Barley Stem Bending Resistance Declines During Maturation, Then Peaks in Ripe, Dry Plants

Barley lodging-specifically stem lodging-occurs when the bending moments from wind and ear weight exceed the culm's load-bearing capacity. Lodging risk decreases as plant height decreases and culm strength increases. Geometry (stem diameter, culm wall thickness) and material strength determine culm bending strength. By studying changes in stem mechanical properties (at three positions along the culm) in two genotypes (grown in a greenhouse), we found that culm strength (assessed with a three-point bending test) slightly diminished through ripening owing to a decline in both area moment of inertia (i.e., strength due to geometry alone) and apparent material strength, presumably due to turgor loss. When the stem segments collected from fully ripe plants were dried to a moisture content typical of harvest maturity, however, strength rose to a maximum. Thus, minimum stem bending resistance occurs during a window in which plants are fully ripe but have not yet reached harvest-dry moisture content. Hence, in the absence of rain-which would severely reduce the mechanical strength of dry, ripe plants-the physiological risk of stem lodging is highest when the crop is fully ripe but not yet harvest-dry. However, the actual lodging risk increases as harvest approaches, because summer storms are frequent at this time of year and dry straw loses rigidity when wetted.