This thesis compares the physiological and morphological
differences between several soybean (Glycine max L.)
cultivars in response to high temperature stresses applied
to either the leaves or the shoots, and the ability of the
leaves to heat harden. In addition, the effects of heat
stress on nitrogen fixation and on the growth of
Bradyrhizobium japonicum and its ability to heat harden
were investigated.
The optimum temperature for the germination of all
cultivars was between 25 and 35°C, while the temperature
range 40 to 42.5°C was the upper limit for germination and
root growth. No growth of any cultivar occurred at 45°C.
Pre-germinating the seeds at lower temperatures before
subjecting them to heat stress resulted in better
germination and growth at higher temperatures.
Leaf chlorophyll fluorescence analysis measured in terms of
Fv/Fm ratio was used to detect differences between the heat
sensitivities of different cultivars. After heat stress at
40°C, the Fv/Fm ratio decreased in all cultivars tested and
after stress at 42.5°C there was a 50-70% decrease in the
ratio. The main difference between cultivars was in their
ability to recover after heat stress. Williams-82 and Sable
were better in this respect than cultivars Bragg, Davis,
Mago-80 or Hardee. There was no recovery from heat stress
treatments at 45°C. Using the cultivar Williams-82 it was
possible to show that heat-acclimation (hardening)
treatments had a significant effect, increasing tolerance
to high temperature stress.
Experiments showed that nitrogen fixation in root nodules
was little affected by root temperatures up to 35°c. At
40°C, small differences were detectable between cultivars
and at 45°C nitrogen fixation was severely inhibited in all
cul ti vars. Nitrogen fixation was also reduced by heat
stress applied to the leaves but this response was
considerably slower.
In pure culture, B.rhizobium japonicum grew well up to
40°C, but growth was very slow at 45°C. Heat-hardening
treatments were apparently effective in permitting faster
growth at high temperatures.
Overall it is concluded that there is a only limited
genetic variability in heat tolerance among the soybean
cultivars examined in this study.