Chapter 6: Plant Adaptations to the Environment All life on Earth is carbo n based The means by which organisms acquire and use carbon represent some of the most basic adaptations required for life Humans gain carbon through ingestion
CO2 – – ultimate source of carbon from which life is constructed Autotrophs – – only organisms that can transform carbon from CO2 into organic molecules and living tissue 1. Chemoautotrophs Convert CO2 into organic matter via oxidation of inorganic molecules (H gas or Hydrogen sulfide) or methane. Dominant primary producers in oxygen deficient environments 2. Photoautotrophs dominant form of autotrophs uses the Sun’s energy to drive the process of CO2 conversion Photosynthesis in green plants, algae, and some types of bacteria Photosynthesis conversion of CO2 into simple sugars energy from the sun (shortwave radiation, PAR) is harnessed in a series of chemical reactions that result in the fixation of CO2 into carbohydrates and release O2 as a by-product
Limits of Photosynthesis: Amount of light Atmospheric gases Amount of water Carbon dioxide concentration Temperature Rubisco Catalyzes the carboxylation reaction C3 Plants Plants that employ a photosynthetic pathway involving the initial fixation of CO2 into the 3carbon PGAs (Calvin-Benson cycle) Common plants Cellular respiration Uses some of the carbohydrates produced in photosynthesis - Also referred to as aero bic respiration Oxidation of carbs to generate energy (ATP) and takes place exclusively in the mitochondria mitochondr ia C6H12O6 + 6O2 6CO2 + 6H2O + ATP Fermentation - Anaerobic respiration respira tion Occurs in the absence of oxygen Enabled cells to convert glucose into lactic acid and ATP Occurs in both plants and animals
Process: 6CO2 + 12H2O C6H12O6 + 6O2 + 6H2O 2 Processes of Photosynthesis: 1. Light reactions Initial photochemical reaction where chlorophyll within the chloroplasts absorb light 2. Dark reactions CO2 is biochemically incorporated into simple sugars CO2 1-carbon molecule
1
+
RuBP 5-carbon molecule
A. Madrona, 2015
2 3 -PGA 3-carbon molecule
C6H12O6 2C3H6O3 + 2ATP Net Photosynthesis Difference in the rates of aerobic and anaerobic respiration is the net gain of carbon
Rates of photosynthesis are typically measured in moles CO2 per unit leaf area (or mass) per unit time
Solar radiation Provides energy required to convert CO2 into simple sugars Light compensation point (LCP) Wherein net photosynthesis is zero The rate of carbon loss due to respiration exceeds the rate of uptake during photosynthesis
Chapter 6: Plant Adaptations to the Environment
Light saturation point (LSP) Value of PAR, above which no further increase into photosynthesis occurs Photoinhibition Result of overloading the processes involved in the light reactions Mesophyll cells Specialized cells where photosynthesis occurs Stomata Point of entry of CO2 into the leaf CO2 enters the leaf by diffusing through these structures Closes to prevent water loss (transpiration)
Water potential () Measure used to describe the free energy of water at any point along the soil-plantatmosphere continuum Pure water (no solute content), which has high free energy, is arbitrarily assigned a water potential of zero ( = 0) units pressure: MPa relative humidity of the atmosphere = 100%, -
As long as the concentration of CO2 in the air outside the leaf is greater than that inside the leaf, CO2 will continue to diffuse through the stomata As long as photosynthesis occurs, the gradient of CO2 concentrations outside of and inside the leaf will remain
Water vapor The rate of diffusion of water vapor depends on the gradient of water vapor from inside to outside the leaf The drier the air, the more rapidly the water inside the leaf will diffuse through the stomata and out into the air
General functions of water in plants: 1. Transport of nutrients from the soil 2. Turgor pressure Force exerted outward on a cell wall by the water contained in the cell Growth rates of plants and efficiency of their physiological processes are at their highest at maximum turgor (fully hydrated) Movement of water from the soil into the roots, from the roots to leaves, and leaves to the atmosphere is a spontaneous reaction Transpiration is driven by the process diffusion The free energy that allows this work to be accomplished is the kinetic energy associated with the random movement (and collision) of the H2O moloecules
2
A. Madrona, 2015
-
atmospheric water potential (atm) = 0 As value drops below 100%, atm = negative Under most physiological conditions, the air within the leaf is or at near saturation As long as the relative humidity of the air is below 100%, a steep gradient of water potential between the leaf (leaf ) and atmosphere (atm) will be the driving process of diffusion A decrease in turgor pressure associated with water loss functions to decrease water potential. The component of plant water potential due to turgor pressure is represented as (p)
Osmotic potential () Increasing concentrations of solutes in the cells associated with water loss that lowers water potential Matric potential (m) Tendency of water to adhere to surfaces which then reduces the free energy of the water molecules, reducing water potential
= p + + m
