What you need to know
- Connect photosynthesis, respiration, mineral uptake, xylem and phloem.
- Explain how wilting affects photosynthesis and growth.
- Explain why transpiration accompanies gas exchange.
- Apply seasonal source–sink changes.
- Evaluate manipulation of plant growing conditions.
The connected plant network
Nitrate absorbed by roots supplies nitrogen for amino acids and proteins. Magnesium is required for chlorophyll. Water is both a photosynthesis reactant and the basis of turgor support.
Water balance, wilting and growth
A plant wilts when water loss exceeds water uptake and cells lose turgor. Guard cells may close stomata, reducing further loss. This protects water balance but also reduces carbon-dioxide entry.
Less carbon dioxide can reduce photosynthesis. Less sugar is then available for respiration, cellulose production, protein synthesis and storage. Cell expansion also depends on turgor, so growth can slow even before permanent damage occurs.
Transpiration is an unavoidable trade-off
Plants need stomata because the waxy cuticle that prevents dehydration also restricts gas diffusion. Opening pores allows carbon dioxide to reach mesophyll cells, but creates a route for water vapour to escape.
Transpiration is not purely harmful. It helps draw mineral-containing water through xylem, supplies leaf cells and can cool the leaf. The plant regulates rather than completely stops water loss.
Benefit of opening
Carbon dioxide enters, supporting a higher photosynthetic rate.
Cost of opening
Water-vapour diffusion increases, risking loss of turgor in dry conditions.
Seasonal sources and sinks
| Stage | Likely sources | Likely sinks |
|---|---|---|
| Early spring | Storage roots, bulbs or stems | New shoots, buds and leaves |
| Active summer growth | Mature photosynthesising leaves | Growing tips, roots, flowers and developing fruit |
| Late season | Mature leaves | Seeds and storage organs accumulating reserves |
The same organ can change role. This flexibility lets perennial plants survive unfavourable seasons and restart growth before new leaves become fully photosynthetic.
Crops and controlled environments
Growers can adjust light, carbon-dioxide concentration, temperature, humidity, water and mineral supply. Increasing a limiting factor may increase photosynthesis and yield, but only until another factor becomes limiting.
| Intervention | Potential benefit | Potential cost or risk |
|---|---|---|
| Supplementary light | Extends photosynthesis in low light. | Energy and equipment costs; excess light can heat leaves. |
| CO₂ enrichment | Reduces carbon-dioxide limitation. | Cost, leakage and little benefit if light or temperature limits. |
| Heating | Raises enzyme-controlled reaction rates towards optimum. | Fuel cost and possible excessive transpiration. |
| Irrigation | Maintains turgor and stomatal opening. | Water demand, waterlogging and salt accumulation. |
| Mineral solution | Prevents deficiency and supports protein and chlorophyll production. | Excess can damage roots and pollute water. |
The most economical setting is not always the one giving the absolute highest biological rate. Growers compare extra yield with resource, labour and environmental costs.
Applying transport data
When interpreting a crop or transpiration graph:
- identify the independent and dependent variables and their units;
- describe the overall pattern before explaining it;
- look for a plateau that suggests a new limiting factor;
- distinguish correlation from evidence of a mechanism;
- check whether variables such as leaf area, plant age and humidity were controlled;
- use repeats, variation and uncertainty when judging differences.
Integrated questions often require a chain: environmental change → stomatal or transport response → photosynthesis or respiration → growth or yield.
Quick retrieval check
1. How are nitrate uptake and photosynthesis connected?
2. Why can wilting reduce photosynthesis?
3. Give one example of an organ changing from sink to source.
4. Why may extra carbon dioxide fail to increase yield?
Exam connection
A greenhouse crop receives more light, but yield does not increase until irrigation is also increased. Explain this result.
Show the mark points
- Light was not the only limiting factor.
- Water shortage caused loss of turgor or stomatal closure.
- Closed stomata reduced carbon-dioxide uptake.
- Photosynthesis could not increase fully despite extra light.
- Irrigation restored water supply, stomatal opening and photosynthetic production for growth.