Suggested answers for Ecophysiology of Plants past exams

2005

1 a) What are the characteristics of short-wave solar radiation and long-wave infrared radiation with respect to the energy budget of a leaf?

Short-wave is 300-3000 nm. Comprises 98% of the sun's radiation. This includes photosynthetically active radiation and UV light. PAR uses less than 1% of the typical energy flux of a leaf (about 5 Wm-2). Infra-red (700-1200 nm) is largely reflected or transmitted. 1200-3000 nm is largely absorbed by water in the leaf.

Long-wave of 3000+ nm. Infra-red heat radiation. Leaves have a very high absorbance of long wave radiation (almost total). Half of the energy absorbed by a leaf is from LR, and can be of the order of 600 Wm-2.

1 b) As wind speed increases, the sensible heat transfer from leaf to air increases, but there is very little effect on evaporative cooling. Why is this the case?

Wind reduces leaf temperature due to an increase in convective cooling, but transpiration is hardly affected. The increase in boundary layer conductance with wind speed is counterbalanced by a decrease in leaf-to-air vapour pressure difference due to the decrease in leaf temperature (Lambers et al. 1998, pp 220).

1 c) What are the best strategies for reduced leaf-air temperature in a high radiation environment?

• Increasing leaf angle
• Decreasing leaf size (reduces boundary layer resistance)
• Increasing reflectance. In /Atriplex mummularia/, low water leads to salt crystallisation, which increases reflectance.
• Increasing evaporation. In mangroves, this could mean more salt intake.

2) The figure attached (see graph for question 2, page 8) shows the leaf water potential of a plant during the onset of drought. Indicate on the graph:

• The likely soil water potential over the period.
• The likely timing and trend of xylem cavitations.
• The likely time course of the stem water potential.
• Indications of stomatal closure.
• The hypothetic time course of the water potential of a mistletoe growing on the plant.

3 a) Using the following graphs define what is meant by the water use envelope of a plant/soil combination? What features limit the envelope? How does stomatal regulation play a role?

Apparently not in the course this year.

3 b) How does the hydraulic architecture of a plant affect cavitation in different parts of the plant? In your answer define cavitation, embolism, and leaf specific conductance.

• Cavitation - the event where xylem pressure goes from negative to zero (a vacuum). Acoustic events can signal cavitations.
• Embolism - Final state, after ingress of water vapour and air, where xylem pressure goes to atmospheric pressure.
• Leaf specific conductance = k/Al = hydraulic conductance/leaf area that the xylem supplies. Lower LSC increases the gradient of psi and makes cavitation more likely. Branches and petioles have low LSC compared to the trunk. This can act as a 'hydraulic fuse'. Root xylem tend to have higher vulnerability, and cavitation can sometimes occur first in roots. Leaf area and root surface area must be accounted for in hydraulic architecture.

Decreased vessel diameter (or pit diameter) reduces cavitation. Bordered pits in tracheids work to prevent the spread of cavitation and embolisms.

Highest resistance to water flow is in the leaves and petioles, indicating cavitation is most likely to occur there.

Stomata are important in preventing runaway cavitation (a positive feedback effect where psi becomes more negative, causing more cavitation) by regulating transpiration rate.

4 a) Explain why arbuscular mycorrhizas are more important in the uptake of nutrients that are immobile in soil (such as phosphorus and zinc) than they are for mobile nutrients like nitrate.

P and Zn diffuse very slowly in soil, so the concentrations close to roots can be very low. Nitrate moves by mass flow in soil solution and can be more readily accessed by roots, though AM may increase the efficiency with which inorganice N is absorbed as well.

4 b) Briefly describe the changes in location and expression of phosphate transporters that take place when a plant becomes arbuscular mycorrhizal.

Plants with arbuscular mycorrhizae switch from expressing epidermis/hair P transporters to arbuscule (points of attachment between fungi and root). Root may switch off direct uptake of nutrients in some cases.

5) Describe the processes that increase the rate of salinisation of soils, and outline plant-based methods that can be used to deal with the problems associated with soil acidity.

