W1, Growth & yield formation

Question 1

Give a short response explaining the critical growth stage for wheat yield

Answer 1

The critical growth stage is the time when the yield is most sensitive to the growing conditions. There is generally two, including the establishment phase, which determines the starting plant density and impacts weed pressure. In wheat, the other critical growth stage is the second half of stem elongation (about 3 weeks before flowering, from flag leaf emergence (~GS37-39)) to about 10 days after flowering, where a lot of the yield potential is determined and the sensitivity of the yield to the growing conditions is greatest.

Question 2

True or False?

Late season cereal varieties have a smaller optimum sowing window than early season varieties.

Answer 2

False.

Question 3

True or false?

The sensitivity of wheat yield is high during grain filling.

Answer 3

False. Compared to other growth phases, grain filling is a relatively conservative phase and yield is affected as much by stress during this time.

Question 4

True or false?

Faba beans are determinate crops.

Answer 4

False. Faba beans (along with canola and most grain legumes (pulses)) are indeterminate crops, meaning that they don’t have separate vegetative and reproductive phases - they can overlap and be extended.

Question 5

True or false?

Would leaf rust be more detrimental (to yield) during tillering or grain filling?

Answer 5

Tillering.

If the plant loses all of its leaves during grain filling, it will still have enough reserves in the stem and roots to continue maturing; whereas if the plant loses a significant portion of its leaves during tilering, the amount of photosynthate being produced to drive the growth required to achieve its yield potential is severely reduced.

Question 6

Explain what biomass partitioning is and how it’s measured/quantified.

Answer 6

Biomass partitioning = how much photosynthate (resources) is being allocated to different structures (particularly reproductive vs vegetative).

Measured as the ‘Harvest Index’, which is:

HI = grain yield / total biomass * 100

Question 7

What would you expect the harvest index of a wheat or barley crop to be in South Australia?

Answer 7

35%-45%

Question 8

What would you expect the harvest index of a canola crop to be in South Australia?

Answer 8

15%-35%

Question 9

What would you expect the harvest index of a faba bean crop to be in South Australia?

Answer 9

20%-30%

Question 10

What is the main reason that dwarf varieties yield more than tall ones?

Answer 10

Biomass partitioning. During the critical growth phase in cereals, there is a high level of resource competition between the stems and the ears, and dwarf varieties are able generally able to allocate more resources to the ears (which are at their highest rate of growth) than the tall varieties.

Question 11

Explain the hierarchy of survival within cereal plants (i.e. the internal competition for resources).

Answer 11

As the season progresses and the crop develops, the level of competition (and stress) increases.

Cereals prioritise the main stem and the first one or two tillers (which you’re likely to get as a minimum, even under considerable stress conditions), and sacrifice the later-formed tillers. By sacrifice, what they actually do is reallocate (some of) the resources stored within the younger tillers to the main stem and oldest tillers.

The severity of tiller loss correlates to the severity of resource competition (i.e. availability).

Question 12

Why are younger tillers more sensitive to stress (or to having the plant reallocate resources away from them during times of stress)?

Answer 12

Because they don’t have their own independent root system (which usually requires at least 3 leaves).

Question 13

What are the major yield compenents of a cereal crop?

Answer 13

• # heads/m2
• # spikelets/head
• # grains/spikelet
• (1000) grain weight
• # spikelets/head * # grains/spikelet = # grains/head
• # heads/m2 * # grains/head = # grains/m2
• # grains/m2 * 1000 grain wt

Question 14

True or false?

The number of grains/m2 represents the post-anthesis growing conditions.

Answer 14

False; it represents the pre-anthesis growing conditions, which is when the number of grains is already determined. The 1000 grain weight represents the post-anthesis growing conditions.

Question 15

True or false?

The number of heads/m² is a major yield component.

Answer 15

True, but the highest yields generally come from having long heads (with a lot of spikelets) rather than more heads.

Question 16

During what developmental stages are the number of heads/m2 determined?

Answer 16

Early stages of development - establishment, tillering, and stem elongation.

Question 17

During what developmental stages are the number of spikelets/head determined?

Answer 17

During the spikelet initiation phase, before stem elongation (by that time, the length of the heads has already been determined).

Question 18

During what developmental stages are the number of grains/spikelet determined?

Answer 18

Just before flowering when the ear is growing most quickly, and at flowering when pollination is an issue.

Question 19

True or false?

The size of one yield component (e.g. heads/m2) affects the size of other components.

Answer 19

True.

• More heads/m2 will generally result in fewer spikelets/head.
• Fewer heads/m2 generally results in larger grain weight.
• More spikelets/head generally results in fewer grains/spikelet.
• More grains/m2 generally results in smaller grain weight.

Question 20

True or false?

