Improving Photosynthetic Efficiency in Sports Turf

By Jeff Haag

Jeff Haag is a native of Crestline, Ohio, and a graduate of Defiance College with a Bachelor of Science degree. He has been in the turf grass profession for the past 25 years, while serving as the sports turf manager/golf course superintendent at Bowling Green State University, and assistant sports turf manager at the University of Louisville. He currently serves as the sports turf specialist at John Carroll University in University Heights, Ohio.
Jeff has published four scientific articles pertaining to the science involved in maintaining intensively manicured turf grass for both sports turf and golf course turf; Preventing Summer Stress, Preventing Turf Grass Cell Damage, Grow playable healthy turf: factors that can damage chloroplasts, and methods to help prevent them, and Playing Defense verse Free Radicals.

• Language: English
• Publisher: Xlibris US
• Release date: Feb 6, 2013
• ISBN: 9781479787555

Jeff Haag

Jeff Haag is a native of Crestline, Ohio, and a graduate of Defiance College with a Bachelor of Science degree. He has been in the turf grass profession for the past 25 years, while serving as the sports turf manager/golf course superintendent at Bowling Green State University, and assistant sports turf manager at the University of Louisville. He currently serves as the sports turf specialist at John Carroll University in University Heights, Ohio. Jeff has published four scientific articles pertaining to the science involved in maintaining intensively manicured turf grass for both sports turf and golf course turf; “Preventing Summer Stress”, “Preventing Turf Grass Cell Damage”, Grow playable healthy turf: factors that can damage chloroplasts, and methods to help prevent them”, and “Playing Defense verse Free Radicals”.

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Copyright © 2013 by Jeff Haag.

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Table of Contents

How Photosynthesis Works

Difference Between C3 and C4 Turf

Turf Stress

Reactive Oxygen Species

Types of Reactive Oxygen Species (Free Radicals) in Turf

Causes of Reactive Oxygen Species in Sports Turf

Managing Reactive Oxygen Species in Turf

Introduction

The photosynthetic process is extremely complex, and involves cooperative measures from each step of the process to be completed properly. In between these steps many things can go array that can limit, or, destroy the process, such as drought, photo-inhibition, freezing temperatures, disease, reactive oxygen species(free radicals), etc., and can ultimately lead to turf grass cell death.

I chose to write this book because I not only want to present the knowledge on photosynthesis from academia, but also to combine academia with everyday experiences that I have had dealing with the photosynthetic process over the past 25 years as both a sports turf manager and golf course superintendent.

This book includes strategies that I have used to enhance photosynthetic efficiency over the past 25 years with great success. It involves both C3(Cool-Season Turf) and C4(Warm-Season Turf), which do have differences, although, almost all Warm-Season sports turf is overseeded with Cool-Season turf, predominately perennial rye grass, for both playability and aesthetic reasons before the Warm-Season sports turf goes dormant during the winter months.

Photosynthesis is a relatively inefficient process, with only a maximum of 8% to 10% of the energy in sunlight being converted to the chemical energy in reduced sugars (Long et al., 2006; Zhu et al. 2010). Further considering carbon losses from autotrophic respiration and limitations by other factors such as water and nutrient limitations, realized conversion efficiencies are typically just 2% to 4% of the energy received in sunlight (Long et al., 2006; Zhu et al. 2010). Therefore, it has been a long-standing aim to increase the photosynthesis of plants to achieve greater conversion efficiencies of available sunlight (Reynolds et al., 2000; Sinclair et al., 2004; Long et al., 2006; Zhu et al., 2010).