Land clearance removes deep-rooted plants, which increases recharge of saline water tables and brings them closer to the surface.

Acidity? We didn't discuss plant-based methods for dealing with acidity.. you can lime the soil, though. May be possible to engineer transgenic plants with better Al tolerance.

6) Explain the following three methods of phytoremediation, highlighting the differences between each and the circumstances in which their use would be most appropriate:

• Phytoextraction - heavy metals are taken up by plants and removed from the system when plants are harvested.
• Phytostablisation - immobilises and inactivates metals through binding to plant roots, reducing leaching through drying of soil. Usually in plants with low root-shoot transfer. Preventing erosion.
• Rhizofiltration - removal of heavy metals from water. Plants are grown hydroponically and harvested when saturated with metals.

Phytoextraction/phytostabilisation must be selected through outcome for the soil (will it be used for agriculture, or is it being revegetated?), and whether hyperaccumulators are available for the particular metals.

7) What are the main effects of acid soils on plant growth and what strategies have tolerant plants developed to enable them to grow under acidic conditions?

Occur in high rainfall areas. Acidity releases Al and Mn ions from gibbsite and manganese oxide (!MnO2, though my lecture notes say MnO), which are insoluble at neutral pH. Metal toxicity results. eg, Al toxicity causes lesions on root surfaces near the root tip. Root growth and plant production are reduced, though the mechanism for this isn't known. Ammonium fertilisers can cause acidification in susceptible soils.

Increasing pH also reduces bioavailability of essential nutrients such as Fe, Mn and Zn (though Mn is a problem because release from insoluble forms is increased and can reach toxic concentrations).

Tolerance is achieved through the release of organic acids such as Malic acid that complex Al. Some cereals do this. Also, uptake and compartmentalisation (eg tea). K+ is also released to maintain pH neutrality.

8 a) What is the heat compensation point, and why is it often exceeded in CAM plants?

The temperature compensation point is where CO2 fixed by photosynthesis is equal to the CO2 released by respiration. Above this, more carbon is lost than gained. Photosynthesis is inhibited by high temperature before respiration. CAM plants are able to exceed it because during the day when temperatures are hottest, stomata are closed and respiration doesn't occur.

8 b) Briefly explain one morphological and one metabolic adaptation of leaves to heat stress.

Morphological - hairy or shiny (waxy) leaf surfaces reflect light (and therefore heat) away. Leaf rolling reduces surface area exposed to light. Vertical orientation of leaves reduces heat input. Small leaves increase convective and conductive heat loss. Dimorphic leaves in Compositae are 'crinkly' and presumably avoid heat somehow.

Metabolic - High temperatures can inhibit ATPases, resulting in a decrease in cytosolic pH. Cytosolic calcium mediates stress, as heat increases Ca2+ levels, which restores pH by activating CaM (calmodulin).

Isoprene is released by plants at higher temperatures to stabilise photosynthetic membranes.

8 c) Heat shock proteins are rapidly synthesized in plant cells in response to heat stress. Suggest why Heat Shock Factors (HSF) and Heat Shock Elements (HSE) are necessary to maintain homeostasis.

Heat shock proteins are produced in response to sudden rises in temperature. They assist (chaperone) heat damaged proteins and help maintain their configuration at high temperatures.

Heat Shock Factors exist in a monomeric state with HSP70 proteins. Heat causes HSFs to dissociate and trimerise. Trimers bind to heat shock elements in the promoter of heat shock protein genes and activate transcription of HSP m!RNAs. Don't think we covered it in much detail in the course.

9 a) Provide a brief definition for each of the following terms: Intensity, Irradiance, PAR and PFD. Which is the best to use when measuring light availability for photosynthesis? Why?

• Intensity - the rate of emission from a source
• Irradiance - radiant energy flux per unit area (at all wavelengths) W m-2. Used for energy balance of leaves.
• PAR - Photosynthetically active radiation. 400-700 nm
• PFD - Photon flux density, or number of photons incident per unit area per unit time. umol m-2 s-1.

Light availability for photosynthesis is based on PFD, because the number of photons is absorbed is more meaningful than energy, since red photons are just as effective as blue.

