BIO2030 Chap.2 Plant Growth, Biomass Partitioning and Growth Indices
Plant Growth, Biomass Partitioning and Growth Indices
Plant Growth, Biomass Partitioning and Growth Indices answers one measurement problem and then spends the rest of the chapter on its consequences: how do you compare the growth of two plants of very different size. The answer is to express growth as a rate per unit of existing plant rather than as an increment, which is what relative growth rate does.
From there the chapter decomposes that rate, works through the indices you will calculate from your own harvested plants, and ends on harvest index, the ratio that explains why shorter cereal varieties fed more people without necessarily growing more. This is the chapter your laboratory report is built on, so the formulae and their units matter more here than anywhere else in the unit.
What this chapter covers
- 01
Why an absolute increase in size is not a comparison, and what a relative rate fixes
- 02
Relative growth rate: the formula, the natural logarithms, and the units of grams per gram per day
- 03
Decomposing the rate: how much leaf a plant has, and how productive each unit of leaf is
- 04
Specific leaf area and leaf area ratio, and the different denominators that separate them
- 05
Root to shoot allocation, and how it shifts under nutrient and water limitation
- 06
Biomass partitioning across the life cycle: where new mass goes at each stage
- 07
Harvest index as economic yield divided by biological yield, and what the definition excludes
- 08
Why dwarf cereal varieties raised yield: a partitioning change, not a growth change
Decide which index answers the question you were actually asked
- +1Identify what the claim is about: light capture per unit of plant invested, which is an allocation question, not a size question.
- +1Rule out total biomass and total leaf area. Both are larger in a plant that is simply bigger, so neither can separate a size effect from an allocation effect.
- +1Choose leaf area ratio, total leaf area divided by total plant dry biomass. It asks what fraction of the whole plant is deployed as light capturing surface.
- +1Say what specific leaf area would add instead: leaf area divided by leaf dry weight tests whether the leaves became thinner, which is a different mechanism from allocating more of the plant to leaves.
- +1State both readings: if leaf area ratio is unchanged the plant is simply larger with the same design; if it rose, allocation genuinely shifted toward leaves, and specific leaf area then tells you whether that came from more leaves or from thinner ones.
Key terms
- Relative growth rate
- New dry biomass produced per unit of existing dry biomass per unit time, calculated from the natural logarithms of biomass at two harvests divided by the interval, in grams per gram per day.
- Specific leaf area
- Total leaf area divided by leaf dry weight, in square centimetres per gram. It describes how thinly a plant builds its leaves, and it says nothing about how much of the plant is leaf.
- Leaf area ratio
- Total leaf area divided by total plant dry biomass, in square centimetres per gram. It describes how much of the whole plant is deployed as light capturing surface.
- Root to shoot ratio
- Root dry weight divided by the combined dry weight of leaves, stem and petioles. It is dimensionless, and it shifts predictably when a plant is limited by something in the soil rather than in the air.
- Harvest index
- Economic yield divided by biological yield, meaning the harvested product as a fraction of above ground dry matter. Roots sit outside the ratio as the unit defines it.
Plant Growth, Biomass Partitioning and Growth Indices FAQ
Why does relative growth rate use natural logarithms?
Because growth compounds. New tissue produced today itself produces tissue tomorrow, so biomass follows an exponential path rather than a straight line, and the natural logarithm is what turns that path into a straight line whose slope is a constant rate. Dividing the difference of the logarithms by the interval gives the average of that slope over the window.
It is also why the units come out as grams per gram per day: mass appears in both numerator and denominator and cancels, leaving an inverse time.
Can I calculate relative growth rate for an individual plant?
Not in this experiment, and the reason is the design rather than the arithmetic. The two harvests are destructive, so the plants weighed at the baseline are not the plants weighed at the end and no individual has both values. The unit therefore tells you to average total dry biomass across the plants at each harvest and use those two means.
Every other index in the chapter is different: those are calculated per plant first, and the mean and standard deviation are taken from the per plant values.
Why did shorter wheat varieties feed more people?
Because they changed where the plant put its growth rather than how much it grew. Harvest index is the harvested product divided by above ground dry matter, so a variety that builds less stem and the same amount of grain has a higher harvest index and yields more from the same total growth. Shorter straw also resists lodging, which lets the crop carry a heavier head and take more fertiliser without falling over.
The lesson generalises: a yield gain can come from more growth or from a different allocation of the same growth, and those are separate levers.
Assessment move
This is the chapter to over prepare, because it is examined twice, once in the first in-class test and once through the laboratory report. Copy the five formulae onto one page with their units beside them and practise until you can write each denominator without hesitating, since that is where the marks are lost.
Take your own harvest spreadsheet and compute every index for one species under both treatments even if you will not use them all, because the pattern across indices is what tells you which story your data can support. Rehearse the decomposition argument in words: a plant can grow faster because it has more leaf or because each unit of leaf is more productive, and the indices separate those.
Then rehearse the two harvest index readings, one about the ratio and one about lodging resistance, because the Green Revolution question in this unit is asked as a mechanism question and not as a history question.
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