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How Photosynthesis Works

Photosynthesis is one of the most important chemical processes in life

A direct connection exists between quantum physics, light and life. It is called photosynthesis and it is essential to almost all life on Earth. There are organisms that exist outside of the photosynthesis loop, but they are comparatively few and rare in comparison to the biomass dependant on photosynthesis.Photosynthesis is a chemical process that is triggered by a fundamental law of physics; the photoelectric effect. As everything is made up of atoms and sub-atomic particles, it is little wonder that chloroplasts and chlorophyll in turf plants utilize an important factor in quantum physics. What is a wonder is that Mother Nature occurred to exploit the photoelectric effect as a source of energy. The fact that most leaves reflect the green wavelength of light should tell us something. The same leaves absorb the blue and red wavelengths of light which just happen to be the types of wavelengths that will trigger hydrogen to lose an electron. These wavelengths are 656.28 nanometers for hydrogen red alpha and 486.13 nanometers for hydrogen blue beta. Hydrogen atoms will absorb and release only light in the blue, ultraviolet and the red end of the electromagnetic spectrum. Once the photon of the correct wavelength contacts the electron shell of the water molecule, it ionizes the molecule and breaks it down in a process called reduction, releasing oxygen molecule and the free radical hydrogen proton is then captured by a carbon dioxide molecule in the process of being converted to sugar. There are four discrete stages in the process of photosynthesis. The first one has been described and this is the stage where high energy molecules are assembled. This is called the Light Reaction stage of photosynthesis.

The process of absorbing certain wavelengths also tells us why photosynthesis is only about six percent efficient. All other wavelengths are either reflected or pass through. This is consistent with the photoelectric effect. Oxygen and carbon, which also play a part in photosynthesis, have many more wavelengths that can be emitted and absorbed. The process of photosynthesis uses only one percent of the electromagnetic spectrum and two percent of the visible electromagnetic spectrum.

The next stage is light independent and it is called the Calvin-Bensen cycle or the Dark Reactions. In this stage, the high energy molecules help to chemically reduce carbon dioxide. This is necessary in order to create the precursors to carbohydrates. During the Light Reaction phase, the pigment chlorophyll absorbs a photon and loses an electron. This electron is passed to a modified form of chlorophyll called pheophytin, which then passes the electron to a quinone molecule, which allows the start of a flow of electrons down an electron transport chain that leads to the ultimate reduction of NADP to NADPH (Nicotinamide adenine dinucleotide phosphate). A proton gradient is created across the chloroplast membrane. The dissipation of the proton is used by ATP synthesis to make ATP (adenosine triphosphate). The chlorophyll molecule regains an electron by taking it from water and releasing an oxygen molecule (O2). The common equation for the chemical reaction of photosynthesis is;

6 CO2 + 12 H2O + photons → C6H12O6 + 6 O2 + 6 H2O

which can also be written as;

carbon dioxide + water + light energy → glucose + oxygen + water

The light independent part requires carbon dioxide, which is captured from the atmosphere. The enzyme, Ribulose-1,5-bisphosphate carboxylase/oxygenase catalyzes the first cycle of carbon fixation. This step is crucial in the manufacture of sucrose, an energy molecule necessary for the fueling of biological processes. This is part of the Calvin Bensen Cycle and three types of molecules are made to make carbon based sugars and starches. This particular protein is considered the most important one on Earth and it is found in all plants and animals. The manufacture of carbohydrates is necessary in the production of cellulose that figures so importantly in cell walls.

The final phase is carbon fixation and the export of stable chemical products to the rest of the cell or for transport to other cells in a complex organism like a tree. These chemical products are used for the growth of new cells, the repair of existing ones or for the eventual production of seeds to propegate the next generation. These final reactions take a millisecond to a second to complete.

Most plants are photoautotrophs, which mean that they can synthesize food directly from inorganic compounds using the parts of the electromagnetic spectrum to drive the process. Water is used as the reducing agent in order to provide the energy to drive the whole process. Thus most light driven plants literally use appropriate photons to drive the whole process of life. These plants are everything from algae, cyanobacteria, annual plants and perennials such as trees and shrubs.

