Lecture 16

Question 1

anther

Answer 1

bears pollen

Question 2

stigma

Answer 2

receives pollen

Question 3

why is physiological ecology different for plants?

Answer 3

• 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)

Question 4

describe photosynthesis

Answer 4

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

Question 5

Net Primary Productivity (NPP)

Answer 5

C gained via photosynthesis - C lost via respiration

Question 6

what happens in synchrony with photosynthesis?

Answer 6

• 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

Question 7

how do photosynthetic structures embody adaptation to environmental stresses?

Answer 7

• leaf size and shape: SA:V ratio important

Question 8

costs and benefits of large leaf SA

Answer 8

• benefits: good fro harvesting light, CO2
• costs: bad for overheating, water loss by transpiration through stomata

Question 9

why and how have plants evolved to overcome the shortcomings of C3 photosynthesis?

Answer 9

1. 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)
2. 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

Question 10

how do plants with large leaves combat overheating?

Answer 10

• growing in shady habitats
• evaporative cooling by opening stomata

Question 11

evaporative cooling needs plentiful water, which is not always available. Plants with large leaves combat water loss by:

Answer 11

closing stomata

Question 12

fundamental trade-off between water conservation and rapid growth

Answer 12

• closing stomata shuts off all gas exchange, including CO2 input, so photosynthesis shuts down.
• the plant stops growing and risks overheating and tissue damage

Question 13

in what plants are the consequences of the water conservation/growth trade-off most obvious?

Answer 13

desert plants

Question 14

Palo Verde (Parkinsonia sp)

Answer 14

= 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)

Question 15

leaves in tropical rainforests vs deserts

Answer 15

• 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

Question 16

Santa RIta prickly pear (Opuntia Santa-rita)

Answer 16

Microphylly taken to extremes: no leaves

Question 17

Saguaro cactus (Carnegiea gigantea)

Answer 17

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

Question 18

describe cactus roots

Answer 18

extensive but shallow
- when it rains in the desert, the water moistens just the top few cm of soil but never penetrates any further

Question 19

describe tropical tree roots

Answer 19

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)

Question 20

Root foraging

Answer 20

plants grow their roots in soils where nutrients are abundant

Question 21

Rebecca Doyle and Legume species (Medicago truncatula):

Answer 21

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

Question 22

can plants evade stress through behaviour?

Answer 22

deciduous habit:
- dropping leaves during dry or cold seasons reduces water stress and tissue damage

Question 23

how does leaf shape influence gas exchange?

Answer 23

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

Question 24

morphological plasticityL sun and shade leaves from one red oak tree

Answer 24

shade leaf: smooth surface, no bumps
- more laminar flow, less cooling
sun leaf: bumpy leaf
- more turbulence, better cooling

Question 25

Monstera deliciosa

Answer 25

• dissected outlines cause turbulent air flow (sun leaf)
• fewer holes in shade leaves to promote laminar flow

Question 26

recursive digression

Answer 26

convective cooling aided by turbulence
- small snow bunny has smooth surface to promote laminar air flow and keep it warm

Question 27

what type of evolution are cacti an example of?

Answer 27

convergent

Question 28

can plants in rainforests be water stressed?

Answer 28

yes, if they’re epiphytes

Question 29

what are epiphytes?

Answer 29

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