9 b) Draw a typical CO2 response curve of photosynthesis for a C3 plant and describe its main features. How would these features differ for a C4 plant? Why?

• C4 saturates at a lower CO2 concentration
• C4 has higher carboxylation efficiency
• Maximum CO2 assimilation at high CO2 concentration is higher in C3.

9 c) With respect to its impact on plants, explain why there is concern about increasing levels of UV-B radiation. Describe two ways in which plants protect themselves against UV-B radiation.

UV-B exposure may disrupt DNA, which with a peak absorbtion of 260 nm can be induced to mutate. Changes can block transcription and prevent DNA replication. Repair can occur, but is sometimes inaccurate. Haploid organisms are more susceptible. It also reduces the activity of enzymes and affects metabolism and causes damage to cell membranes and organelles (lipids).

Protection is provided by:

• Surface reflectance, such as waxes and hairs
• Flavonoid pigments in the epidermis that screen out UV (230-380 nm) and protect underlying cells

9 d) Describe what happens during the 4 phases of CAM. What advantages and disadvantages does CAM photosynthesis provide? Why?

• Nocturnal opening of stomata. Fixation of CO2 by PEPC producing malate. Malate transported into the vacuole.
• Early morning - Transition between early morning and day. Stomata still open. CO2 fixation by both PEPC and Rubisco.
• Daytime - stomata close. Malic acid efflux from vacuole. Decarboxylation of malate by NADP-ME (malic enzyme). Refixation of CO2 by Rubisco in chloroplasts.
• Late afternoon - Exhaustion of malic acid store in vacuole. Stomata open late in the day. CO2 fixation by Rubisco. CO2 exchange increases, but decreases as light availability drops off.

Advantages - increases CO2 concentration around Rubisco. Decreased photorespiration due to recycling of respiratory CO2. Increased water use efficiency (as stomata are closed during the day) of 2-10 times C4, or up to 100 times C3. Less N required. Some CAM plants can revert to C3 when water availability is high. Under extreme water stress, stomata may close completely. CAM plants can maintain carbon balance by recycling respiratory CO2.

Disadvantage - less productive than C3 and C4. Low photosynthetic surface area

9 e) Draw a typical light response curve of photosynthesis and describe its main features. How would these features differ in plants grown in high and low light? Why?

• A net CO2 release exists at zero PFD due to photorespiration. C4 has no dark respiration.
• Light limited and CO2 limited regions.
• Light compensation point where photosynth = respiration + photorespiration.
• Pmax photosynthetic capacity
• Low light plants would have same slope (efficiency is not changed) but lower Pmax. They also have a lower light compensation point.

10 a) Using diagrams, describe the operation of the xanthophyll cycle and explain how it protects plants from excess light.

Carotenoid pigments located in the antenna complexes of PSII. Interconversion is regulated b y light via the pH of the lumen. In low light, violaxanthin is an accessory light harvesting pigment. In excess light, antheraxanthin and zeaxanthin enhance the thermal dissipation of light.

Now look at the graphs below and explain the observed responses. /Ulva rotundata/ is a marine alga found in intertidal zones. CAP is an inhibitor of chloroplast-mediated protein synthesis.

In fig. 1B, PSII yield is reduced significantly by the high light exposure. CAP-treated plant recovers poorly compared to control. This suggests that photodamage has occured, as the plant is unable to synthesise replacement proteins. In fig. 1B the CAP-treated plant eventually recovers. It appears that reduction in yield was mostly due to the action of the xanthophyll cycle (which doesn't involve protein synthesis) and that little photodamage occured at the lower light level.

10 b) Using diagrams, describe how the C4 pathway of photosynthesis works.

Two spatially separated carboxylation steps.

• Fixation of CO2 in mesophyll by PEPC (HCO3 + PEP -> C4 acid)
• Transport of C4 acid to bundle-sheath cell
• Fixation of CO2 in bundle-sheath cells by Rubisco (Calvin cycle). CO2 + RuBP -> 2 PGA
• Regeneration of PEP

Now look at the table below. Which photosynthetic pathway does each species use? Discuss the responses of the two species and suggest how each might be affected by future climate change (consider both temperature and rainfall), and rising atmospheric CO2 concentrations.

