Suggested answers for Ecophysiology of Plants past exams
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.
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).
Apparently not in the course this year.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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:
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
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.
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.
Two spatially separated carboxylation steps.
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.
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.
Heat shock proteins, calcium mediation (mention these two for best results), hairy/waxy surfaces, leaf rolling, vertical orientation, small leaves, dimorphic leaves.
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.
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.
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)
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.
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.
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.
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).
See 2005 8 b)
(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.
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.
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.
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.
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.
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.
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.
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.
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/.
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.
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.
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.
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.
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.