F. Plants: Anatomy, Growth, and Function · Plants: Anatomy, Growth, and FunctionGrowth Factors: Light, Water, Nutrients, Temperature and Soil pH
F3.4 F2.2 F2.1 A1.6 A1.13About 70 minutes
Learning goals- I can explain how light quantity, light quality and day length affect plant growth and flowering.
- I can explain the roles of the major plant nutrients and identify common deficiency symptoms.
- I can explain how temperature, water retention and percolation, and soil pH affect growth.
- I can apply the idea of a limiting factor and read a fertilizer label.
Words to know
limiting factor
The resource or condition in shortest supply relative to need, which limits the rate of growth.
macronutrient / micronutrient
Mineral elements plants need in large amounts (N, P, K, Ca, Mg, S) / in tiny amounts (such as Fe, Mn, Zn, B, Cu).
chlorosis
Yellowing of leaves caused by a lack of chlorophyll, often from a nutrient deficiency.
photoperiodism
A plant's response to the relative lengths of day and night, especially in timing flowering.
short-day / long-day plant
A short-day plant flowers when nights are longer than a critical length; a long-day plant flowers when nights are shorter than a critical length.
water retention / percolation
How much water a soil holds / how fast water drains down through it.
soil pH
A measure of how acidic or basic soil is; it affects which nutrients plants can absorb.
N-P-K
The three numbers on a fertilizer label: the percentages by mass of nitrogen (N), phosphorus (as P₂O₅) and potassium (as K₂O).
Why does the same seed grow differently?
Plant the same tomato seeds in a sunny garden in Leamington and a shady balcony in Thunder Bay, and you'll get very different plants. A plant's genes set its possibilities, but its environment decides how close it gets. In this lesson you'll look at the main outside factors: light, water, nutrients, temperature and soil pH. You will use them in the next lesson to design your own growth inquiry.
Remember from Grade 9 and this unit
Photosynthesis needs light, CO₂ and water; cellular respiration needs oxygen. Roots absorb minerals by active transport (Lesson 2), and drought makes plants close their stomata using ABA (Lessons 4 and 12).
Light: how much, what colour, and for how long
Quantity (intensity). More light usually means faster photosynthesis, up to a point. At high intensity, the rate levels off (light saturation) because something else, such as CO₂ or temperature, becomes limiting. Too little light gives tall, thin, pale (etiolated) plants.
Quality (colour). Chlorophyll absorbs mostly blue and red light and reflects most green light, which is why leaves look green. LED grow lights for greenhouses and indoor farms often give mostly red and blue light. Plants also sense the ratio of red to far-red light: under other leaves, far-red is relatively high, so a shaded plant grows taller to escape the shade.
Duration (day length). Many plants time their flowering by photoperiod, using a pigment called phytochrome. What they really measure is the length of the night:
- Short-day plants (chrysanthemums, poinsettias) flower when nights are long, in fall.
- Long-day plants (spinach, lettuce, many northern wildflowers) flower when nights are short, in early summer.
- Day-neutral plants (tomatoes, cucumbers) flower regardless of day length.
Canada's long summer days at high latitudes suit many long-day crops.
Nutrients: what plants need from the soil
Plants make their own sugar, but they need mineral nutrients from the soil to build proteins, DNA, chlorophyll and ATP. The table summarizes the most important ones.
| Nutrient | Main roles | Deficiency symptoms |
|---|
| Nitrogen (N) | Proteins, nucleic acids, chlorophyll | Slow growth; older leaves yellow first |
| Phosphorus (P) | ATP, DNA, cell membranes; root growth and flowering | Stunted growth, dark or purplish leaves, poor roots |
| Potassium (K) | Opening and closing stomata, enzyme activity | Scorched or yellow leaf edges on older leaves, weak stems |
| Magnesium (Mg) | Centre of the chlorophyll molecule | Yellowing between veins on older leaves |
| Iron (Fe) | Making chlorophyll, electron transport | Yellowing between veins on young leaves |
| Calcium (Ca) | Cell walls, cell division | Damaged growing tips; blossom-end rot in tomatoes |
Fertilizer labels. A bag labelled 20-10-10 contains 20% nitrogen (N), 10% phosphate (expressed as P₂O₅) and 10% potash (expressed as K₂O) by mass. Too little fertilizer limits growth; too much can burn roots (salty soil draws water out of them by osmosis), wastes money, and washes into lakes and rivers, where extra nitrogen and phosphorus cause algal blooms.
Limiting factors. Growth is limited by whatever is in shortest supply relative to need, an idea often pictured as a barrel with staves of different heights (Liebig's law of the minimum). Adding more of a factor that isn't limiting does nothing; adding the limiting one gives a big response, until something else becomes limiting.
