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July 2002, No. 3
TRANSPIRATION
LEADS TO WILT
"We never know the worth of water till the well is dry."
- Thomas Fuller, M.D. (1732)
As I re-think my turf tips for this month, another possibility
of a timely rainfall has past. I had several topics of tips that
were and still are afflicting the turf managers throughout the
state, (remnants of take-all patch, Rhizoctonia patch,
turfgrass ataenius, localized dry spot and even some Pythium)
but the lack of rainfall and the continual need to irrigate has
inspired me to review water and its relations in the turfgrass
plant.
THE HYDROLOGIC CYCLE
The hydrologic cycle is simply the endless circulation of earth's
water molecules from ocean to atmosphere to land and back to ocean.
Winds roll over the earth's surfaces converting liquid water to
vapor. The vapors are condensed back to liquid droplets forming
clouds and sometimes fall to the back to the earth's surface in
a process known as precipitation. Some of this water falls on
land surfaces (not my fairways), some falls back into the oceans
and some even evaporates directly back into the atmosphere. Of
the water reaching land, some sinks into the soil and rocks, some
runs into surface lakes and rivers. Some of the water absorbed
by the land is accumulated in large underground reservoirs (aquifers)
and some eventually seeps back to the ocean. The turfgrass plant
fits into this cycle through a process known as transpiration,
and is very important to understand especially during this time
of year.
TRANSPIRATION
Transpiration is water loss through the plant to the atmosphere
by evaporation. It is a mechanism that the plant uses to cool
itself. The human body perspires as a means of evaporative cooling;
plants transpire. In turfgrass, most occurs through the leaves,
however any plant part exposed to the atmosphere can lose water
through transpiration. Two primary types of transpiration occur
from the leaf blade. The first takes place from the external wax-like
layer or cuticle of the leaf blade. The amount of water loss through
the cuticle depends on many factors including the thickness of
the cuticle. The thickness of the cuticle is primarily determined
by the turfgrass species and the environmental conditions during
leaf development.
Over 90% of the total transpirational water loss from the turfgrass
plant takes place through stomata. Stomata are structures found
within the leaf blade and are responsible for facilitating the
gaseous exchange of CO2 and O2 during photosynthesis. The gas
exchange function of the stomata also leads to the loss of plant
water through transpiration. There are three primary environmental
factors that affect the amount and rate of water loss through
stomatal openings: radiation, water and temperature.
The amount of water in the turf plant at any given time is termed
plant water status. The plant water status is the physiochemical
condition of the water relative or plant growth or activity. During
normal weather, transpiration can influence rates of different
metabolic processes in the plant through the manipulation of the
plant water status. Transpiration directly benefits the turfgrass
plant through the evaporative cooling process. This process is
imperative to the health of our cool-season turfgrass during high
temperatures. However, this same process can be detrimental to
the plant when the water loss through transpiration exceeds the
absorption rate of water through the plant's root system. This
is known as a plant water deficit. A plant water deficit will
always reduce growth. When environmental and cultural conditions
are prime for a water deficit to occur the result is wilting and
desiccation of the turfgrass plant. These conditions have been
common this July, but there's more…………
PLANT WATER STRESS
Every growth function is affected by plant water status. A plant
water stress occurs whenever the plant water status is potentially
harmful to the plant (deficit). Specific growth processes have
different sensitivities to water deficits. Cell turgor and growth
(expansion) are immediately affected by a plant water stress.
This is the reason plants begin to wilt at the onset of a water
stress.
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WHAT IS TURGOR?
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As a turfgrass grows, each cell accumulates or manufactures
soluble materials. The accumulation of these materials
(salts, sugars, proteins, etc) lowers the cell's water
potential relative to that of the "outside".
(Water content is lower in the cell because of the accumulation
of materials) The permeable membranes enclosing the
cell protoplasm restrict solute movement out but permit
free exchange of water. This allows water movement into
the cell in response to the osmotic potential gradient.
(From greater water content to lower water content)
The cell walls' mechanical resistance to stretching
causes an internal hydraulic or turgor pressure to build
as water enters the cell. (Much like filling a balloon
with water………as the amount of water increases in the
balloon, the surface tension of the balloon increases
and becomes more rigid.) Turgor pressure is very important
because it is the driving force behind cell expansion,
tissue support, stomatal opening, and metabolic activities.
Turgor pressure can be reduced by the movement of water
out of the cell (occurs during a water deficit) and
the cell tissue becomes wilted (droops) because of the
pressure drop. (If the water-filled balloon were punctured,
water would move out and the walls of the balloon would
be thinner and sag.
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As the stress continues, photosynthesis slows, protein synthesis
ceases, nitrate reductase levels lessen, respiration slows, sugar
accumulation diminishes and injury and death occurs. The actual
sensitivity of the turfgrass plant to water stress depends on
many plant (genus, species, preconditioning etc) and stress factors
(intensity, duration, frequency, etc).
One plant factor that adds to the troubles this summer is known
as preconditioning. The term preconditioning can be defined as
"to put in a desired condition in preparation". To precondition
a turfgrass for a hot and dry summer, a turf would need to prepare
itself by going through a warm and dry spring. It has been shown
that a moderate soil moisture stress will increase overall rooting
depth. Most of our turfgrass plants went through a preconditioning
period of spring training for a fairly brief, cool, summer laden
with timely rainfall. (Primarily between the hours of 10pm and
4am so as not to affect play). The preconditioning of this spring
discouraged plants to grow a deep and extensive root system. Cool
temperatures and weekly weekend rains set the table for shortened
root systems. Lush-top growth was much more in vogue and necessitated
frequent mowing. When the weather jumped into summer (it was summer,
by the way) the turf had no preconditioning at all and plants
quickly showed signs of water stress.
