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Physiological parameters such as Green Leaf Area Index,
Dry biomass, Chlorophyll concentration, were concurrently measured
at the same interval throughout the growing season, After every
15 days, all the plants from one square meter area were harvested
at ground level and Leaf area was measured by an automatic leaf
area meter (Model Hayashi Denkoh and Co. Japan), the plants were
oven dried to give leaf and stem dry weights of the plants, All
weights were presented on a per square meter ground area basis.
Leaf area was used for commuting leaf area index. Chlorophyll
concentration was measured using the method of Arnon (1949).
3, RESULTS AND DISCUSSIONS
The red and IR reflectance were plotted against the
number of days after sowing (Figure 1). Figure 1 (a) gives the
variation of red reflectance of wheat crop throughout the growing
season, In the beginning, the red radiance decreases very rapidly
with time because of increased chlorophyll absorption, This
increase in absorption is due to increase in chlorophyll concen-
tration in the plants and also because of increased green leaf
area index, The red radiance decreases until the growing season
wanes and senescence begins. The red radiance for irrigated wheat
crop shows a saturation behaviour fram 55 to 100 days. A. rapid
increase in the red radiance occured after 100 days. due to chloro-
sis of the maturing crop. The red reflectance for stressed (un-
irrigated) plants was always higher than that of irrigated one.
The infrared reflectance versus days after sowing curve
(Ficure 1 b) showed a gradual increase with time to a maximum at
70 days (for irrigated plot) followed by a decrease due to senes-
cence, The peak for unirrigated plot comes earlier than the
irrigated plot. This is becuase of the fact that the unirrigated
crop matures earlier. In the early growing season the IR radiance
for irrigated plot is less than that for the unirrigated one after-
ward the trend is reversed. This is because the chlorophyll
concentration in the stressed plant increases in the beginning and
afterwards it reduces very fast. Figure 2 a gives the variation
of IR/red ratio with number of days. lt again shows maxima, Here
Separation in the level of the two maxima (irrigated and unirriga-
ted crop) is more, Again the maximum for unirrigated crop occurs
early, The difference in the IR/red ratio value of irrigated and
stressed crop is more in between 60 and 100 days. That means the
descriminability to irrigated and stressed plants is more during
this period, In order to detect stress the remote observation may
be taken in this period. The maximum difference in the ratio
value for the stressed and nomal crop is as high as 40%.
Normalized difference ND = (IR-red/IR+red) is plotted
against the days after sowing in figure 2 b. In the beginning,
ND increased very fast as the crop cover increases (upto 55 days).
Once the green vegetative cover is completed, ND reaches a plateau.
During the period of crop maturation and dry down, ND declines
gradually. The ND plateau for unirrigated plot is at lower level
than that for irrigated one, This is because the principal
factors influencing ND measurements are chlorophyll density and
the green leaf area index. The stress conditions restricts the
plant growth by affecting chlorophyll density or leaf area index
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