Chlorophyll is contained in organelles which the plant moves to the light source of sunward side of the leaf. The cells in the interior tissues of a leaf, called the mesophyll, can contain between 450,000 and 800,000 chloroplasts for every square millimeter of leaf. This maximizes the potential for the capture of photons and for the creation of the surplus of energy required for the creation of sugars, starches and the eventual creation of protiens. Photosynthesis is affected by the quantity of light, the temperature and the availability of trace elements that assist in plant growth and the production of chlorophyll. There are many complex processes involved in turning light into useful protiens and starches, but the foundation is light of specific wavelengths that causes electron emission.

Photosynthetic Quantum Yield

For well over half a century, it has been known that the energy conversion efficiency of incident photons to chemical energy by leaves is wavelength dependent (Hoover, 1937). This is due to several processes that can be divided into two classes. First, the absorption of incident irradiance by a leaf is wavelength dependent due to the different absorptance spectra of the different leaf pigments. Second, even on an absorbed light basis, different wavelengths have different quantum yields for CO2 fixation or O2 evolution: Red light (600 to 640 nm) has the highest quantum yield, whereas blue and green light (400 to 570 nm) are considerably less efficient in driving photosynthesis (McCree, 1972b;Inada, 1976; Evans, 1987). Maximum quantum yields for C3 leaves were found to be close to 0.093 mol CO2 fixed (Long et al., 1993) or 0.106 mol O2 evolved (Björkman and Demmig, 1987) per mol absorbed photons.

Three major causes for the wavelength dependence of the quantum yield for absorbed photons have been identified (i.e., absorption by photosynthetic carotenoids, absorption by nonphotosynthetic pigments, and an imbalanced excitation of the two photosystems) (Terashima et al., 2009). Photosynthetic carotenoids have absorption maxima for blue wavelengths and differ in their efficiency (35 to 90%) for excitation energy transfer to chlorophylls, depending on the type of carotenoid and its position within the photosynthetic apparatus, whereas the energy transfer efficiency in the antenna complexes from chlorophyll to chlorophyll is 100% (Croce et al., 2001; de Weerd et al., 2003a, 2003b; Caffarri et al., 2007). Nonphotosynthetic pigments, such as flavonoids and free carotenoids, also absorb light, predominantly in the UV region but also in the blue and green part of the spectrum (e.g., anthocyanins). Nonphotosynthetic pigments do not transfer any absorbed energy to the photosynthetic apparatus. Finally, the pigment composition and absorbance properties differ for photosystem I (PSI) and photosystem II (PSII); consequently, the balance of excitation between the two photosystems is wavelength dependent (Evans, 1986,1987; Chow et al., 1990; Melis, 1991; Walters and Horton, 1995). Any imbalance in excitation of the two photosystems results in quantum yield losses (Pfannschmidt, 2005). However, a quantitative understanding of the relative contribution of each of these factors causing quantum yield losses is still lacking.

Changes in weather and sun angle and in the longer term when leaves become shaded by other leaves or when shaded leaves become exposed to full sun (e.g., after canopy gap formation). The degree of shading by other vegetation strongly affects both the light intensity and spectrum to which a leaf is exposed. Spectral changes can directly alter the photosynthetic quantum yield via changes in the relative absorbance by the different pigments and via changes in photosystem excitation balance. Acting on a time scale of minutes, state transitions are believed to redirect excitation energy from one photosystem to another (Haldrup et al., 2001), although in intact leaves no subsequent increase in the quantum yield for CO2 fixation has been found (Andrews et al., 1993).

Difference Between C3 and C4 Turf

C3 Turf (Cool-Season Turf)

C3 Photosynthesis

Turf plants which use only the Calvin cycle for fixing the carbon dioxide from the air are known as C3 plants. In the first step of the cycle CO2 reacts with RuBP to produce two 3-carbon molecules of 3-phosphoglyceric acid (3-PGA). This is the origin of the designation C3 or C3 in the literature for the cycle and for the plants that use this cycle.

