and oxygen released, but more In order to extract CO2 from importantly, it provides the chemi- the air, rubisco needs to have a actions occur in cells called bundle sheath cells not in direct contact cal energy to fix CO2 into carbohy- high affinity for it (CO2 makes up drate in the carbon reactions. only 0.04 percent of air). However, with the air. Plants with C4 photo- rubisco also binds significantly to synthesis are able to achieve high • Carbon or dark reactions. The rates of photosynthesis with their O2 gas which makes up a much carbon reactions occur in the higher percentage of air (21 percent). stomata only slightly open which matrix of the chloroplast called minimizes water loss. They often When rubisco binds O2, a wasteful the stroma and uses protein types process called photorespiration (not look better than C3 plants during called enzymes. The most im- to be confused with respiration) hot dry conditions because they portant enzyme in this process is occurs. Photorespiration diminishes are able to protect the plant from called rubisco. Rubisco is the most photosynthetic output because it high water loss by closing their abundant protein in plants and actually produces CO rather than stomata. therefore, the major consumer of 2 fixing it into carbohydrate. The • CAM photosynthesis. Plants with nitrogen. This is why when plants negative effects of photorespiration are deficient in nitrogen, they are crassulacean acid metabolism result from a plant’s reduced ability to (CAM) photosynthesis, such as not productive and turn yellow be- maintain a favorable CO gradient into 2 cacti, all succulents, and purslane, cause they stop photosynthesizing. the leaf air spaces. Most types of plants The dark reaction creates three car- minimize photorespiration (C3 plants, see below) respond to and water loss by keeping their bon sugar products that leave the increased photorespiration by opening chloroplast for use in respiration stomata completely closed during their stomata more to compensate the day so no water is lost (Figure and sucrose (common table sugar) for the unfavorable CO gradient. synthesis. The sucrose is converted 2 8). This also means they cannot This has severe consequences for C3 take in CO during the day. To get to starch or shipped out to other plants under hot or water restricted 2 parts of the plant for storage or around this, CAM plants take in conditions. CO through the stomata at night growth through the phloem. 2

© The Board of Regents of the University of Nebraska. All rights reserved. 7 and store it as a molecule called malate. Then during the day, with the sun shining and the stomata closed, rubisco coverts the malate to useful carbohydrate. Although this enables CAM plants to grow in extreme heat and be extremely water efficient, they have low photosynthetic productivity — they grow slowly!

Respiration

Respiration takes sugar either directly from photosynthesis or from breakdown of storage compounds like starch or lipids (oils), and uses its stored chemical energy to make energy currency (ATP). The whole process of respiration can be divided into several Figure 8. CAM plants are very slow growers. Because they grow in different steps. The first part is called water-limiting environments, they have adapted in other ways, out- glycolysis which literally means sugar side of their CAM photosynthesis, to conserve water. These adaptions splitting. This occurs in the cytoplasm include reduced or no leaves, light gray or green color to reflect light, of the cell and does not use oxygen vertical stems and leaves, and thorns for protection from predators. and produces a small amount of ATP. As a result, cacti, aloe, agave, and other CAM plants can go weeks Glycolysis also serves as the central with little to no watering. primary metabolic pathway on which most other secondary metabolic pathways depend. This means that crucial plant biomolecules such as proteins, lipids, starch, cellulose, DNA, RNA, chlorophyll, other pigments, plant hormones, and many others are all intricately related with metabolic flux through glycolysis. The other parts of respiration occur in specialized organelles called mitochondria. This is where the bulk of the ATP is released in processes requiring oxygen.

Plant Productivity and Respiration

During seed germination, seed storage proteins, carbohydrates, and lipids must all be broken down to support the germinating seedling, and aerobic respiration is a crucial Figure 9. Most plants, except aquatic plants, can only survive short part of these processes. When seeds periods of time underwater. Extended periods of low or no oxygen in the soil results in anaerobic respiration, root death, and eventually plant death. The dead turf areas indicate locations where water had stood for less than 72 hours. (Photo courtesy of Zac Reicher.)

8 © The Board of Regents of the University of Nebraska. All rights reserved. are planted in the spring, timing and it also results from an unfavorable temperature are important. First, balance between photosynthesis and Summary planting time should be past the respiration. As temperature and light danger of frost damage. Second, soil availability increase, photosynthetic Plant growth and development temperatures should be sufficiently output eventually plateaus because relies on water for transpiration, warm. The reason for this is that the chloroplasts have a finite light photosynthesis, and respiration. The respiration increases substantially absorption capacity. On the other unique ability of water to regulate with increased temperature. If the soil hand, the rate of respiration keeps temperatures, dissolve molecules of temperature is too cold, respiration increasing as it gets hotter, which life, and allow gas exchange, is essential will be too low to metabolize seed burns more and more carbohydrate. for all life on earth. Transpiration storage reserves, and the seed cannot The more respiration increases, the is essential for evaporative cooling, germinate (Figure 9). less net photosynthetic product there CO2 acquisition, maintaining plant is. The effect of high temperature on turgor, and mineral nutrient uptake. The fact that respiration increases respiration is most severe at night Photosynthesis converts CO2 into with temperature also has a profound when there is no photosynthesis. If simple carbohydrates. Respiration effect on adult plant productivity. nighttime temperatures are very high, releases energy obtained from When it is very hot, many plants all the carbohydrate made by the photosynthesis. Respiration also acts grow very slowly because of reduced plant during the day can be used up as a central metabolic hub, ultimately productivity. This usually results in respiration and there may be no net resulting in the complex organic from lack of water to support growth (Figures 10 and 11). molecules that form the basis of plants. transpiration and CO2 uptake. But In addition to carbon derived from

CO2, many of these complex carbon molecules also incorporate mineral nutrients acquired from the soil.

Figure 10. Cultural practices, such as scalping or mowing at lower than recommended mowing heights, will reduce photosynthetic rates because of the lack of green tissue available for photosynthesis. This practice will reduce the amount of carbohydrates stored because they will be directed toward new growth to repair the damaged turf. This will put unneeded stress on a turfgrass stand and may result in thin- ning or weed encroachment. Most turfgrasses in Nebraska should be mowed no lower than 2.5 inches. (Photo courtesy of Zac Reicher.)

Growth potential

Photosynthates used in respiration Increasing rate of life processes

Increasing temperature

Figure 11. The rate of photosynthesis and respiration generally increase as temperature increases. When the rate of photosynthesis exceeds the rate of respiration, plants grow. If respiration exceeds photosynthesis, then growth declines, photosynthate reserves are used, and plants become more susceptible to biotic and abiotic stresses. Optimal growth temperatures vary; cool-season plants, such as Kentucky bluegrass, tall fescue, broccoli, and radish prefer temperatures between 40-75°F. Warm-season plants, such as buffalograss, peppers, and tomatoes prefer temperatures between 65-90°F.

This publication has been peer reviewed.

UNL Extension publications are available on- line at http://extension.unl.edu/publications.