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A wheat plant that grows a heavier ear of grain can also fall over under its own weight. That simple mechanical problem sat at the centre of Norman Borlaug's career. His answer, a shorter and sturdier wheat, became one of the foundations of the Green Revolution. This post covers who he was, the plant science and physics behind the short stem, how he sped up breeding, and what a student can take from his way of working.
Who was Norman Borlaug?
The Nobel Prize biography records that Borlaug was born on 25 March 1914 on a farm near Cresco, Iowa. He studied forestry at the University of Minnesota, then plant pathology, earning a master's degree in 1939 and a doctorate in 1942.
In 1944 he took charge of the Cooperative Wheat Research and Production Program in Mexico, a joint project of the Mexican government and the Rockefeller Foundation. The biography describes him as a pragmatic scientist who dropped methods that did not work and avoided what he called "academic butterflies". He later directed the wheat programme of CIMMYT, the International Maize and Wheat Improvement Center, and trained young scientists from many countries.
The Nobel facts page gives the 1970 Peace Prize motivation as "for having given a well-founded hope - the green revolution". He died on 12 September 2009 in Dallas.
Why did tall wheat become a problem?
In his Nobel lecture, Borlaug explained that as fertiliser use rose in Mexico and yields climbed to about 4,500 kilograms per hectare, "lodging (falling over of the plant) began to limit further increases in yields". More fertiliser meant heavier ears on the same tall, thin stems, and the plants toppled before harvest.
The physics is the physics of torque. A sideways push from wind on the ear acts at the top of the stem, and the turning effect at the base is the force multiplied by the height:
Torque at the base = force on the ear x height of the ear.
If a stem is half as tall and the push on the ear is the same, the torque at the base is halved.
So a shorter stem can carry a heavier ear with less risk of bending over. Our post on torque and equilibrium in ladder problems uses the same idea with forces at different heights.
Where did the dwarfing genes come from?
The team searched wheats from around the world for a source of "genetic dwarfness". In the lecture, Borlaug names it: "Norin 10, an extremely dwarf wheat from Japan, proved to be a suitable source." Through crosses and re-crosses begun in 1954, the short stature was bred into the high-yielding Mexican types, giving the dwarf Mexican varieties.
He was careful not to oversell them. The new varieties, properly cultivated, had in general double or triple the yield potential of the highest-yielding older tall-strawed wheats, but he added: "There are no miracles in agricultural production." The yield came from the variety together with fertiliser, water and a package of farming practices.
How did he breed wheat twice as fast?
Plant breeding is slow because you wait a full season to see each generation. The lecture describes Borlaug's fix. Two generations of breeding material were grown every year. One was sown near sea level in Sonora, at about 28 degrees north, in the autumn, when days grow shorter. The second was sown near Toluca, at about 18 degrees north and 2,500 metres altitude, in the summer, when days grow longer.
This doubled the pace of selection. It also had a side effect he turned into an advantage: plants that did well in both places were insensitive to day length, so the varieties suited a wide range of latitudes and resisted a broad range of diseases.
What does the short stem do inside the plant?
The genetics were explained decades later. In a 1999 paper in Nature, Peng and colleagues wrote that Green Revolution wheats "are shorter, increase grain yield at the expense of straw biomass, and are more resistant to damage by wind and rain". They are short "because they respond abnormally to the plant growth hormone gibberellin", due to mutant dwarfing versions of the Reduced height-1 genes, Rht-B1 and Rht-D1.
Gibberellin normally promotes stem elongation. A plant that responds less to it grows a shorter stem. If you are studying plant hormones for school, our guide to plant growth regulators covers what gibberellins do.
What happened when the wheat reached India?
The lecture gives the numbers. Trials of dwarf Mexican varieties began in India and Pakistan in 1963 and 1964. In 1965, 250 tons of seed went to India for wide-scale testing on farms, and India imported 18,000 tons in 1966. Indian wheat production rose from a record 12.3 million tons in 1964-65 to 20.0 million tons in 1970.
A quick check of what that means: 20.0 divided by 12.3 is about 1.63, so production rose by roughly 63 per cent in about five harvests. Our post on M. S. Swaminathan and the Green Revolution tells the Indian side of the story.
Borlaug also warned against complacency. In the same lecture he said the Green Revolution "has won a temporary success in man's war against hunger and deprivation; it has given man a breathing space."
What can a student take from how he worked?
Find the real limit. Yields were not limited by the grain but by the stem falling over. Naming the true bottleneck changed the whole programme.
Speed up the feedback loop. Two generations a year meant twice as many chances to learn. In study terms, frequent short tests beat one big test at the end.
Do not claim more than the evidence shows. He gave credit to the whole package of practices and refused to call his wheat a miracle.
When a mechanics idea such as torque is easier to see than to read, the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read.
This week, draw a stem as a lever, mark where the wind pushes and where it pivots, and work out how the torque changes if the stem is half as tall.
