The harvest index (HI) is the ratio of harvested (economic) yield to total above-ground biomass (or total plant biomass). It measures how efficiently a plant converts total dry matter into the product we want to harvest .
Key insight: Two fields with the same total biomass can have very different harvestable yields depending on how efficiently biomass is partitioned to harvested organs. Increasing harvest index is often a more achievable breeding goal than increasing total biomass .
Modern wheat: 0.5-0.6
Traditional wheat: 0.3-0.4
Rice: 0.5-0.6
Maize: 0.5-0.6
Soybean: 0.4-0.5
Pea: 0.4-0.5
Faba bean: 0.4-0.5
Chickpea: 0.3-0.5
Potato: 0.6-0.8
Cassava: 0.6-0.7
Sweet potato: 0.6-0.8
Tomato: 0.3-0.5
Pepper: 0.3-0.4
Cucumber: 0.4-0.5
Lettuce: 0.8-0.9
Spinach: 0.8-0.9
Cabbage: 0.7-0.8
Rapeseed: 0.2-0.3
Sunflower: 0.3-0.4
Niger seed: 0.2-0.3
Assimilate partitioning refers to how photosynthetically fixed carbon (assimilates) is distributed among different plant organs. Plants must constantly balance investment in:
Partitioning is dynamic and changes throughout development:
Harvest index is under genetic control. Major genes affecting plant height (e.g., Rht in wheat, sd1 in rice) have dramatic effects on HI by reducing competition from stems .
Stress during reproductive growth reduces harvest index more than stress during vegetative growth:
Harvest index is highest when sink demand matches source supply. Too few sinks (poor fruit set) → low HI. Too many sinks (excessive fruit load) → competition and smaller individual organs .
Very high plant densities increase competition, reducing HI. Optimal density balances total biomass and HI .
Harvesting too early or too late reduces HI. Maximum HI occurs at physiological maturity .
| Factor | Effect on HI | Management |
|---|---|---|
| Drought during grain fill | Decreases HI | Irrigation, drought-tolerant varieties |
| High nitrogen | Can decrease HI (excess vegetative growth) | Balanced fertilization, split applications |
| Optimal plant density | Maximizes HI | Species-specific spacing |
| Dwarfing genes | Increase HI | Use semi-dwarf varieties |
The Green Revolution of the 1960s and 1970s dramatically increased cereal yields worldwide, particularly wheat and rice. A key factor was the introduction of semi-dwarf varieties with higher harvest index .
Traditional wheat varieties were tall (1.5m) with long straw. Under high fertility, they would lodge (fall over), reducing yield. Norman Borlaug introduced semi-dwarf genes (Rht) from Japanese varieties, creating short, stiff-strawed wheat that:
Borlaug won the Nobel Peace Prize in 1970 for this work, which saved millions from famine .
Similarly, the sd1 gene in rice (deficient in gibberellin synthesis) created semi-dwarf varieties like IR8 that:
IR8 yielded 5-10 t/ha compared to traditional varieties' 1-2 t/ha .
Root and tuber crops have naturally high harvest indices because the harvested organ is a major sink. However, partitioning can still be optimized:
Fruit crops have lower HI than grains or tubers because much biomass remains in the perennial structure (trunk, branches, roots). However, annual partitioning to fruit can be optimized:
Thinning excess fruits early in development reduces sink number but increases size of remaining fruits. This improves marketable yield and HI .
Pruning balances vegetative and reproductive growth, optimizing the source-sink relationship for fruit production .
Dwarfing rootstocks (e.g., M9 for apple) reduce vegetative growth, directing more assimilates to fruits and increasing HI .
Apple trees on dwarfing rootstocks (M9, M26) have much higher harvest index than those on vigorous rootstocks (seedling). The dwarfing rootstock restricts root and shoot growth, redirecting assimilates to fruit production. This is why modern orchards use dwarfing rootstocks with high-density planting .
Teff (Eragrostis tef) has a relatively low harvest index (0.2-0.3) because:
Faba bean HI is typically 0.4-0.5. Drought during flowering and grain filling reduces HI significantly. Improved varieties with better stress tolerance could stabilize HI .
Enset (false banana) is unique—harvested for starch in pseudostem and corm. The entire plant is used, so HI concepts need adaptation. Understanding partitioning between corm, pseudostem, and leaves could help optimize harvest timing .
Niger seed has low HI (0.2-0.3), typical of oilseeds. Improving HI through breeding could significantly increase oil production without expanding area .
| Strategy | How it works | Crop examples |
|---|---|---|
| Dwarfing genes | Reduce stem height, redirect assimilates to grain | Wheat, rice, sorghum |
| Reducing tillering | Fewer, more productive tillers | Rice, wheat |
| Fruit thinning | Reduces sink number, increases individual fruit size | Apple, peach, citrus |
| Dwarfing rootstocks | Restrict vegetative growth of scion | Apple, pear, citrus |
| Optimizing planting density | Balances competition and resource use | All crops |
| Stress management | Protects reproductive growth | Irrigation, pest control |
Key insight: There's often a trade-off between HI and total biomass. Maximum yield is achieved by optimizing both, not maximizing one at the expense of the other .
Problem: A wheat field produces 8 t/ha of grain and 10 t/ha of straw. What is the harvest index?
Solution:
Interpretation: 44% of the total biomass is in the harvested grain. This is typical for modern wheat .
Problem: A potato crop produces 40 t/ha of fresh tubers (80% water) and 10 t/ha of dry vine biomass. Calculate HI on a dry weight basis.
Solution:
Note: Fresh weight HI would be 40/(40+10) = 0.80, which is misleading because of high water content in tubers .
| Crop type | Typical HI | Key factors |
|---|---|---|
| Cereals | 0.4-0.6 | Dwarfing genes, stress during grain fill |
| Root/tuber crops | 0.6-0.8 | High natural HI, stress during bulking |
| Fruit vegetables | 0.3-0.5 | Fruit set, thinning, pruning |
| Leafy vegetables | 0.8-0.9 | Harvest timing, nitrogen |
| Oilseeds | 0.2-0.4 | Low natural HI, breeding potential |
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