Yield depends more on grain size than grain number.

Answer 20

False. Grain weight doesn’t affect yield a whole lot, but there is a high correlation between #grains/m² and yield.

Question 21

What is the effect of increasing the amount of available N on the various yield components of cereals?

Answer 21

• Can cause haying off (large amount of biomass early on, then runs out of water at just before flowering and during grain filling).
• # heads/m2 increases
• # spikelets/ear increases
• # grains/spikelet decreases
• grain wt decreases

Question 22

Why might it be a good idea to limit biomass production early on in the season by holding back on the fertiliser?

Answer 22

The number of tillers/m2 generally declines during stem elongation up to flowering as the plant decides on how to allocate its available resources. So rather than wasting resources on tillers that will eventually die, if you limit the number of tillers that are formed in the first place, the resources that you allocate to the plant can be utilised more efficiently.

Question 23

Explain the difference between determinate vs indeterminate crops

Answer 23

Determinate crops like cereals have distinct vegetative and reproductive phases that do not overlap, whereas indeterminate crops like canola and most grain legumes exibit overlapping vegetative and reproductive phases, as well as overlapping reproductive and grain filling phases.

Question 24

Why are the top 3 leaves (flag leaf, flag leaf -1, flag leaf -2) considered so important?

Answer 24

They’re the main leaves that provide the photosynthesis for the growth of the crop leading up to flowering, and filling the grain after flowering, particularly the flag leaf.

Question 25

How often (generally) do leaves emerge on cereals?

Answer 25

~1/week.

Question 26

How much photosynthate comes from the ear, flag leaf, stems and leaf sheath, and the lower leaves?

Answer 26

ear: 10-60%

flag leaf: 10-50%

stems and leaf sheath: 10-60%

lower leaves: 0-20%

Question 27

Explain what water soluble carbohydrates are and why they’re important.

Answer 27

Basically a temporary store of photosynthate in the stems. They’re important for maintaining the growth of the plant after anthesis, and they’re also an important component (quality criteria) for oaten hay production because it determines the digestibility and metabolisable energy, hence why people cutting hay try not to let the crop get into grain filling, because that’s when those WSCs are being utilised by the plant.

Question 28

True or false?

As rainfall decreases (i.e. in drier environments), the stem and ear are more important sources of photo-assimilates than the top 3 leaves.

Answer 28

True. The stem and ear can account for up to 80% of photo-assimilate production.

Reflects the plant’s ability to maintain a large green leaf area.

Question 29

What is the problem with overlapping growth stages (i.e. indeterminate crops), particularly around flowering?

Answer 29

• Competition between vegetative growth (stem elongation) and flowering
  
  • = lower pod set and yield penalties.
• Its been shown in lupins and faba beans that a large amount of the early flowers don’t form pods because of the amount of competition.
• Can be good to restrict vegetative growth at this stage

Question 30

Define physiological maturity, and explain why it is important for windrowing canola.

Answer 30

• Physiological maturity = seed has reached its maximum weight
  
  • from that point on its just losing water (dehydrating).
  • mostly evaluated by colour
• In canola, windrowing has to match the point of physiological maturity.
• For every week too early that canola is windrowed, there’s a ~3% loss in yield.

Question 31

True or false?

Canola is more sensitive to grazing than cereals. Explain.

Answer 31

True; canola’s reproductive structures remain above the soil where they can be easily damaged or removed, whereas cereal reproductive structures remain below the soil until around GS30 which protects them from damage/grazing.

Question 32

True or false?

In canola, the yield potential is already determined by the time of flowering.

Answer 32

True.

At the start of flowering:

• Max leaf area reached
• 50-75% total N taken up
• Stems ~50% max length

Question 33

True or false?

Canola pods are photosynthetic.

Answer 33

True.

By the pod filling stage the pods produce a majority of the photosynthate (followed by the stems). The reason for this is that by this stage a lot of the leaves have been shaded out and killed off, so the stems and pods become the sources of photosynthate.

Question 34

At what point can you be pretty sure that the pods you see are going to continue on to maturity?

Answer 34

By the time the pods are ~2cm long, you can be pretty sure that they’re going to continue on to maturity even if they encounter stress.

Question 35

True or false?

Those flowers and pods produced later are more likely to set seed and be successful.

Answer 35

False; the earliest flowers are the ones that get set.

75% of the pods at maturity are set within the first 11 days after the start of flowering.

Flowers produced later are competing for resources with pods that have already been set, and at the later the season gets the more heat and drought stress the plants are likely to encounter.

Question 36

When is canola yield the most sensitive to stress, and what yield components are most affected?

Answer 36

During the first 10 days of pod set (after the begining of flowering).

Stress has minimal impact on seed weight, but a large impact on seed number, which is what affects yield.