ENVIRONMENTAL FACTORS AFFECTING TRANSPIRATION
When turfgrass plants begin to wilt, a water deficit has occurred.
Simply stated, there is more water going out (through transpiration)
than coming in (through root absorption). As previously mention
there are three main factors affecting the plant's primary transpiration
mechanism (stomatal opening) in the environment. These three factors
are radiation, water and temperature. Two of the three can be
confusing.
Radiation is the transfer of energy through free space, not just
the part of the electromagnetic radiation that we see and term
as light. In terms of plant use, radiation can be characterized
in three different ways. Radiation is measured in wavelengths
and describes the quality that a turf plant may absorb. The amount
of radiation is measured as irradiance (quantity of photons on
a given surface area). The duration of radiation or photoperiod,
measures the dose or length of time a plant is exposed.
As the sun rises in the morning, it warms the ground and the ground
warms the air in contact with it by conduction. Conduction is
the passing of heat energy from one molecule to the next. Air
is a poor conductor of heat (reason that insulation is filled
with air space) and most of the heat would stay a few inches above
ground except for when convection occurs. Convection is the transfer
of heat energy by the mass movement of a fluid (air or water).
Free movement of the individual molecules in air and water create
thermals. When cooler air molecules bounce into warm air molecules
(near the earth's surface), the cooler molecules begin to warm
by conduction. This heated air expands and becomes less dense
than the surrounding cooler air. Thermals or large bubbles of
warm air form and rise throughout the cooler air. These thermals
help to redistribute heat upward throughout the air.
Around noon, the sun's rays are most intense but the turfgrass
is still warming for a few more hours. This occurs because the
heat coming in from the sun still exceeds the heat leaving the
atmosphere. On clear days (cloud free) the maximum temperature
may occur between 3:00 and 5:00 pm. When it is cloudy or there
is a haze the temperature maximum will occur an hour to two earlier.
Clouds or water vapor reflect back some of the incoming solar
radiation into the atmosphere. This is why cloudy days are often
cooler than sunny days.
Next to radiation, water vapor or humidity is most often misunderstood.
Humidity refers to any number of ways of specifying the amount
to water vapor in the air:
- The mass of water vapor over volume of air is known as absolute
humidity (grams of water vapor per cubic meter of air)
- The mass of water vapor over the mass of air is known as specific
humidity (grams of water vapor per kilogram of air)
- The mass of water vapor over the mass of remaining dry air
is known as mixing ratio (grams of water vapor per kilogram
of air)
However, the term that is most frequently used and most misunderstood
is relative humidity. Relative humidity is a ratio of the amount
of water vapor actually in the air compared to the maximum amount
of water vapor required for saturation at that particular temperature
(and pressure). It doesn't describe the actual amount of moisture
in the air, only how close the air is to complete saturation.
It is strongly affected by two factors: the water vapor content
of the air and the temperature. A change in the air's water vapor
content can change the air's actual vapor pressure. If the temperature
remains constant and the water vapor content increases, the air's
water vapor pressure will increase and so will the relative humidity
(the converse is true as well). As water vapor is added to the
air (at constant temperature), the relative humidity increase,
and as water vapor is removed from the air, the relative humidity
lowers. When a change in temperature is thrown into the mix, the
air's saturation vapor pressure changes (and so does the relative
humidity). As the temperature increases, the air's saturation
vapor pressure increases allowing it to hold more water and increasing
the point where the air will become saturated. If there is no
increase in water vapor content the relative humidity will decrease.
Recall the warmest part of the day, the afternoon when the effects
of the sun are still warming the air. If there is no increase
in the water vapor content during this time of day the relative
humidity decreases (See graph).
It is at this time when the plants' water needs are greatest because
of the drop in relative humidity and increase in air temperature.
Remember transpiration is an evaporative cooling process. As the
air temperature increases, water vapor pressure is decreased and
the relative humidity is reduced. This creates a greater need
for available moisture to the plant's root system for transpiration.
The plant will use more water during these times and this is why
it is not uncommon for plants to wilt during the afternoon, especially
this year.
MANAGING WILT
Wilting is a sign of a water deficit. Symptoms of a turfgrass
wilting include; changing color from green to purples and sometime
blues. Often a wilting turf will easily footprint, caused by the
drop in cell turgor pressure. The internal cell water has been
pulled from the cell for transpiration. The root system does not
have an adequate supply of water to replace the water lost to
transpiration and wilt occurs.
Shallow-rooted turfs are more prone to wilt. Any condition that
inhibits rooting such as lack of aeration, compaction, waterlogged
soil (throughout this spring), excessive nitrogen fertilization,
high amounts of thatch, root feeding insects and nematode damage
may reduce the wilt tolerance of any turfgrass. Any of these conditions,
combined with high temperatures, wind movement, solar radiation,
and low humidity increase the chances of turfgrass wilt.
Increasing the amount of available water to the plants root system
will help to decrease the incidence of wilting. Water absorption
is increased by healthy root system. Any cultural practice that
encourages an extensive root system will help to reduce wilting.
These include cultivation, irrigating to the depth of the root
system, judicious use of fertilizer, and use of wetting agents
for increased water penetration.
When wilt is apparent corrective measures should be taken as soon
as possible. The turf should be irrigated, or what is referred
to as syringing, to decrease the water deficit and allow transpirational
cooling to function without negative impacts to the turf.
Luke Cella, CGCS, Editor
Tom Voigt, U of I Turfgrass Specialist
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