Agriculture Reference
In-Depth Information
imposing drought tolerance. Higher RWC is a good indicator of drought
stress tolerance (Shaw et al., 2002).
9.3.8 PLANT CANOPY TEMPERATURE OR TRANSPIRATIONAL
COOLING
It has long been recognized that leaf or canopy temperature is highly de-
pendent on the rate of transpiration and therefore, can be used as an indi-
cator of stomatal opening. Accordingly, infrared thermometry has been
developed as a means for irrigation scheduling. Plant canopy tempera-
ture is directly correlated to stomatal conductance and transpiration. As
long as the plants continue to transpire through open stomata the canopy
temperatures could be maintained at metabolically comfortable range oth-
erwise higher temperature would destroy the vital enzyme activities. Sto-
matal closures for a considerable period of time are known to increase
the leaf temperature. The thermal imagery system is a powerful tool as it
can capture the temperature differences of plant canopies fairly quickly
and instantly. The trend in canopy temperature and differences in tem-
peratures between canopy and air (T
c
-T
a
) is an indicator of the plant water
stress. The relationships between canopy temperature, air temperature and
transpiration is not simple, involving atmospheric conditions (vapor pres-
sure deficit, air temperature and wind velocity), soil (mainly available soil
moisture) and plant (canopy size, canopy architecture and leaf adjustments
to water deficit). These variables are considered when canopy temperature
is used to develop the crop water stress index (CWSI), which is gain-
ing importance in scheduling irrigation in crops. Relatively lower canopy
temperature in drought stressed crop plants indicates a relatively better
capacity for taking up soil moisture and for maintaining a relatively better
plant water status by various plant constitutive or adaptive traits. Besides,
it should be noted that canopy temperature is dependent on climatic pa-
rameters and internal plant water status. High crop canopy temperature in
water-stressed plants may also be related to decreased transpiration rate
and leaf water retention capacity values Tan (1993). Drought resistant
genotypes show higher values for T
c
-T
a
. There seems to be a positive link
between yield and transpiration rate.
