Agriculture Reference
In-Depth Information
use one deeper sensor, at an equal distance
from the plant and dripper, to control the
deep water content and the drainage
(Fig. 11.3). Furthermore, another sensor can
be placed at the border of the wet bulb and
at the same depth as the first one, to check
the proper distribution of water.
plant's water status. The crop water stress
index (CWSI) has been proposed as a stress
index (Idso
et al
., 1981) (see Appendix 1
section A.8.1).
The measurement of the leaf tempera-
ture is usually done with an infrared ther-
mometer, although thermistors have also
been used to plug in to the 'phytomonitor-
ing' equipment (section 11.6.5)
The crop temperature is not an instant
indicator of water stress, as the temperature
increase due to partial closure of the sto-
mata takes place much later than the inci-
dence of stress in other processes such as
leaf expansion, so its use in horticultural
crops sensitive to water stress, such as veg-
etables, is of little interest (Gallardo and
Thompson, 2003b).
Methods based on plant parameters
The use of plant sensors is a direct method
that indicates when to irrigate, knowing the
moment at which the plant starts to suffer
water stress. After irrigation, the plant sen-
sor can detect if the performed irrigation
was insufficient or not, making it possible
to adjust the water supplies.
In the past, in order to know the water
status of the plants, manual measurements
were performed (leaf and stem water poten-
tial, stomatal conductance) and plant sen-
sors were only used in research. Nowadays,
there are sensors that measure indirectly the
water status of the plants, automatically,
and it is possible to use them in commercial
crops (phytomonitoring). This will be dis-
cussed later in this chapter.
s t e m
d i a m e t e r
.
When the plant starts to
transpire, at the beginning of the day, a
water content decrease in the leaves and
stems is caused, as the water absorption
by the roots is slower than transpiration,
causing a gap that is maintained through-
out most of the day. When the evaporative
demand decreases, in the afternoon, this
gap decreases and the water absorption
by the roots continues until all the tis-
sues are rehydrated (Kramer, 1983). The
variations in the stem water content
caused by this gap between transpiration
and root water absorption can be meas-
ured with sensors called dendrometers or
lineal variable displacement transducers
(LVDTs). These operate continuously and
automatically.
Over a 24 h period the greatest stem
diameter occurs at the end of the night,
when hydration is at the maximum, and the
minimum value is reached at noon (Gallardo
and Thompson, 2003b). In order to use the
stem diameter as an indicator of water stress
for irrigation scheduling, it is necessary to
have proper data interpretation methods.
w a t e r
p o t e n t i a l
.
Water potential is measured
with a pressure chamber and characterizes,
directly, the water status of the plant. In
order to schedule irrigation it is necessary
to know the threshold values from which
the crop suffers water stress. Measurement
of water potential has had no use in horti-
cultural crops, as it is not adapted to auto-
mation, among other reasons.
c a n o p y
t e m p e r a t u r e
.
The canopy or leaf tem-
perature is another indirect indicator of the
s a p
f l u x
.
The water flux through the stem is
a direct measurement of the plant's trans-
piration and a very sensitive indicator of its
water status. It allows the crop water
requirements to be detected, although its
use in vegetables is still limited to the
research stage.
Fig. 11.3.
Suggested location of soil moisture
sensors in vegetable crops.
