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(trying to store the heat within the plant). Thermal diffusivity
can be described as the ratio of these two properties; the
thermal conductivity divided by the material’s volumetric
heat capacity.
To achieve a full picture the correct operation of this thermal
diffusivity sensor depends upon good thermal contact with the
surrounding plant tissue. Although poor thermal contact may
not neccesarily damage the plant, it causes its temperature
to rise more than normal as it attempts to push the same
amount of heat through a far smaller contact area.
The sensor is in place to detect and reports this error.
Vapor Pressure Deficit
Another means of assessing the value of VPD is through
assessing the atmospheric demand for water or the ‘drying
power’ of the air. VPD is usually measured in pressure
units, in most cases either millibars or kilopascals, with the
data gathered basically a combination of temperature and
relative humidity, grouped together as a single value. VPD
values run in the opposite way to RH vales, so when RH
is high VPD is low.
The higher the VPD value, the greater the potential the air
has for sucking moisture out of the plant,
with VPD providing what is, for the grower, a more accurate
picture of how plants relate to temperature and humidity.
Awell positioned sensor measuring the air temperature and
humidity close to, or just below, the crop canopy is adequate
for providing a good indication of actual leaf conditions.
Water Dynamics
The effects of plant water dynamics on the thermal diffusivity
sensor can be described as two-fold, although, in both
instDnces the SODnt sensor is sensitiYe to both sDS ÁoZ Dnd
tissue water content.
Plant water content changes during the day as plant water
status changes, a fact that researchers have been aware for
decades, thanks to the use of dendrometers, which have in
the past proven that stem diameters shrink during periods
of maximum transpiration.
7he tests cDrried oXt reDIfirPed the theory thDt SODnt tissXe is
capable of “relaxing “ and even “recharging” overnight from
the stresses caused by daily transpiration. Theoretically the
grapevine’s tissue density increases, making for a steadily
decreasing thermal diffusivity, and as a result. The component
of heat stored by the extra water in the surrounding tissue
causes the tissue density to increase.
These changes in the local cell water content have a knock-
on eIIect on the Zood to ZDter rDtio Zhich ZiOO inÁXence
tissue density, causing changes in the local volumetric
heat capacity.
'DtD IroP fieOd triDOs oI the sensor in grDSeYines shoZed
that unstressed vines have a CWS of over three hours,
and stressed vines have a CWS measured only in minutes.
Results and discussion
(Ificient irrigDtion rePDins D Yery iPSortDnt IDctor in
grapevine production in water-short regions. In order to
improve irrigation management in vineyards a plant based
technique capable of diagnosing the onset and severity of
water stress is proposed.
This technique, based on the proven actuality that stem
water potential can play a part in providing a reliable
indicator of grapevines water status, although not without
its disadvantages,
7he Xse oI therPDO diIIXsiYity Dnd YDSor SressXre deficit
sensors is a procedure that requires to be assessed
continuously over a sustained period, with particualr emphasis
on measuring the close relationship between mean daily
sDS ÁoZ decreDse Dnd steP ZDter SotentiDO
According to data gathered during this more than year
long trial, the information can be used as an accurate and
simple signal for real-time irrigation scheduling in vineyards,
requiring only a small number of sensors generating data
which can be easily gathered and simple to understand.
Possible discrepancies of the method due to seasonal
ÁXctXDtions in 93' Dnd eYDSorDtiYe dePDnd Dre e[Sected
to be negligible due to the inherent ability of grapevines to
regXODte their trDnsSirDtion rDte thXs e[erting fine controO oYer
transpiration under completely different evaporative
conditions.
Despite the data gleaned being limited, the amount of
useful information gathered suggests a strong relationship
between soil moisture conditions and levels of crop water
stress in vineyards. ■