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The entire process, from light energy capture to sugar production occurs within the chloroplast. The light energy is captured by the non-cyclic electron transport process which uses the thylakoid membranes for the required electron transport.

About 85% of plant species are C3 plants. They include the cereal grains: wheat, rice, barley, oats. Peanuts, cotton, sugar beets, tobacco, spinach, soybeans, and most trees are C3 plants. Most sports turf grasses such as rye, bluegrass, and fescue are C3 plants.

C3 plants have the disadvantage that in hot dry conditions their photosynthetic efficiency suffers because of a process called photorespiration. When the CO2 concentration in the chloroplastsdrops below about 50 ppm, the catalyst rubisco that helps to fix carbon begins to fix oxygen instead. This is highly wasteful of the energy that has been collected from the light, and causes the rubisco to operate at perhaps a quarter of its maximal rate.

The problem of photorespiration is overcome in C4 plants by a two-stage strategy that keeps CO2high and oxygen low in the chloroplast where the Calvin cycle operates. The class of plants calledC3-C4 intermediates and the CAM plants also have better strategies than C3 plants for the avoidance of photorespiration.

C4 Turf (Warm-Season Turf)

The C4 photosynthetic carbon cycle is an elaborated addition to the C3photosynthetic pathway. It evolved as an adaptation to high light intensities, high temperatures, and dryness. Therefore, C4 plants dominate grassland floras and biomass production in the warmer climates of the tropical and subtropical regions (Edwards et al., 2010).

In all plants CO2 is fixed by the enzyme Rubisco. It catalyzes the carboxylation of ribulose-1,5-bisphosphate, leading to two molecules of 3-phosphoglycerate. Instead of CO2, Rubisco can also add oxygen to ribulose-1,5-bisphosphate, resulting in one molecule each of 3-phosphoglycerate and 2-phosphoglycolate. Phosphoglycolate has no known metabolic purpose and in higher concentrations it is toxic for the plant (Anderson, 1971). It therefore has to be processed in a metabolic pathway called photorespiration. Photorespiration is not only energy demanding, but furthermore leads to a net loss of CO2. Thus the efficiency of photosynthesis can be decreased by 40% under unfavorable conditions including high temperatures and dryness (Ehleringer et al., 1991). The unfavorable oxygenase reaction of Rubisco can be explained as a relict of the evolutionary history of this enzyme, which evolved more than 3 billion years ago when atmospheric CO2 concentrations were high and oxygen concentrations low. Apparently, later on, it was impossible to alter the enzyme’s properties or to exchange Rubisco by another carboxylase. Nevertheless, plants developed different ways to cope with this problem. Perhaps the most successful solution was C4 photosynthesis.

The establishment of C4 photosynthesis includes several biochemical and anatomical modifications that allow plants with this photosynthetic pathway to concentrate CO2 at the site of Rubisco. Thereby its oxygenase reaction and the following photorespiratory pathway are largely repressed in C4 plants. In most C4plants the CO2 concentration mechanism is achieved by a division of labor between two distinct, specialized leaf cell types, the mesophyll and the bundle sheath cells, although in some species C4 photosynthesis functions within individual cells (Edwards et al., 2004). Since Rubisco can operate under high CO2concentrations in the bundle sheath cells, it works more efficiently than in C3plants. Consequently C4 plants need less of this enzyme, which is by far the most abundant protein in leaves of C3 plants. This leads to a better nitrogen-use efficiency of C4 compared to C3 plants, since the rate of photosynthesis per unit nitrogen in the leaf is increased (Oaks, 1994). Additionally C4 plants exhibit better water-use efficiency than C3 plants. Because of the CO2 concentration mechanism they can acquire enough CO2 even when keeping their stomata more closed. Thus water loss by transpiration is reduced (Long, 1999).

