1.3 Growth Curves
Plants have different life cycles ranging from a single season to many years. Based on their life spans, plants are categorised into annual, biennial, and perennial plants. Plant growth can be monitored and represented graphically using growth curves that track parameters such as dry mass or height over time. Understanding these curves reveals how food storage, photosynthesis, environmental factors, and seasonal changes influence plant development at different stages of life.
Learning Objectives
- Classify plants based on their life cycles into annual plants, biennial plants, and perennial plants, with relevant textbook examples.
- Describe the characteristics and stages of the sigmoid growth curve of an annual plant.
- Explain the shape and features of the growth curve of biennial plants consisting of two combined sigmoid curves.
- Explain the growth curve of perennial plants consisting of a series of small sigmoid curves influenced by seasonal changes.
- Describe how dry mass is determined experimentally to investigate the growth curve of a plant (such as Zea mays).
- State the function of an auxanometer in measuring the rate of plant elongation.
Comparison of Growth Curves in Annual, Biennial, and Perennial Plants
Each life cycle type produces a distinctive growth curve shape, driven by food storage, photosynthesis and seasonal changes.
Single Sigmoid Curve
Stage A: cotyledon food use. Stage B: photosynthesis boom. Stage C: maturity plateau. Stage D: senescence and leaf shed.
Two Combined Sigmoid Curves
Season 1: vegetative growth and tuber storage. Winter dip. Season 2: flowering and fruiting using stored reserves.
Series of Small Sigmoid Curves
Staircase-like growth rising in spring/summer and flattening in winter, repeated across many years.
Short Explanation
What is a growth curve?
A sigmoid curve is an S-shaped curve that represents the growth of an organism over time. In plants, the shape of the growth curve depends on the type of life cycle: annual, biennial or perennial.
Annual plants
Annual plants complete their life cycle in one season or year. Examples include paddy, pumpkin and watermelon plants. Their growth curve is a single complete sigmoid curve with four stages: A, B, C and D.
Biennial plants
Biennial plants take two years and two seasons of growth. Examples include cabbage, carrot and silver cock's comb. Their growth curve consists of two combined sigmoid curves.
Perennial plants
Perennial plants live for more than two years. Examples include grass, hibiscus and mango plants. Their growth curve is a series of small sigmoid curves linked together year after year.
Measuring plant growth
Plant growth can be measured by determining dry mass over time, or by using an auxanometer to measure the rate of stem elongation.
Try to Answer First
Answer in your mind, then press "Check Answer".
Why does the dry mass of an annual plant decrease during Stage A of its growth curve?
What occurs during the first growth season of a biennial plant such as a carrot?
Why does the growth rate of perennial plants decrease during winter?
Common Mistakes
- Confusing the causes of Stage A and Stage D in an annual growth curve: Stage A is caused by the utilisation of stored reserves in the cotyledon for germination, not by death or leaf shedding.
- Assuming biennial plants complete their life cycle in one year. Biennials require two growing seasons across two years, where reproduction only occurs in the second year.
- Thinking perennial plants grow at a constant speed all year round. Perennial growth rate fluctuates seasonally, being high during spring and summer and dropping during winter.
- Confusing fresh mass with dry mass in plant growth experiments. Measuring plant growth using dry mass requires drying the plant at 100 °C until a constant mass is reached to eliminate water content fluctuations.
Concept Misunderstandings
Perennial plants have a straight linear growth curve because they live for many years.
Perennial growth curves are not straight lines; they are composed of a series of small sigmoid curves occurring each year, reflecting seasonal variations in light intensity and temperature.
Photosynthesis begins immediately upon planting the seed.
Photosynthesis only begins after foliage leaves emerge (Stage B). During germination (Stage A), the embryo relies strictly on organic reserves stored in the cotyledons or endosperm.
The auxanometer measures the total dry mass of a plant.
An auxanometer specifically measures the elongation or increase in height of a plant shoot over time, not dry mass.
Summary
- Classification of Plants by Life Cycle: Annuals complete their life cycle in one season/year (e.g., paddy, pumpkin, watermelon). Biennials complete their life cycle in two years across two seasons (e.g., cabbage, carrot, silver cock's comb). Perennials live for more than two years, flowering and fruiting repeatedly (e.g., grass, hibiscus, mango).
- Growth Curve Patterns: Annuals show a single sigmoid curve featuring Stage A (decreasing mass via cotyledon use), Stage B (exponential increase via photosynthesis), Stage C (plateau at maturity), and Stage D (decrease via senescence, leaf fall, and seed dispersal). Biennials show two combined sigmoid curves. Perennials show a series of small sigmoid curves reflecting active growth during spring/summer and slowed growth during winter.
- Measurement: Growth can be investigated by measuring dry mass over time using an oven at 100 °C (Zea mays experiment) or stem elongation using an auxanometer.
Short Activity
Objective: classify plants by life cycle, explain growth curve stages, and identify measurement methods.
A. Objective Quiz
Which of the following plants is correctly matched with its life cycle type?
What is the primary reason for the rapid increase in dry mass during Stage B of an annual plant's growth curve?
B. Fill in the Blanks
An apparatus used to measure the rate of plant stem elongation is called an .
The growth curve of a biennial plant consists of two curves that are combined together.
C. Matching / Drag and Drop
Match each plant classification or growth stage to its correct textbook description. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Growth Stage or Type to Its Description
Write the correct growth stage or type next to each description.
| Characteristic Description | Growth Stage or Type |
|---|---|
| 1. Dry mass decreases because stored cotyledon food is consumed for germination. | |
| 2. Dry mass decreases slowly due to aging, leaf fall, and seed dispersal. | |
| 3. Vegetative growth occurs and synthesized food is stored in tubers. | |
| 4. A series of small sigmoid curves repeated across multiple years. |
Answer choices: Stage A in annual plants • Stage D in annual plants • First season of biennial plants • Growth curve of perennial plants