Water, soil texture and percolation
Plants need water for photosynthesis, for transport and to keep their cells firm (turgid). Too little leads to wilting and closed stomata; too much drowns roots (Lesson 2). The soil decides how well water is balanced with air:
- Sand has large particles and large pores: water percolates (drains) quickly and little is retained. Plenty of air, but plants dry out fast and nutrients wash away.
- Clay has tiny particles and tiny pores: it retains lots of water but drains slowly and can become waterlogged and airless.
- Loam, a mix of sand, silt and clay with organic matter, holds water while still draining and letting air in. Most crops grow best in loam. Adding compost improves both sandy and clay soils.
Temperature and soil pH
Temperature. Photosynthesis, respiration and growth are controlled by enzymes, which work fastest within an optimum range. Too cold, and reactions slow down; too hot, and enzymes begin to lose their shape, while stomata close to save water. Cool-season crops (peas, lettuce, spinach, cabbage) grow well in Canadian spring and fall and tolerate light frost. Warm-season crops (tomatoes, corn, squash, beans) are killed by frost and are planted after the last spring frost date.
Soil pH. Most garden plants grow best in soil with a pH of about 6 to 7 (slightly acidic to neutral), because that is where most nutrients are easy for roots to absorb. In strongly acidic soil, some nutrients become scarce and toxic aluminium becomes more available; in basic (alkaline) soil, iron and other micronutrients become hard to absorb, causing chlorosis. Some plants are specialists: blueberries need acidic soil (about pH 4.5 to 5.5). Bigleaf hydrangea flowers turn blue in acidic soil (where they can absorb aluminium) and pink in more alkaline soil. Gardeners raise pH with lime and lower it with sulfur or acidic organic matter.
Worked exampleLet's diagnose a nutrient problem
A student's tomato seedlings are small, and their oldest leaves have turned pale yellow all over while the newest leaves are still green. Another student's seedlings have young leaves that are yellow between green veins. Suggest a likely deficiency for each, and a fix.
- Student 1: uniform yellowing starting on older leaves, plus slow growth, fits nitrogen deficiency. The plant moves mobile nitrogen from old leaves to new growth, so old leaves yellow first. Fix: add a fertilizer with nitrogen (or compost).
- Student 2: yellowing between veins on young leaves fits iron deficiency. Iron is not easily moved, so new leaves show it first. Check the soil pH: if it is high (alkaline), iron may be present but unavailable; lowering the pH or using chelated iron helps.
- Confirm with a soil test if possible: symptoms can overlap.
Student 1: likely nitrogen deficiency. Student 2: likely iron deficiency, possibly caused by high soil pH.
Worked exampleLet's read a fertilizer label
A 2.0 kg bag of lawn fertilizer is labelled 24-4-8. (a) How many grams of nitrogen does it contain? (b) The instructions say to apply 2.0 kg per 200 m². How much nitrogen is that per m²? (c) Why do lawn fertilizers usually have a high first number?
- (a) 24% of 2.0 kg = 0.24 × 2000 g = 480 g of nitrogen.
- (b) 480 g ÷ 200 m² = 2.4 g of N per m².
- (c) Nitrogen is needed for leafy growth (proteins and chlorophyll), and a lawn is mostly leaves, so lawn fertilizers are high in N.
(a) 480 g N; (b) 2.4 g N/m²; (c) nitrogen drives green leafy growth.
Just for funPoinsettias are very serious about bedtime. To turn red for the holidays, they need about 14 hours of total darkness every night for several weeks in the fall. A greenhouse worker who switches on the lights at midnight can delay the whole crop.
Hands-on activityPercolation and water retention test
You need: 3 clear plastic bottles (500 mL to 2 L), 3 coffee filters or squares of paper towel, elastic bands, a measuring cup, a timer, and three soils: sand (play sand), potting soil, and garden soil with clay if you can find it. A grown-up should cut the bottles.
- A grown-up cuts each bottle in half. Turn the top half upside down into the bottom half like a funnel. Fix a filter over the neck with an elastic band.
- Put the same volume of each soil (e.g. 250 mL) into each funnel, dry or equally damp.
- Pour 250 mL of water into each, all at the same time. Start the timer.
- Record when water first drips out, and the volume collected after 5 minutes.
- Calculate the volume retained = 250 mL − volume drained. Make a bar graph.
What to look for: sand drains fastest and retains least; clay soil drains slowest (the water may sit on top) and retains most; potting soil or loam is in between, and holds water while still draining. Discuss which soil would suit a cactus, a vegetable garden, and a wetland plant. Wash hands after handling outdoor soil.
Your turn
1Explain why leaves look green, using what chlorophyll absorbs.
Chlorophyll absorbs mostly blue and red light and reflects most green light, which reaches our eyes.
2What do short-day plants actually measure, and when do they flower?
They measure the length of the night; they flower when nights are longer than a critical length, usually in late summer or fall.
3Give the main role of nitrogen, phosphorus and potassium in plants.
N: proteins, nucleic acids, chlorophyll (leafy growth). P: ATP, DNA, membranes (roots and flowering). K: stomata opening and closing, enzyme activity.