Species A is C3 because it performs much better at high CO2 concentrations. Species B is C4 because it would already be CO2 saturated at ambient levels, so increasing the concentration has a less significant effect. Species B is also less affected by droughted conditions, which is a C4 characteristic.

Climate change involving more CO2 or higher water availability are likely to favour C3. Higher temperatures and high light will favour C4. Some species respond differently, though.

11) Devise a hypothesis about galling thrips and the ecophysiology of the Acacia host to explain why only some trees of the host species become galled. Describe how you would test your hypothesis using your knowledge of plant ecophysiology.

From JG's lecture. Water and temperature stress are interrelated. Discuss.

Water used for transpiration. When stomata closed in times of water stress, temperature increases. Temperature is normally decreased through transpiration and latent heat of evaporation.

List five adaptations to heat stress

Heat shock proteins, calcium mediation (mention these two for best results), hairy/waxy surfaces, leaf rolling, vertical orientation, small leaves, dimorphic leaves.

2004

1 a) What are the plant physiological ways in which the radiation terms and nonradiation terms of a leaf’s energy budget are affected?

Radiation terms: (Proportion of long and short wave radiation inputs and outputs depend upon position in the canopy and the condition of SR ie cloudy and sunny day) Leaf angle has a large effect on SR absorbed. Leaves can track the sun such that rays are parallel to the surface (paraheliotropic) or perpendicular (diaheliotropic), and this can change depending on water status. Reflection with hairs, wax or salt crystals.

Non-radiation terms: water vapour flux controlled by stomata. Leaf size also affects energy budget for boundary layer reasons.

1 b) Using the following graphs, explain why leaf-air temperature difference (DT) changes as a function of leaf dimension and stomatal conductance. Explain in terms of LR, C, and lE.

Leaf dimension: Larger leaves have a larger boundary layer resistance, which decreases leaf heat loss (by LR), leading to higher leaf-air temperature differences. Lambda E fairly constant because stomata are assumed to be open (unless it's a desert, in which case it may be trying to conserve water). C proportional to leaf-air temperature during the day.

Stomatal conductance: Increase in stomatal conductance causes a decrease in leaf-air temperature due to an increase in evaporative cooling. Lambda E (evaporative loss) is increased. LR and C decreases with increased stomatal conductance because leaf-air temperature difference is decreased by transpiration.

2 a) What is the evidence for the function of aquaporins in plants? What significance do they have for water flow into or out of plant cells, and through the plant?

Cell membranes/walls alone let little water through. Aquaporins allow cells to selectively conduct water and direct pathways of water through tissues. Dependence on aquaporins can be demonstrated by inhibitors (eg HgCl2) that slow down the rate of equilibration, low temperature dependence and variable water permeability that may be controlled by various cell factors.

Affect the transmission of water potential changes through a plant.. can buffer a plant against changes in water potential (eg brackish inlet where potential can change quickly)

2 b) What is the significance of the volumetric elastic modulus in plant cell water relations and how does it relate to the bulk modulus of elasticity?

Wall elasticity determines the change in turgor pressure for a change in cell volume. The coefficient is the volumetric elastic modulus. High values - walls are inelastic. Low values mean a large volume change occurs for a change in turgor pressure, which can be important in water storage. Lower osmotic pressure also allows large volume changes.

Bulk elastic modulus is the ratio of change in pressure to the corresponding change in relative volume of the leaf tissue. Bulk modulus is the volume-weighted elastic modulus for a tissue (measure with pressure bomb), while the volumetric modulus is for a single cell (measured with pressure probe)

Bulk modulus varies linearly with turgor. This gives a 'stress hardening' effect at high turgor. This can also be seen on Hoefler diagrams, where the slope of pressure potential (psi p) is not constant with changes in RWC.

Some drought-adapted plants (eg /Olea/) have a low bulk elastic modulus (in both wet and dry conditions) which means that they lose turgor less readily (for a given loss of water). Non-adapted plants have a higher elastic modulus, and that increases in drought. However, the mechanism for this is not known - plants may also use osmoregulation to maintain turgor, so there aren't really any rules.

