Timing of Cold Stress Impacts Germination
Early planting often exposes seeds to hydration with cold water, which can cause direct physical damage. When the dry seed imbibes cold water as a result of a cold rain or melting snow, imbibitional chilling injury may result. The cell membranes of the seed lack fluidity at low temperatures, and under these conditions, the hydration process can result in rupture of the membranes. Cell contents then leak through this rupture and provide a food source for invading pathogens. Cold water can similarly affect seedling structures as they begin to emerge. The degree of damage ranges from seed death to abnormalities such as corkscrews or fused coleoptiles (Figure 6).
Figure 6. Abnormal mesocotyl and coleoptile development due to cold stress in an early-planted Illinois field.
To help understand the importance of the timing of cold stress, two hybrids with stress emergence scores of 4 (below average) and 7 (above average) were allowed to germinate in rolled towels for 0, 24, or 48 hours at 77°F (25°C). The hybrids were then subjected to a stress of melting ice for three days and allowed to recover for four days at 77°F (25°C). Hybrids were evaluated for the number of normal seedlings reported as percent germination (Figure 7).
Both hybrids showed significant stand loss when the cold stress was imposed immediately (0 hours). However, the hybrid with a higher stress emergence score had a higher percent germination than the hybrid with a low stress emergence score. Germination rates for both hybrids were greatly improved if allowed to uptake water and germinate at warmer temperatures for at least 24 hours before the ice was added.
Figure 7. Germination of two hybrids with stress emergence scores of 7 (above average) and 4 (below average) following imbibitional chilling induced by melting ice. Ice was applied immediately after planting (0 hours) or after 24 hours or 48 hours of pre-germination in warm conditions.
Planting just before a stress event such as a cold rain or snow can cause significant stand loss. The chances of establishing a good stand are greatly improved if seeds are able to germinate at least one day in warmer, moist conditions before a cold-stress event. Also, choosing a hybrid with a higher stress emergence score can help moderate stand losses due to cold stress.
Snowfall soon after planting imposes a very high level of stress on corn emergence due to seed imbibing chilled water or prolonged exposure to cold, saturated soils.
One reason why temperature during imbibition is critical to corn emergence is the fact that seed imbibes most of the water needed for germination very rapidly. To illustrate the rapid timing of water uptake, seed was submerged in 50°F water for 3 hours and weighed at intervals of 30, 60, 120 and 180 minutes to determine water uptake (Figure 8).
Figure 8. Amount of water uptake by corn seed during the first three hours after submersion in 50°F water.
The data show that seed imbibes the most water within the first 30 minutes after exposure to saturated conditions. If this early imbibition occurs at cold temperatures, it could kill the seed or result in abnormal seedlings. Growers should not only consider soil temperature at planting, but also the expected temperature when seed begins rapidly soaking up water. Seed planted in warmer, dry soils can still be injured if the dry period is followed by a cold, wet event.