4Why might adding more water to a field not increase growth?
If water is not the limiting factor, adding it doesn't help: growth is held back by whatever is in shortest supply (for example, nitrogen or light). Too much water could even harm roots.
5Compare how sand and clay soils handle water.
Sand: large pores, water percolates quickly, little retained, well aerated but dries fast. Clay: tiny pores, retains lots of water, drains slowly, can become waterlogged and airless.
6Why do most garden plants grow best at a soil pH of about 6 to 7?
At this pH, most nutrients are available for roots to absorb; in very acidic or basic soil, some nutrients become unavailable or toxic.
7A fertilizer bag is labelled 10-20-10. What does each number mean?
10% nitrogen (N), 20% phosphate (as P₂O₅) and 10% potash (as K₂O), by mass.
More practice: warm-up
Yellowing of leaves from a lack of chlorophyll.
2Which element is at the centre of a chlorophyll molecule?
3Name one cool-season crop and one warm-season crop.
Cool-season: peas, lettuce, spinach or cabbage. Warm-season: tomatoes, corn, squash or beans.
4What soil pH do blueberries prefer?
Acidic, about 4.5 to 5.5.
5Which soil type is a mix of sand, silt and clay with organic matter?
More practice: on your own
1Seedlings grown on a dim windowsill are tall, thin and pale. Explain and suggest a fix.
Low light: the seedlings put energy into stem elongation to reach light (etiolation) and make little chlorophyll. Fix: move them to brighter light or use a grow light close above them, 12 to 16 hours a day.
2Explain why nitrogen deficiency shows first in older leaves.
Nitrogen is mobile in the plant: when it is short, the plant breaks down proteins in older leaves and moves the nitrogen to new growth, so old leaves yellow first.
3A farmer applies too much fertilizer and the seedlings wilt even though the soil is moist. Explain.
Excess fertilizer makes the soil water very salty (high solute concentration). Water moves out of the roots, or at least not into them, by osmosis, so the plants suffer drought stress ("fertilizer burn").
4A 5.0 kg bag of 15-15-30 fertilizer: how many grams of N, of P₂O₅ and of K₂O?
N: 0.15 × 5000 = 750 g. P₂O₅: 750 g. K₂O: 0.30 × 5000 = 1500 g.
5Why do Ontario gardeners wait until after the last spring frost date to plant tomatoes but can sow peas weeks earlier?
Tomatoes are warm-season plants: frost kills them and cold soil stunts them. Peas are cool-season plants that germinate in cool soil and tolerate light frost.
Challenge
1Light intensity on a crop is doubled, but growth increases only slightly. CO₂ is then raised and growth increases a lot. Explain using limiting factors.
At first, light was not the main limiting factor (or became non-limiting quickly), so doubling it had little effect: photosynthesis was near light saturation. CO₂ was the limiting factor; increasing it raised the rate of photosynthesis and growth until another factor (such as light, temperature or nutrients) becomes limiting. This is why some greenhouses add CO₂.
2A gardener wants pink hydrangea flowers but they keep coming out blue. Explain what is happening and what she could do.
Bigleaf hydrangeas turn blue when they absorb aluminium, which is more available in acidic soil. Her soil is acidic. To get pink flowers, she can raise the pH with garden lime (to about 6 to 6.5 or a bit higher), making aluminium less available; the change takes a season or more.
3A poinsettia grower's crop doesn't turn red in time. A security light was switched on in the greenhouse every night for a few minutes at 1 a.m. Explain.
Poinsettias are short-day plants: they flower (and their bracts turn red) only after a long, uninterrupted night. A flash of light in the middle of the night splits the long night into two short ones, so the plants respond as if nights were short and don't start flowering.
Quiz
1. Chlorophyll absorbs mainly:
2. A plant with yellowing older leaves and slow growth is most likely short of:
3. Which soil retains the most water but drains slowly?
4. Short-day plants flower when:
5. A 10 kg bag of 20-5-5 fertilizer contains how much nitrogen?
6. If nitrogen is the limiting factor, which change will increase growth the most?
Big idea: F3.4 asks students to describe factors affecting plant growth (growth regulators and tropisms were covered in Lessons 12 and 13; here: sunlight, water, nutrients, temperature, soil acidity). F2.2 names nutrient quantity, light quantity and quality, temperature, and water retention or percolation as factors to investigate; the percolation test practises this.
How to help: Garden centres sell inexpensive soil pH test kits and fertilizer bags with N-P-K labels, which make good real-world examples. Help cut the bottles for the percolation test. Houseplants with yellow leaves are a great diagnosis puzzle.
Common difficulties:
- Thinking more fertilizer always means more growth (limiting factors and fertilizer burn).
- Thinking photoperiod is about day length. Plants actually measure night length.
- Confusing water retention (how much soil holds) with percolation (how fast it drains).