Lecture 4 - adaptations to light in tropical rain forests

Question 1

how have plants adapted to optimise their use of sunlight?

Answer 1

they have the ability to acclimate to changes in irradiance

Question 2

what is acclimation?

Answer 2

the process of adapting to a new environment

Question 3

what does the potential of acclimation to changes in irradiance enable plants to do?

Answer 3

they can exploit more variable environments than plants with a narrower range of responses to light

Question 4

what are sunflecks?

Answer 4

rapid changes in irradiance - plants exposed to short bursts of sunlight

Question 5

what is the impact of sunflecks?

Answer 5

the dynamic irradiance can vary in frequency and intensity and plants can adapt to use this energy when available - they provide a major source of energy for maintenance and growth in the understorey

Question 6

what contribution do sunflecks have?

Answer 6

contribute 10-85% of the total daily light exposure and can enhance C gain by 60-70% for understorey plants

Question 7

why is the ability to acclimate to longer term changes in irradiance also important?

Answer 7

important for climax and shade tolerant species which start life in low sunlight and then need to adapt to the change in sunlight as they emerge from the canopy into full sunlight - this ability of acclimation is important to compete, establish and survive

Question 8

describe begonia erythrphyllas adaptation

Answer 8

• evolved epidermal cells that behave like lenses that focus light onto the chloroplasts - 15 times greater irradiance - maximises light interception in the understorey

Question 9

what are the benefit of anthocyanins?

Answer 9

some monocots and dicots possess red or purple anthocyanin pigments on the underside of leaves which increases the efficiency of light capture by reflecting back absorbed light into the leaf

Question 10

what is blue iridescence?

Answer 10

• a result from microscopic anatomical features that interfere with light and increase capture of photosynthetically active radiation at the red end of the spectrum

Question 11

what taxa is blue iridescence seen in?

Answer 11

seliginella
melastomatacae
begoniaceae

Question 12

what does plagiotropic mean?

Answer 12

horizontal branches

Question 13

what does orthotropic mean

Answer 13

more vertical growth of branches

Question 14

describe understorey trees

Answer 14

plagiotropic with fairly large well spaced leaves - self shading can be minimised by leaf shape and size

Question 15

describe trees in the sun

Answer 15

orthotropic with small leaves orientated further from the horizontal to avoid damage from excess irradiance (photoinhibition)

Question 16

how can trees adapt to changes in irradiance

Answer 16

trees can exhibit changes in form and leaf angle at different stages of their lifecycle

Question 17

describe the pioneer tree macaranga gigantia

Answer 17

• produces large leaves near the stem
• expansion takes 3 weeks but petioles continue to extend for 91 days which allows the ageing leaf to extend beyond newly produced leaves just avoiding shading them

Question 18

describe the differences in leaves/ plants growing in the sun and shade

Answer 18

• anatomically and metabolically different
• quantum yield is the same in both sun and shade plants although the light compensation point is higher in the sun plant
• sun plants achieve a higher rate of light-saturated photosynthesis than shade plants

Question 19

describe the cells of sun plants

Answer 19

• large cells
• small chloroplasts
• low chlorophyll/rubisco ratio
• high chlorophyll a/b ratio
• high N content
• high xanthophyll cycle pigments to protect against damage

Question 20

cells of shade plants

Answer 20

• small cells
• large chloroplasts
• high chlorophyll/rubisco ratio
• low chlorophyll a/b ratio
• low N content
• low xanthophyll cycle pigments

Question 21

leaves of sun plants

Answer 21

• small and thick
• high stomatal density
• high rate of transpiration
• low specific leaf area

Question 22

leaves of shade plants

Answer 22

• large and thin
• low stomatal density
• low rate of transpiration
• high specific leaf area

Question 23

sun plants

Answer 23

• vertical leaf orientation
• leaf area index higher
• more canopy layers
• short leaf lifespan and high turnover
• high photosynthetic capacity

Question 24

shade plants

Answer 24

• horizontal leaf orientation
• leaf area index lower
• fewer canopy layers
• long leaf lifespan and low turnover
• low photosynthetic capacity

Question 25

why is a thicker cuticle advantageous in sun leaves

Answer 25

contributes to great leaf rigidity and reduces wilting and susceptibility to drought

Question 26

describe the plant s. leprosula

Answer 26

light hardwood with a faster growth rate - requires a higher light to regenerate and its seedlings dont survive long in deep shade

Question 27

describe the plant D.lanceolota

Answer 27

medium hardwood - seedlings survive in a wide range of light conditions from deep shade to large canopy gaps

Question 28

describe the experiment involving d.lanceolota and s.leprosula and the results

Answer 28

• seedlings were planted under 3 contrasting light conditions
• D.lanceolota had a greater ability to aclimate to shade and put a high carbon allocation into growth of just a few long branches increasing total leaf area of the plant - forages for light horizontally
• S.leprosula had a greater ability to aclimate to high light with high rates of height and branch growth - forages for light vertically

Question 29

what were the conclusions of the experiment between the two plants?

Answer 29

both are climax species but with different competitive advantages in high and low light conditions - allows for nice differentiation between the two species with respect to the timing of disturbance events that increase light availability
S.L = superior growth in high light
D.l = characteristics of longevity in low light