Lecture 6

Question 1

Describe the annual rainfall patterns in Australia.

Answer 1

High contrast between centre of Australia and its margins - central areas receive less than 200 mm per annum whereas coastal margins can receive up to 2000 +.

Question 2

Average daily temperatures increase the further _____ in Australia you go. Roughly the upper third of the country has an average daily maximum of __ degrees C.

Answer 2

north; 27.

Question 3

Many parts of Australia (mostly in the centre of the country) have maximum average daily temperatures of 40 degrees C + in the ______ months.

Answer 3

Summer.

Question 4

Dry matter production (a measure of productivity) ______ when precipitation ______.

Answer 4

Increases; increases.

Question 5

Less than _% of plant water use is in reactions of photosynthesis. There is a net ___ of water in photosynthesis.

Answer 5

1; use.

Question 6

Gas exchange is regulated via _________. Gas exchange requires open ______.

Answer 6

stomata; stomata.

Question 7

The pore size of stomata ________ when the turgor of the guard cells increases.

Answer 7

increases.

Question 8

When turgor of guard cells decreases, the pore size of the stomate _______.

Answer 8

decreases.

Question 9

Is there a big gradient between CO2 concentration inside C3 plants and CO2 concentration outside? How does this affect the rate of CO2 diffusion?

Answer 9

No. Thus CO2 diffusion is slow.

Question 10

Is there a big gradient between H20 concentration inside plants and H20 concentration outside? How does this affect the rate of flow of H20 from the plant?

Answer 10

Yes. High humidity inside plant; relatively low humidity outside plant. Thus when stomata are open, flow of humid air from inside the plant to outside is rapid.

Question 11

What key process drives the movement of water from the roots of a plant to the stems?

Answer 11

Transpiration.

Question 12

List the three photosynthetic pathways in order of least to most water efficient.

Answer 12

C3, C4, CAM.

Question 13

Are ‘drought avoidant’ plants necessarily more water efficient?

Answer 13

No.

Question 14

What is the main way that drought avoidant plants survive during droughts?

Answer 14

Through maximising water supply.

Question 15

Give some examples of drought avoidant plants and the way in which they exhibit this characteristic.

Answer 15

• River red gum (deep roots to access ground water)
• Sturt’s desert pea (reproduces quickly after rain)
• Annual/ephemeral plants (spend dry times as seeds and reproduce quickly after rain)

Question 16

What is the other name for drought tolerant plants?

Answer 16

Xerophytes.

Question 17

What are the two main types of xerophytes?

Answer 17

Succulents and sclerophylls.

Question 18

Sclerophylls are ___-leaved plants.

Answer 18

Hard

Question 19

Are succulents or sclerophylls more common?

Answer 19

Sclerophylls.

Question 20

Which conditions favoured the evolution and dominance of sclerophylls in the Australian landscape?

Answer 20

• increased aridity and fire frequency

- weathering of soils

Question 21

Sclerophylls contain lots of ______ tissue. They have ____, rigid leaves that do not wilt when water stressed.

Answer 21

structural; hard.

Question 22

List three adaptations of sclerophylls.

Answer 22

• small leaf size, short internodes
• proportionally thick leaves
• reduced surface area/volume ratio (sometimes through leaf rolling/leaves circular in cross section).

Question 23

List three groups of sclerophylls (including one type of grass).

Answer 23

• Acacia
• Ericaceae (southern heaths are sub-family Epacridoideae).
• Triodia (spinifex) grass

Question 24

List 4 anatomical features of sclerophyll leaves.

Answer 24

1) THICK WALLED CELLS

Sclerophylls lay down additional layers of carbohydrates (lignin) inside the primary cellulose cell walls to form thick secondary cell walls in fibre and sclereid cells. The secondary walls become thicker and thicker as additonal layers are added and the space for living contents diminishes. The cells are actually non-living at maturity (functioning as structural support).

2) THICK CUTICLE AND WAXY COATING
Makes leaf water tight except for where stomata are.