4 a) Some plant species (such as tomato and wheat) do not respond positively to colonisation by AM fungi when grown alone in pots, suggesting that they receive no nutritional advantage from the symbiosis. Suggest reasons for the persistence of mycorrhizal colonisation in such plants, giving brief explanations to support your suggestions.

Nutritional benefit can be a 'community based' parameter, and lack of growth response in pots may not mean that there are no advantages in communities. The AM may help a plant to outcompete non-host plants where resources are scarce. AM may also have a benefit for drought tolerance and increased tolerance to some diseases.

4 b) Describe an experimental approach to determine the actual contribution of AM fungi to plant P uptake.

In a pot, separate part of the soil with a mesh too small to allow roots to pass through, but large enough for AM. Add 33P to this soil and compare ratio of 33P:normal P in plants with and without AM. Only AM will be able to access the 33P, so an increase the ratio will indicate the contribution of AM. Can't look at total P uptake alone, as AM may inactivate direct P uptake by roots.

5 a) The advantages of using phytoremediation for decontaminating soil.

• Inexpensive compared to engineering solutions
• May be seen as a "green" technique
• May allow contaminants to be extracted in commercial quantities

5 b) The difficulties and limitations associated with phytoremediation.

• May take a long time to be effective, as suitable plants may have low biomass and growth rates.
• Genetic engineering may be required to produce plants with desirable qualities, which could have problems with community support
• Where chelating agents must be added to the soil, there is a chance that metals could be mobilised and released into ground water.

6 Considered in the context of plant growth, give reasons why soil analysis is inappropriate for assessing the levels in soil of Nitrogen

• N is mostly in organic form in soil and requires mineralisation
• Mineralisation depends on climate
• N retained in soil dependent on climate (eg high rainfall -> N leached)
• Models to preduct N requirement incorporating climatic factors more successful

.. of Trace metals

• Uptake is highly dependent on the plant species/cultivar
• Amounts taken up small, and there are analytical problems in determining trace levels in soil
• The cost of correcting deficiencies is small (just throw some trace metals onto the soil), so high precision isn't required

7) A group of ‘treehuggers’ argue that a proposed mine will have serious adverse effects on the physiology and ecology of the local vegetation due to acid mine drainage. Explain how this might occur.

Pyrite FeS2 reacts with water and oxygen to produce Fe(OH)3 and H2SO4. At low pH, biological conversion by /Thiobacillis ferrooxidans/ also occurs. Soil acidification can have catastrophic effects on the environment by solubilising metal-containing minerals, leading to toxicity, especially from Mn and Al. Effects can be wide-ranging if water courses are nearby -> toxic levels of metals in farm animals, effects on terrestrial and aquatic plants, frogs, invertebrates.

Mine would need to prevent infiltration of oxygen and water needed for pyrite oxidation, but it's not that easy to achieve (aside from not digging it up).

8 a) Discuss three (3) adaptations of plants to heat stress.

See 2005 8 b)

8 b) Explain how water stress and heat stress are interrelated.

(JG's question) Water used for transpiration. When stomata closed in times of water stress, temperature increases. Temperature is normally decreased through transpiration and latent heat of evaporation.

8 c) Explain the CAM plant compromise as a result of high tissue temperature.

Seems like a poorly worded question. CAM plants avoid high tissue temperatures by succulence, as water content buffers against ambient temperature changes. Transpiration doesn't occur during the day to prevent evaporative water loss. They compromise by fixing CO2 (to malate) at night (storing it as malic acid in the vacuole) and decarboxylating stored malic acid during the day.

9 b) Explain the roles of antenna complexes and reaction centres. Which types of pigments would you expect to find in each? How would the ratio of antenna complexes to reaction centres vary in plants grown in high or low light?

