Lecture 16 Flashcards
anther
bears pollen
stigma
receives pollen
why is physiological ecology different for plants?
- sessile, with little scope for behaviour; animals can escape adverse conditions, but plants must tolerate them
- autotrophic; they make their own food through photosynthesis
- all plants need the same few things to grow; light, CO2, water, and soil nutrients (esp NPK)
describe photosynthesis
CO2 + H2O –(light)–> carbohydrate + O2
- plants must bring together CO2, water, and light in functioning photosynthetic tissues
- enzymes also require a certain temperature
- for growth, plants have to acquire more carbon through photosynthesis than they lose through respiration; carbon balance is thus key
Net Primary Productivity (NPP)
C gained via photosynthesis - C lost via respiration
what happens in synchrony with photosynthesis?
- photosynthetic (green) structures are usually leaves (but can be stems)
- plants take in CO2 through stomata
- but plants also transpire; they lost water through stomata
how do photosynthetic structures embody adaptation to environmental stresses?
- leaf size and shape: SA:V ratio important
costs and benefits of large leaf SA
- benefits: good fro harvesting light, CO2
- costs: bad for overheating, water loss by transpiration through stomata
why and how have plants evolved to overcome the shortcomings of C3 photosynthesis?
- most plants fix carbon by C3 photosynthesis:
- rubisco is the enzyme that accepts CO2
- but at high temperatures, rubisco often captures O2 instead of CO2, which is bad for plants (photorespiration) - some plants have evolved:
- C4 photosynthesis: the enzyme PEP carboxylase first accepts CO2, reducing photorespiration
- CAM photosynthesis: plants close stomata during the day to reduce water loss, open stomata at night to let in CO2; photosynthesis still needs light, so they store CO2 as malate until daytime
how do plants with large leaves combat overheating?
- growing in shady habitats
- evaporative cooling by opening stomata
evaporative cooling needs plentiful water, which is not always available. Plants with large leaves combat water loss by:
closing stomata
fundamental trade-off between water conservation and rapid growth
- closing stomata shuts off all gas exchange, including CO2 input, so photosynthesis shuts down.
- the plant stops growing and risks overheating and tissue damage
in what plants are the consequences of the water conservation/growth trade-off most obvious?
desert plants
Palo Verde (Parkinsonia sp)
= green stick
- microphylly
- photosynthetic bark on trunks and branches makes up for this
- can grow without incurring heat load and water loss through leaves
- deciduous tree in response to drought (drops leaves)
leaves in tropical rainforests vs deserts
- tropical rainforests: warm and wet
- leaves very big
- deserts: hot and dry
- leaves very tiny
this is because rainfall is abundant in tropical rainforests, so plants can afford to have very big leaves and lots of transpiration
Santa RIta prickly pear (Opuntia Santa-rita)
Microphylly taken to extremes: no leaves
Saguaro cactus (Carnegiea gigantea)
restricted to Sonoran desert and adapted to episodic rains
- grows to 15m, 200yr, 5+ tonnes
- CAM photosynthesis
- extensive, shallow roots
- accordion-pleated trunk allows expansion
- can absorb 800L water from one storm, use it gradually for growth
describe cactus roots
extensive but shallow
- when it rains in the desert, the water moistens just the top few cm of soil but never penetrates any further
describe tropical tree roots
extensive but shallow
- have a shallow layer of nutrient-rich soil due to rain leaching
- extensive, shallow roots are an adaptation to acquire scarce nutrients (phosphorous, nitrogen, etc)
Root foraging
plants grow their roots in soils where nutrients are abundant
Rebecca Doyle and Legume species (Medicago truncatula):
split root experiment
- low N vs high N
- good vs bad NF bacteria
- legumes forage for both soil nitrogen and nitrogen-fixing bacteria
- legume roots grow larger in more nitrogen and make more nodules with good bacteria
can plants evade stress through behaviour?
deciduous habit:
- dropping leaves during dry or cold seasons reduces water stress and tissue damage
how does leaf shape influence gas exchange?
through laminar vs turbulent air flow over surfaces
- laminar: airflow unimpeded, moves in smooth fashion over a surface
- causes stagnant layer of air called a boundary layer to build up, preventing gas exchange
morphological plasticityL sun and shade leaves from one red oak tree
shade leaf: smooth surface, no bumps
- more laminar flow, less cooling
sun leaf: bumpy leaf
- more turbulence, better cooling
Monstera deliciosa
- dissected outlines cause turbulent air flow (sun leaf)
- fewer holes in shade leaves to promote laminar flow
recursive digression
convective cooling aided by turbulence
- small snow bunny has smooth surface to promote laminar air flow and keep it warm
what type of evolution are cacti an example of?
convergent
can plants in rainforests be water stressed?
yes, if they’re epiphytes
what are epiphytes?
plants than grow on trees, so they aren’t able to put their roots into the soil, leading to water stress and nutrient shortages
- some cacti grow epiphytically on trees