Osmotic and matric potentials will ALWAYS have a NEGATIVE value (-) Turgor pressure can be POSITIVE or NEGATIVE (+/-) Total potential can be POSITIVE or NEGATIVE (+/-) Values of total water potential at any point along the continuum are typically NEGATIVE Movement of water through the continuum depends on maintaining a gradient of increasingly negative water potential at each point along the way atm
< leaf < root < soil
Chapter 6: Plant Adaptations to the Environment Water loss through transpiration continues as long as: 1. Amount of energy striking the leaf is enough to supply necessary latent heat of evaporation 2. Moisture is available for roots in the soil 3. Roots are capable of maintaining a more negative water potential than that of the soil Water-use efficiency The ratio of carbon fixed (photosynthesis) per unit of water lost (transpiration)
Boundary layer - A layer of still air (or water) adjacent to the surface of each leaf Carbon gained in photosynthesis is allocated to the production of plant tissues Carbon balance Focuses on the balance between uptake of CO2 in photosynthesis and its loss through respiration
Stomata must be open to carry out photosynthesis but the plant also loses water through transpiration The balance between photosynthesis and transpiration is an extremely important constraint that governed the evolution of terrestrial plants
Aquatic plants lack a stomata CO2 diffuses from the atmosphere into the surface waters then mixed with the water column CO2 reacts with the water and forms bicarbonate (HCO3-) Some species can utilize bicarbonate as a carbon source 2 ways of conversion of CO2 into enzyme carbonic anhydrase: 1. Active transport of bicarbonate into the leaf followed by conversion to CO2 2. Excretion of the enzyme into adjacent waters and subsequent uptake of converted CO2 across the membrane Increase in temperature = increase in photosynthesis and respiration rates Initially, photosynthesis increases faster than transpiration As temperature further increases, photosynthetic rates increase until it reaches a maximum and then eventually declines as temperature reaches critical levels Temperature of the leaf controls the rate of photosynthesis, not the air
Modes of heat exchange in plants Terrestrial plants: 1) convection evaporation - Aquatic plants: Convection
3
A. Madrona, 2015
and
2)
How net carbon is gained will majorly influence plant survival, growth, and reproduction Variation in the physical environment (salinity, depth, flow of water, spatial and temporal patterns in climate, variations in geology and soils) resulted in a wide array of plant adaptations Plants must maintain a positive carbon balance to survive
Plants growing in shaded environments Lower LCP and LSP, maximum rate of photosynthesis Have lower rubisco concentration Leaf respiration of seedlings are significantly lower Leaves have a greater specific leaf area (broader and thinner leaves) Shade-intolerant species adapted to high-light environments sun-adapted species Shade-tolerant species adapted to low-light environments shade-adapted species lower maximum rates of net photosynthesis, leaf respiration, and relative growth rate Evergreen rhododendrons respond to moisture stress by inward curling of the leaves
Prolonged moisture stress – inhibits chlorophyll production causing leaves to turn yellow Tropical regions – species evolved to drop their leaves at the onset of dry season (drought deciduous)
Chapter 6: Plant Adaptations to the Environment C4 and CAM plants Evolved a modified form of photosynthesis to increase water-use efficiency Involves an additional step in the conversion of CO2 into sugars C4 plants Have two distinct types of photosynthetic cells where photosynthesis is divided: 1. Mesophyll cells 2. Bundle sheath cells Cells that surround the veins/vascular bundles Have CO2 react with PEP carboxylase (3-carbon compound) within the mesophyll cells Typically fix more carbon Great advantage in hot, dry climates Higher energy expenditure because of the need to produce PEP and PEP carboxylase Mostly grasses native to tropical and subtropical regions and some shrubs in arid and saline environments CAM plants Found in deserts - A pathway similar to C4 pathway in that CO2 initially reacts with PEP Photosynthesis occurs in the mesophyll and bundle sheath cells but at separate times Open their stomata at night and are closed during the day Slow and inefficient in CO2 fixation compared to C4 Macronutrients needed by plants: Carbon Phosphorus Hydrogen Magnesium Oxygen Sulfur Nitrogen Potassium Calcium Micronutrients needed by plants: Chlorine Copper Iron Molybdenum Manganese Zinc Boron Nickel (Check p.113, Table 6.1 for major functions)
4
A. Madrona, 2015
Wetland environments Soils are saturated with water for most or all of the year
Plants accumulate ethylene in their roots in response to anaerobic conditions
Ethylene gas Growth hormone Highly insoluble in water Normally produced in small amounts in the roots Stimulates cells in the roots to self-destruct and separate to form interconnected gas filled chambers (aerenchyma) Aerenchyma Chambers that allow some exchange of gases between submerged and better-aerated roots - Allow oxygen to diffuse between the plant parts above the water and the submerged tissues Halophytes Plants that take in water containing high levels solutes - Accumulate high levels of ions within their cells, especially in the leaves