• LHC/antenna complexes - responsible for 'harvesting' light. Contain pigments such as Chl A/B. Absorb light and donate electrons to RC.
• RC - used for light reactions. Sites at which photosystems are located. Accept electrons and carry out work.
• Sun plants - low LHC:RC - sun plants maximise light use with more RC (ie, also more Rubisco, more Chl A than Chl B (more PSI than PSII)). Thick leaves.
• Shade plants - high LHC:RC - shade plants maximise light capture with more LHC. More ChlB than A. Thin leaves (larger surface area)

10 a) Using diagrams, describe what occurs during the four phases of CAM. Now look at the table below and give an explanation for the data.

Mean PFD is increasing (sort of) and rainfall is decreasing over time. In January and February, relatively low morning malate concentrations and little difference between day and night. In March, a lot of malate is accumulated overnight and this seems to have been 'used up' during the day. Appears that the plant has switched from C3 (Jan/Feb) to CAM (March) in response to water and light availability.

10 b) Using diagrams, describe the operation of the xanthophyll cycle and explain how it protects plants from excess light. Now look at the graphs below and explain the observed responses. T. australis is an Australian rainforest plant.

See 2005 10 a)

In high light, the water stressed plants changed more V to A and Z to avoid photodamage. This can also be seen with the corresponding reduction in PSII efficiency. While violaxanthin levels had recovered at 180 minutes, yield had not, indiciating that some photodamage did take place. Water stress makes plants have a lower capacity for photochemistry, and a corresponding drop in max PFD at which photodamage occurs.

11) Australia has a high diversity of hemiparasitic plants including mistletoes (stem parasites) and quandongs (root parasites). What impact do you think rising atmospheric CO2 concentrations and the predicted increases in temperature are likely to have on this group of plants. In answering this question, you should consider the mechanisms that hemiparasites use to extract water and nutrients from their hosts as well as the impacts of climate change on the hosts themselves.

• Hemiparasites extract water by retaining a more negative water potential through a high transpiration rate, haustorial resistance and low osmotic potential (holoparasites have very low transpiration rates).
• Hemiparasite stomata are often insensitive to light, VPD and CO2.
• Rising CO2 impacts host plants with increased photosynthesis but decreased stomatal conductance, transpiration and water use efficiency. That'll probably lead to less water being available for parasites.
• Temperature and CO2 may lead to a shift in C3/C4 plant balance - loss/movement of host plants will affect distribution of parasites directly.
• C4 plants may be advantaged by increase in CO2. More C4-attaching parasites?

2003

1 i) What can leaf temperature tell us about the microclimate and physiological status of a leaf? What else would we need to know to determine the degree of stress a leaf may be experiencing?

High leaf temperature could indicate water stress if transpiration has stopped during the day. Would need to investigate transpiration rate.

Boundary layer thickness also influences heat loss - would need to examine size of leaf, wind speed.

1 ii) What are the best strategies for reduced leaf-air temperatures in a high radiation environment? What might be the best strategy for a very salt tolerant mangrove in the tropics?

Increase in leaf angle, decrease in leaf size, increase in reflectance and increase in mangroves.

Salt tolerant mangrove may be able to crystallise salt on leaves to increase reflection.

2 i) How are water potential gradients established through a terrestrial plant? What are the components of total water potential that dominate in each part of the soil plant atmosphere continuum (include root cells, xylem, leaf cells)?

• Psi soil > root > stem > leaf >>> air
• Root - low resistance and high surface area
• Xylem - low resistance to flow
• Leaf cells - loss of water to atmosphere leads to water potential that draws water from rest of the plant. Must overcome gravity in large plants.
• Atmosphere - very low water potential - water readily lost into atmosphere from plant (during transpiration)

4) List the problems that plants face in acquisition of phosphorus from soil. Use a carefully labelled diagram to show the ways in which mycorrhizas help to overcome such problems.

P is very insoluble in the soil, present in very low concentrations in the soil solution, moves slowly by diffusion and may be present as insoluble organics. Fungi have the capacity to absorb soluble inorganic phosphate and hydrolise many organic P compounds. They also increase the surface area over which nutrients can be absorbed.

6 a) What is the ‘photoelectric effect’ and what is its significance for photosynthesis?