In the mesophyll cells of C4 plants CO2 is converted to bicarbonate by carbonic anhydrase and initially fixed by phosphoenolpyruvate (PEP) carboxylase (PEPC) using PEP as CO2 acceptor. The resulting oxaloacetate is composed of four carbon atoms, which is the basis for the name of this metabolic pathway. Oxaloacetate is rapidly converted to the more stable C4 acids malate or Asp that diffuse to the bundle sheath cells. Here, CO2 is released by one of three different decarboxylating enzymes, which define the three basic biochemical subtypes of C4 photosynthesis, NADP-dependent malic enzyme (NADP-ME), NAD-dependent ME (NAD-ME), and PEP carboxykinase (PEPCK). The released CO2 is refixed by Rubisco, which exclusively operates in the bundle sheath cells in C4 plants. The three-carbon compound resulting from CO2 release diffuses back to the mesophyll cells where the primary CO2 acceptor PEP is regenerated by pyruvate orthophosphate dikinase by the consumption of, at the end, two molecules of ATP (Hatch, 1987).

The two other biochemical subtypes differ from the NADP-ME type by the transport metabolites used and the subcellular localization of the decarboxylation reaction. In NAD-ME plants Asp, which is synthesized in the mesophyll cytosol, is used as transport metabolite. After deamination and reduction, the resulting malate is decarboxylated by NAD-ME in the bundle sheath mitochondria. Plants of the PEPCK type use Asp as well as malate as transport metabolites. Asp is synthesized in the cytosol of mesophyll cells and decarboxylated in the cytosol of bundle sheath cells by the combined action of Asp amino transferase and PEPCK. As in NADP-ME-type C4 species, malate is synthesized in the mesophyll chloroplasts but decarboxylated by NAD-ME in the mitochondria of bundle sheath cells. This reaction produces NADH that is used in the mitochondria to produce the ATP needed to drive the PEPCK reaction (Hatch, 1987). If Asp is used as transport metabolite, usually the three-carbon decarboxylation product, pyruvate, is partially transported back to the mesophyll cells in the form of Ala to maintain the ammonia balance between the two cell types (Hatch, 1987).

Compared to C3 plants the bundle sheath cells of C4 plants have expanded physiological functions. This is reflected by the enlargement and a higher organelle content of these cells in most C4 species. For the efficient function of the C4 pathway a close contact between mesophyll and bundle sheath cells is indispensable and they are tightly interconnected to each other by high numbers of plasmodesmata (Dengler and Nelson, 1999). To ensure a direct contact between bundle sheath and mesophyll cells, C4 plants possess a characteristic leaf anatomy. The bundle sheath cells enclose the vascular bundles and are themselves surrounded by the mesophyll cells. The high vein density in the leaves of C4 plants leads to a nearly one-to-one ratio of the volumes of mesophyll and bundle sheath tissues. The internal anatomy of a C4 leaf is often composed of a repeating pattern of vein-bundle sheath-mesophyll-mesophyll-bundle sheath-vein. Because of its wreath-like structure this type of leaf anatomy was termed Kranz anatomy by the German botanist G. Haberlandt (1904). Kranz anatomy is found with more or less considerable variations in nearly all monocotyledonous and dicotyledonous lineages that use the two-cell mode of C4 photosynthesis.

What Limits C3 Photosynthesis

It has been known for some time that a large proportion of the limitation to carbon assimilation in plants using the C3 cycle is due to the catalytic properties of the enzyme Rubisco (Portis and Parry, 2007). During the 1990s, metabolic control analysis was used to explore the possibility that enzymes other than Rubisco may also have a role in determining rates of carbon flux through the C3 cycle (Stitt and Schulze, 1994; Raines, 2003). To undertake metabolic control analysis of a pathway, it is necessary to be able to reduce specifically the amount of an individual enzyme in that pathway; the effect of this reduction on flux can then be compared to the control. The flux control coefficient can vary from 0, for an enzyme that makes no contribution to control, to 1, for an enzyme that exerts total control. One fundamental difference between this approach and that based on the kinetics of individual enzymes is that metabolic control analysis allows for all enzymes in a pathway to share control of flux in that pathway. The flux control value for any single enzyme is not a constant and can change depending on the conditions under which the analysis was carried out.

