| dc.description.abstract | Seeds of Phaseolus vulgaris L. cv GLP2 were
sown in two plots over two dry seasons; January to March
and July to September 1990. A line source sprinkler
system was used to supply and control the amount of water
received by the crop. During the first season the high
and low water treatments, received 699.6 mm and 421.5 mm
of water respectively. Reference crop evaporation
was 329.3 mm over the season. During the
second season the high treatment received 255.7 mm of
water while the low water treatment received 123.8 mm and
Etq was 210.8 mm.
Measurements bf dry weight, leaf area and radiation ,,
were made after everyone week. Soil moisture levels were
measured-over the season at depths of 10cm, 30cm and
later, 50cm as the roots grew deeper. Leaf area in
many cases was significantly different at p<0.05 between
treatments for both seasons. SoiL moisture levels during
the first season were significantly different at almost
all depths between 33 days after sowing (DAS) and 70 DAS.
For the second 'season there was significant difference
from 37 DAS upto the end of -the season with only one
exception. Differences in dry weight between treatments
increased with time in both seasons and generally, plants
of the low water treatment absorbed less PAR than those
of the high water treatment in both seasons.
Photosynthetic efficiency, e, was estimated as a
regression of the dry matter accumulated over the growing
season upon the cumulative GR intercepted and PAR
absorbed over the same time period and those based on
PAR converted into a dimensionless energy term. The
economic efficiencies were calculated using only the
economically important part of the plant (.i.e the final
dry seed) as ratios of the final seed-weight per metre
squared to the total GR intercepted and PAR absorbed over
the season. Those for PAR absorbed were also expressed as
a dimensionless energy term. Photosynthetic
efficiency was also estimated as a product of the
proportion of incident PAR absorbed (e.••) and
the photochemical efficiency with which the absorbed PAR
is converted
into dry matter for three
growth stages in each of the two seasons.
a drop in pho\osynthetic efficiency of
about 20% in the low water treatment for both seasons (in
terms of the dimensionless eaer9.Y term) while for the
economic efficiency, the drop was about 20% and 501. for
the first and second seasonsYespectively. The yield in
the first season decreased by 21% in the low water
treatment compared to the high water treatment. The
average final seed-weight for the treatments were 12.9
g/plant and 15.8 g/plant respectively. In the second
season the average yield decreased by 42% In the low
water treatment. The method of estimating e values
from the components and differed
from that of regression in that the former was based on
incident PAR and on growth stages and a different
grouping of dry matter accumulation and radiation.
Differences of efficiencies calculated differently are
explained in the text.
An investigation of the performance of PAR sensors,
i .e. the tube solarimeters and a line quantum sensor,
showed that each tube solarimeter under-read the line
quantum sensor by the same amount irrespective of its
orientation.
Yield in beans under water stress is confirmed to be
reduced through a decreased assimilatory leaf area
leading to a decline in the amount of PAR absorbed as ,,
well as by a reduced rate of photosynthesis as indicated
by the dr-op in" photosynthetic efficiency. The latter
means that the efficiency of conversion of absorbed
radiant energy into dry matter needs to be taken into
consideration in the search for plant varieties suitable
for growth in the marginal semi-arid areas | en |
