Freeze injury in wheat impacts management options
click image to zoom The table below summarizes freeze injury symptoms and yield loss by growth stage. Diagnosis of freeze injury requires knowledge of plant parts most vulnerable at each growth stage, their location and their appearance after freezing as well as when they are normal.
Emergence to tillering – Zadoks 10-25. During seedling to early tillering stages the growing point is below the soil surface and protected from freeze injury. Most damage occurs to leaves which may have distinct light-yellow bands and which become chlorotic or necrotic and usually twisted.
Jointing – Zadoks scale 31-39. Leaves of freeze-injured plants develop damage symptoms similar to those of the tillering stage. The most serious injury can occur to the growing points. The growing point in a stem is located just above the uppermost node you can feel when you run the stem between your thumb and forefinger. To observe the growing point, split the stem lengthwise with a sharp blade to expose the developing head. A normal uninjured growing point is bright pearl white to yellow green and turgid. Freeze injury causes the growing point to turn dull white or brownish and water-soaked. Injury to the growing point can occur in plants that appear to be otherwise normal because the growing point is most sensitive to cold. When the growing point is injured stem elongation stops, but later uninjured tillers continue to grow masking the damage. Expect normal and late tillers, uneven maturity and decrease in grain yield. Stem discoloration is associated with reduced metabolite transport through the nodes.
Boot – Zadoks scale 41-49. Freezing may cause heads to be trapped inside the boot so that they cannot emerge. The heads may remain in the boot, split out the side of the boot or emerge from the boot base first. Often the peduncle or stem supporting the head continues to elongate normally, causing crimps in the stem that can inhibit normal transfer of photosynthates. The result is low test weight grain. Often the head appears normal from the outside even though the anthers are dead. Because wheat (barley and oats) are self-pollinated, male sterility causes poor seed set and low grain yield. Anthers are more sensitive to freezing temperatures than female flower parts. Normal anthers are light green, full of developing pollen grains and turgid. They turn yellow when they mature and shed pollen. Freeze injury causes anthers to turn white and shrivel. It usually prevents them from extruding and shedding pollen. Anthers should be examined during this stage as leaves and stems may appear normal.