Agriculture Reference
In-Depth Information
The maintenance of the turgor of the
cell and tissues, by means of water, is fun-
damental for the elongation of the tissues
and growth. Therefore, proper levels of
water content must be maintained in the
plant, to avoid inducing growth limitations
that negatively affect the yield.
Transpiration depends on the intercepted
solar radiation and on the environmental
humidity at the level of the leaf boundary layer
(see Chapter 6). Transpiration shows a hyster-
esis in relation to solar radiation, from sunrise
until noon. At night, the rehydration may
involve, on cloudy days, a relevant percentage
of the daily water use (Medrano, 1999).
When air humidity decreases, the VPD
(water vapour pressure deficit) increases and
transpiration increases. If this transpiration
increase is large, and the water supply by the
roots is low, the stomata will progressively
close to avoid tissue dehydration. Therefore,
in climate controlled greenhouses, an exces-
sive increase of the VPD must be avoided.
The water movement through the soil-plant-
atmosphere continuum is governed by its
overall potential, so the alteration of the
water potential conditions in the soil or in
the plant affect the whole set.
With high ambient humidity and low
levels of radiation the transpiration rate is
very low. As nutrient absorption is linked to
the transpiration rate, these conditions (low
radiation and high ambient humidity) may
induce nutrient deficits, especially for ele-
ments like calcium whose mobility in the
plant is very strongly linked to transpiration.
Under Mediterranean conditions, the
maximum transpiration value for a green-
house crop such as tomato is 6 mm day
−1
(Jolliet, 1999), although some authors place
it in general between 6 and 9 mm day
−1
, or
even 1-1.5 mm h
−1
(Kempes, 2003).
decreases quickly as the soil surface dries out
and it is not rehydrated. When crop develop-
ment is slow, and the plants only slightly
shade the soil (intercepting little radiation),
the evaporation can be very important. When
the crop is well developed and shades the
soil completely, the evaporation is very lim-
ited, because the plants intercept most of the
solar radiation preventing it from being used
in the evaporation of water from the soil.
Soil mulching, such as sand mulch
('enarenado') or with plastic materials,
totally or partially avoid the evaporation of
water from the soil.
In greenhouses, with crops grown in the
soil, drip irrigation minimizes the evaporation
(E), wetting much less of the soil surface than
for example sprinkling irrigation systems.
In soilless crops, the substrate is usu-
ally covered by a plastic film, so the E com-
ponent of the ET is virtually nil.
11.4.1
Introduction
The water status of the soil is characterized
by its capacity to retain water and by its
water potential (energy status of the water
in the soil).
the soil water stress
Water potential
The most precise way of quantifying the
water available in the soil for absorption by
the plant's roots is by means of the water
potential in the soil (y
soil
). The water poten-
tial is normally measured in pressure units,
using MPa and kPa (1 kPa = 0.001 MPa).
The water potential is a measure of the
free energy of the water and has four com-
ponents (matrix (matric) potential, gravita-
tional potential, osmotic potential and
pressure potential). The matrix potential
(y
m
) is caused by the forces that retain the
water in the soil. The osmotic or solutes
11.3
Evapotranspiration
Evapotranspiration (ET) is the sum of the
water evaporation from the soil surface (E)
and the transpiration (T) or water evapora-
tion through the plants.
The evaporation (E) from the soil is high
when the soil is wet (e.g. after irrigation) but