Photons encountering metal atoms dislodge electrons and generate a charge when they are of a certain energy (wavelength). In photosynthesis, this is used to 'capture' energy. Consequence is that light must be of a certain wavelength to engage photosynthesis - with less energy, not even large amounts will trigger it.

6 b) What is photorespiration? Explain why it is more significant at higher temperatures.

Photorespiration occurs when Rubisco fixes O2 instead of CO2. As Rubisco is a rate determining step in photosynthesis, this reduces the efficiency of photosynthesis considerably. At high temperatures, Rubisco's affinity for O2 increases, making this more significant. /Probably need some more information here about what photoresp is/.

6 c) At night CAM plants use PEPC to fix CO2 which is then stored as malate in the vacuole. How do they prevent Rubisco and PEPC from competing for the released CO2 during light periods?

CAM increases the concentration of CO2 around Rubisco. PEPC is regulated during the day by inactivation due to the presence of malate. Probably need more info here too.

6 d) Parasitic plants need to maintain a water potential gradient across the haustorial interface with the host plant. Describe the ways in which this is achieved for both hemi- and holoparasites.

Hemiparasites have very high transpiration rates that allow a water potential gradient to be maintained. Both hemi and holo- may lack adaptations that prevent water loss such as a waxy coating and cuticle. Active transport in the haustorium may also aid in the transfer of water to achieve a gradient.

7) Using diagrams, describe how the C4 pathway of photosynthesis works. Now look at the graphs below and explain the responses of the two species.

• C4 plants have a higher photosynthetic nitrogen use efficiency as less Rubisco is required to fix a given amount of CO2. Slope of C4 is higher in both charts.
• C3 plants have lower efficiency at high temperature as Rubisco affinity for CO2 is decreased, therefore lower slow at 35C.
• C4 plants improved by high temperatures due to increased efficiency of photosynthesis at high temperature and maintenance of high CO2 concentration around Rubisco leading to low photorespiration.

8 i) Around 10 million years ago, stable carbon isotope ratios of tooth enamel from fossil remains of herbivorous mammals in Africa showed a significant shift from low values associated with a predominantly C3 diet to higher values indicative of a change to C4 plants. Discuss the possible reasons for such a shift in diet.

Increased temperature in historic instances of climate change are predicted to have increased the abundance of C4 plants, as these are favoured by high temperatures.

8 ii) The IPCC has predicted that atmospheric CO2 concentrations will double by the turn of the next century if emission rates do not decline. What are the possible implications of this for communities dominated by C4 or CAM plants? Why?

Increased CO2 may improve the growth of C3 plants as this increase will reduce photorespiration, though ther is evidence that prolonged exposure leads to a reduced capacity as plants reduce their Rubisco content. This is probably species-dependent. Effects on the composition of micro-organisms, pests and diseases haven't been quantified.

9 i) Indicate on a diagram of the mitochondrial electron transport chain where NADH, FADH2 and NADPH are oxidized. What are some of the sources of these metabolites?

9 ii) Describe one alternative electron transport pathway in plants. What is a possible function of the alternative pathway?

Cyclic e- transport. Involves only PSI. No H+ or O2 from H20. No NADPH formed, but one H+ is translocated per e-. ATP synthesis continues.

Pseudocyclic electron flow (Mehler reaction). Electrons flow from H2O to O2. No net O2 production, no NADPH. ATP still produced. Alternative sink for e- if CO2 fixation is slow or blocked. If the Calvin cycle stops, oxygen radicals accumulate. Mehler may use these up. eg if stomata close on a hot day, light energy is still being absorbed and has to go somewhere.

9 iii) Why cannot CAM plants cool by transpiration? Explain the compromise CAM plants have made as a result of high tissue temperatures?

9 iv) Discuss one of the following in relation to heat stress in plants:

• cytosolic calcium
• heat shock proteins
• isoprene

Other things

C4 cycle

• Carboxylation of PEP produces a C4 acid (catalysed by PEPC)
• Transport of C4 acid to bundle sheath cell
• Decarboxylation of C4 acid (CO2 removed) and subsequent fixation by Rubisco in Calvin cycle
• Regeneration of PEP