Heavy Rains Exclude Oxygen Needed For Seedling Health From Soils
COLUMBIA, MO.
Planting high quality seed
does not guarantee obtaining
a good stand.
Several bad things can happen
to good seeds. Seed germination
begins with water
absorption. Water absorption
changes the seed from a
nearly dormant organism into
a living, functioning seedling. As seed tissues
imbibe water, enzymes necessary for growth are
activated, stored reserves break down, and cell
division and expansion occur. The “bad thing”
that may become apparent this spring is low
oxygen availability.
Heavy rains this spring have resulted in rapid
and sustained water runoff. Flash floods warnings
have been numerous this year. These conditions
usually mean that water inundates
portions of fields, but subsides relatively
quickly. Near creeks and rivers, a longer lasting
flood may cover fields for days. Even if flooding
is not a problem, many of our fields have low
areas in which water collects. Sometimes the
“ponding” is not visible with water above soil,
but the soils are water logged with water content
above what we call “field capacity”. Some
soils are more prone to water logging than others,
for example claypan soils of NE Missouri or
soils high in clay content.
All of these conditions have one characteristic
in common that affects germinating seeds and
developing seedlings – water sits in spaces between
soil particles and aggregates that should
hold air. Water in soil pores excludes oxygen
needed for seedling growth. Initially, water absorption
by seeds is not dependent on oxygen.
In fact, both dead and live seeds absorb water.
But, once water content of seeds exceeds 35-50
percent continued water absorption depends on
energy released by seed respiration. More importantly,
all of the life processes the seedling
needs to stay alive depend on respiration.
Oxygen demand by the seedling increases rapidly
and that oxygen must come from air within
soil pores. The heaviest demand for oxygen is
centered in the growing point. Rapid cell division
and elongation depends on adequate oxygen.
Four factors interact to determine if
seedling health will be impacted by low oxygen:
seed quality, water temperature, water motion,
and location of the growing point in the
seedling.
Seeds with low vigor are less likely to withstand
short exposures to low oxygen availability.
Companies only sell high quality seed, but
saved seed or seed that was not stored or handled
properly might possess poor quality. Warm
soil and water temperatures increase seedling
respiration. So, soil oxygen is depleted more
quickly if water is warm. Moving water creates
turbulence which mixes air into the water. Although
moving water can lodge plants, there
will be slightly more oxygen in moving water
than in still water. Corn plants exhibit hypogeal
emergence, so the growing point stays below
ground for at least three weeks. That means
that the center of oxygen need is usually located
where oxygen is the least available in water
logged soils. Soybean plants possess epigeal
emergence where the growing point is at the tip
of the stem and the stem elongates above
ground. This may be an advantage because the
growing point may remain above the water surface.
Anaerobic respiration produces small
amounts of energy and may keep the seedling
alive for several days. Most seedlings can tolerate
3 or 4 days of flooding, but will often succumb
to periods longer than 7 days. Plant
structures experiencing reduced oxygen availability
produce several toxic substances.
Ethanol is harmful to organelle membranes and
the enzymes necessary for life. Lactic acid reduces
the pH within the cell. If pH becomes too
acidic, enzymes precipitate out of solution and
the cell dies.
There is not much that we can do to help
seedlings experiencing reduce oxygen availability.
Protecting seeds and seedlings with seedapplied
fungicides and insecticides might be
beneficial. These chemicals do not improve oxygen
availability or reduce the formation of toxic
substances, but they protect the seedlings from
opportunistic microbes and insects. These organisms
may cause greater harm to weakened
seedlings and increase plant death. Δ
DR. WILLIAM WIEBOLD: Professor Division of
Plant Sciences, University of Missouri