3) SUNKEN STOMATA
Creates a pocket of humid air just outside of guard cells to reduce loss of water vapour to atmosphere. Lowers concentration difference of H20 between inside and outside of guard cells.

4) LEAF HAIRS/FORMATION OF ‘BOUNDARY LAYER’
Dense mat of short interlocking hairs on the undersurface of leaves to trap water vapour from nearby stomata and reduce evapotranspiration. Water vapour accumulates in the hairs and creates a ‘boundary layer’ of humid air just near the external surface of the leaf. (Reduces concentration difference of H20 inside and outside guard cells).

Question 25

How does leaf-rolling in protect grass sclerophylls during dry conditions?

Answer 25

Only outer surface is displayed. The inner surface is highly invaginated and contains lots of hairs. The stomata lay below the invaginations and are thus protected from the very hot/dry external environment. Again, hairs reduce concentration different of H20 between outside and inside of guard cells. Pocket of humid air/boundary layer created.

Question 26

How does a lack of phosphorus affect plant growth?

Answer 26

A lack of nutrients can limit plant cell growth and metabolism. Phosphorus is critically important as it is the backbone for nucleic acids, is a component of ATP and cell membranes.

Plants in low phosphorus conditions exhibit slow growth and small leaves and internodes. Carbohydrates are channeled into lignin in order to create thick cell walls.

Question 27

What was Beadle’s hypothesis? What piece of historic evidence supported his claim?

Answer 27

Beadle hypothesized that sclerophylly was primarily an adaptation to infertile soils, even where rainfall was relatively high, because sandy soils dry out and can cause water stress in summer. Sclerophylly is well developed in banksias (family Proteaceae), which were present in the early Paleogene when rainfall was high and fires infrequent. Beadle and other workers suggested that sclerophyll plants formed wet heathlands on the margins of rainforest where soil fertility was low. Heathlands occur today on the most extremely infertile soils, in both wet and dry climates.

Beadle suggested that low levels of soil phosphorus were particularly important in limiting the growth of rainforest species and favouring sclerophylls. He argued that low phosphorus levels affect physiological processes and lead to a reduction in the number of cells formed and length of stem internodes. Such changes result in smaller leaf sizes and smaller plants, which would have been able to survive low and variable rainfall as aridity increased, and also droughts of cool montane environments during the onset of the ice ages.

Sclerophylls of the modern flora are characterised by a number of features in addition to small leaves and short internodes: thick cuticle on the leaf epidermis, which may overarch guard cells of stomata; stomata sunken in grooves or pits, or protected by a dense covering of hairs; high proportion of thick, lignified cells such as fibres and sclereids.

Question 28

What are the main features of succulent plants?

Answer 28

Plants fleshy, with large cells filled with a watery sap; drought and salt tolerant.

Question 29

Which family of succulent plants is common in Australia?

Answer 29

Chenopodiaceae.

Question 30

What are some adaptations of Atriplex (saltbush) in the Chenopod family?

Answer 30

Surface colour reflects radiation thus reducing change of overheating.
Covered with bladder cells to excrete salt.

Question 31

How do some succulent plants deal with living in very saline environments?

Answer 31

They take up the salt dissolved in water and store it in particular areas so that it won’t do damage to their tissues. Salt bush uses bladders to do this.

Question 32

Does arid Australia have large succulent plants?

Answer 32

No.

Question 33

Are Boab trees succulent? How many species are found in Australia?

Answer 33

Yes. 1.

Question 34

Do Boab trees retain their leaves throughout the year?

Answer 34

No. They are drought deciduous.

Question 35

What is the genus name of Prickly Pear?

Answer 35

Opuntia.

Question 36

Why did Prickly Pear spread so quickly after it was introduced with the cochineal bug?

Answer 36

• Drought resistant (CAM, succulence, cuticle)
• Reproduces sexually and asexually
• Pads and fruit can root (e.g. during floods)
• Yummy fruit encouraged seed dispersal by e.g. emus

Question 37

Which 2 methods were used extensively to try and control Prickly Pear before biological control?

Answer 37

• slash and burn

- vast amounts of arsenic pentoxide