For horticulturists and farmers, yield is the ultimate measure of success. At its core, yield represents the conversion of sunlight energy into harvestable plant material through photosynthesis .
Key insight: Increasing yield means optimizing each term in this equation—capturing more light, converting it more efficiently to biomass, and partitioning more of that biomass to harvestable parts .
Before photosynthesis can happen, light must reach the leaves. Factors affecting light interception include:
Total leaf area per ground area. Optimal LAI for most crops is 3-5 (enough leaves to intercept 95% of light). Too high LAI shades lower leaves, reducing efficiency .
Leaf angle affects light penetration. Erect leaves (like grasses) allow light deeper into canopy; horizontal leaves intercept more at top .
Too sparse → light wasted on ground. Too dense → competition, lower leaves shaded. Optimal density balances interception and competition .
How long the canopy covers the ground. Early vigor and delayed senescence increase total light interception over the season .
Modern maize hybrids have been bred for erect upper leaves and more horizontal lower leaves, optimizing light distribution throughout the canopy. This increases photosynthesis in lower leaves and contributes to higher yields .
Radiation Use Efficiency is the amount of biomass produced per unit of intercepted light (typically g biomass per MJ of light). RUE varies by:
| Crop type | Typical RUE (g biomass/MJ) | Example crops |
|---|---|---|
| C3 cool season | 1.0-1.5 | Wheat, barley, potato |
| C3 warm season | 1.2-1.6 | Soybean, cotton |
| C4 | 1.5-2.0 | Maize, sorghum, sugarcane |
| High-input crops | Up to 2.5 | Intensive horticulture |
Harvest index (HI) is the proportion of total plant biomass that is harvestable product (grain, fruit, tuber, etc.).
| Crop | Harvest Index | Notes |
|---|---|---|
| Cereal grains | 0.4-0.6 | Modern wheat up to 0.55 |
| Root crops | 0.6-0.8 | Potato, cassava |
| Fruit vegetables | 0.3-0.5 | Tomato, pepper |
| Leafy vegetables | 0.8-0.9 | Lettuce, spinach |
Breeding and management can increase HI by:
The Green Revolution dramatically increased wheat and rice yields by introducing semi-dwarf varieties. These shorter plants invested less biomass in stems and more in grain, raising harvest index from ~0.3 to ~0.5 .
Let's calculate potential yield for a hypothetical crop:
Given:
Calculation:
This matches typical wheat yields in good growing conditions. To increase yield, we could improve any of the three components .
At high light intensities, photosynthesis plateaus because the Calvin cycle can't keep up with the light reactions. This is called light saturation. C4 plants saturate at higher light levels than C3 plants .
Current atmospheric CO₂ (~420 ppm) is limiting for C3 photosynthesis. This is why CO₂ enrichment in greenhouses boosts yield—it pushes the Calvin cycle faster .
Each crop has an optimal temperature range for photosynthesis. Outside this range, enzymes slow or denature, and photorespiration increases .
Even mild water stress causes stomatal closure, reducing CO₂ uptake. This is often the most limiting factor in rainfed agriculture .
Nitrogen is part of chlorophyll and rubisco. Magnesium is in chlorophyll. Iron, manganese, and other micronutrients are enzyme cofactors. Deficiencies directly reduce photosynthetic capacity .
| Strategy | Target | Example |
|---|---|---|
| Optimize planting density | Light interception | Maize planted at 70,000-90,000 plants/ha |
| Row orientation | Light interception | North-south rows in high latitudes |
| Irrigation | RUE (prevents stomatal closure) | Drip irrigation maintains optimal water status |
| Fertilization | RUE (nutrient supply) | Nitrogen applied at key growth stages |
| CO₂ enrichment | RUE (greenhouses) | 800-1000 ppm CO₂ in protected cultivation |
| Pest/disease control | RUE (protect leaf area) | Integrated pest management |
| Variety selection | All components | High HI, adapted to environment |
Modern greenhouse tomato production achieves yields of 50-80 t/ha—much higher than field tomatoes (20-30 t/ha). This is achieved by optimizing all components:
Most Ethiopian smallholders face multiple constraints that limit photosynthetic yield:
Research shows that Ethiopia's maize yields (currently ~3-4 t/ha on average) could potentially reach 6-8 t/ha with improved management—closing the "yield gap" through better agronomy. This represents a doubling of production without new land .
Scientists are working on multiple fronts to increase photosynthetic efficiency:
Creating faster rubisco or rubisco with better CO₂/O₂ discrimination
Speeding up recovery from photoprotective states (NPQ) when clouds pass
Engineering shortcut pathways to recycle photorespiratory products more efficiently
Introducing C4 photosynthesis into rice (see Unit 2.1.3)
| Component | What it measures | How to improve |
|---|---|---|
| Light interception | % of sunlight captured by canopy | Optimal planting density, row orientation, early canopy closure, long green leaf duration |
| Radiation use efficiency | Biomass per unit light captured | Optimal water, nutrients, temperature; C4 pathway; CO₂ enrichment; pest/disease control |
| Harvest index | % of biomass that is harvestable | Variety selection (semi-dwarf), source-sink management, stress reduction during grain fill |
Discuss your answers in the course forum.