Wheat and the Shortened Stalk

Non-Shattering Ears and Why Dwarf Straw Prevented Global Famine

Wild grasses scatter their seeds by snapping their own spines. In wild einkorn (Triticum boeoticum), the central axis of the grain head is called the rachis. As the plant dries in early summer, cell walls along the rachis turn brittle and shatter at the slightest breeze. Individual spikelets fall to the ground equipped with barbed awns that drill the seed into soil cracks. For human foragers, however, a shattering ear is a nightmare to harvest.

In 1990, archaeobotanists Gordon Hillman and Stuart Davies tested wild einkorn stands on Mount Karacadag in southeastern Turkey. They measured what happened when Neolithic gatherers swung flint-bladed sickles through wild grain around 8500 BCE. Beating a wild stand with baskets knocked loose only the ripe, brittle-rachis seeds. Swinging a sickle or uprooting entire stalks, by contrast, selectively captured rare mutant ears whose rachis stayed tough at maturity. Gatherers carried those intact ears home and sowed part of the grain the next autumn. Because only tough-rachis ears survived the sickle, the mutant allele multiplied rapidly. Within twenty to thirty generations, a single recessive mutation at the Btr1 locus fixed non-shattering wheat as a domesticated crop. Once domesticated, the plant could no longer disperse its own seeds without human hands.

Ten thousand years later, domesticated wheat hit a second mechanical wall. In the early twentieth century, industrial chemists Fritz Haber and Carl Bosch learned to synthesize ammonia fertilizer from atmospheric nitrogen. Farmers spread heavy doses of synthetic nitrogen onto traditional wheat varieties. In response, the stalks grew nearly five feet tall and produced heavy, plump grain heads. Under rain and wind, those slender hollow stems buckled at the base and flattened across the mud. Agronomists call this structural failure lodging. Once a wheat field lodged, the damp ears rotted on the ground and mechanical combines could not lift them.

In October 1944, American plant pathologist Norman Borlaug arrived in the sun-baked Yaqui Valley of Sonora, Mexico. Mexico was importing half its wheat after three consecutive harvests were destroyed by stem rust (Puccinia graminis). To breed rust resistance fast enough to outrun the fungus, Borlaug defied agronomic orthodoxy. He launched a two-climate "shuttle breeding" program across ten degrees of latitude. Every autumn he planted wheat at sea level in Sonora. He harvested the survivors in spring and drove the seeds south to Toluca at 2,600 meters elevation for a summer crop. Growing two generations a year cut breeding time in half. It also inadvertently eliminated day-length sensitivity, creating wheat lines that could flower at any latitude on Earth.

When Borlaug's rust-resistant Mexican wheat received heavy nitrogen fertilizer, however, its tall stalks still lodged. In 1953, Borlaug obtained seeds of Norin 10, a short-strawed Japanese wheat variety collected by Cecil Salmon in occupied Japan in 1946. Norin 10 carried two recessive dwarfing genes, Rht-B1b and Rht-D1b. In conventional wheat, the plant hormone gibberellin binds nuclear receptors and triggers the destruction of DELLA growth-repressing proteins. That destruction allows stem cells to stretch rapidly. The Rht mutations alter the DELLA protein so gibberellin cannot break it down. Stem elongation halts early, and the plant diverts its photosynthetic sugars directly into extra grain kernels rather than tall straw.

The mechanical stability of a wheat stalk under wind load obeys the cantilever bending moment equation at the root crown:

Here $M$ is the overturning torque at the base of the stem, $\rho$ is air density, and $v$ is wind velocity. The variable $C_d$ represents the drag coefficient, $A$ is the projected area of the grain head, and $h$ is the stalk height from soil to ear. Breeding Norin 10 genes into Mexican wheat reduced stalk height $h$ from 130 centimeters to 70 centimeters. That reduction cut the lever arm nearly in half while thickening the lower stem walls. Describing these new varieties to skeptical farmers, Borlaug explained that the short, stiff-strawed plants refused to lodge even when farmers applied heavy doses of nitrogen fertilizer and irrigation water. By halving stalk height with two gibberellin-insensitive genes, Borlaug reduced wind-induced bending torque by half and tripled Mexico's wheat harvest per hectare between 1945 and 1965.