C3 turf plants with reductions in Rubisco protein levels produced using an antisense construct were used to assess the relative contribution that Rubisco imposed on carbon flux. This approach has demonstrated clearly that the limitation imposed by Rubisco on C3 carbon fixation is greatest in high light and temperature conditions (Stitt and Schulze, 1994). In contrast, as expected, this is reduced in turf plants grown in elevated CO2 (Masle et al., 1993). Importantly, these C3 turf studies also showed that Rubisco does not limit the C3 cycle in all conditions and that enzymes of the regenerative phase of the cycle also play a role in determining the rate of photosynthesis (Raines, 2003, 2006; Stitt et al., 2010).

Two properties of Rubisco are targets for improvement: its low catalytic turnover rate and the use of O2 as an alternative substrate in catalyzing a wasteful side reaction known as photorespiration. An additional approach to improve plant photosynthesis lies in increasing the activation state of Rubisco under moderate heat stress. Rubisco activation state decreases at elevated temperatures primarily due to the thermolability of Rubisco activase. Therefore, it is expected that an improvement in Rubisco carboxylase activity, specificity or the thermostability of Rubisco activase will increase photosynthetic rates of plants grown at certain conditions.

Rubisco

The most abundant protein, Rubisco [ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase; EC 4.1.1.39] catalyses the assimilation of CO2, by the carboxylation of ribulose-1,5-bisphosphate (RuBP) in photosynthetic carbon assimilation. However, the catalytic limitations of Rubisco compromise the efficiency of photosynthesis (Parry et al., 2007). Compared to other enzymes of the Calvin cycle, Rubisco has a low turnover number, meaning that relatively large amounts must be present to sustain sufficient rates of photosynthesis. Furthermore, Rubisco also catalyses a competing and wasteful reaction with oxygen, initiating the process of photorespiration, which leads to a loss of fixed carbon and consumes energy. Although Rubisco and the photorespiratory enzymes are a major N store, and can account for more than 25% of leaf nitrogen, Rubisco activity can still be limiting. Furthermore, growth studies with transgenic plants with decreased amounts of Rubisco have confirmed that, under field conditions with intense or variable irradiance, photosynthetic rate is highly correlated with the amount of Rubisco. This relationship cannot be ignored in attempts to improve resource use, particularly of nitrogen and water (Parry et al., 2005, 2007; Parry and Reynolds, 2007).

In higher plants, Rubisco has a hexadecameric structure, being composed of eight large, chloroplast-encoded subunits arranged as four dimers and eight small, nuclear-encoded subunits. This is also known as Form I Rubisco. Each large subunit has two major structural domains, an N-terminal domain and a larger C-terminal domain which is an alpha/beta barrel. Most of the active site residues (that interact with substrate and/or substrate analogues) are contributed by loops at the mouth of the alpha/beta barrel with the remaining residues being supplied by two loop regions in the N-terminal domain of the second large subunit within a dimer. The availability of high resolution 3-D structures has provided detailed insight into the catalytic mechanisms of the enzyme and enabled properties to be related to sequences (Parry et al., 1987; see comprehensive reviews by Andersson, 1996, 2008). Rubisco is highly regulated to control flux through the photosynthetic carbon reduction cycle in response to short-term fluctuations in the environment. The potential Rubisco activity is determined by the amount of Rubisco protein which, in turn, is determined by the relative rate of biosynthesis and degradation. These processes are regulated by gene expression, mRNA stability, polypeptide synthesis, post-translational modification, assembly of subunits into an active holoenzyme, and various factors which impact upon protein degradation. In the short term, regulating Rubisco activity is essential to match the capacity for RuBP regeneration with the prevailing rate of RuBP utilization. This is not achieved solely by the availability of substrate since Rubisco in excess of that needed to