Potassium Research and Agricultural Production - The International ...

In our view the fact that the K ion efflux was not affected as expected by the Ca ion at
low temperature (under cold stress), is certainly worthy of attention. It is conceivable
that there was very little or no endogenous Ca in the cell. In this case, of course, the
permeability of the membranes under cold stress increases considerably, resulting in an
appreciable K ion efflux.
At any rate these data support the view that Ca plays a very important role in the structure
of the membranes, and particularly in its function of K ion uptake. The above results
allow a ready explanation of the effect caused by the addition of Ca ion at a later
stage of the K ion uptake experiment (fig. I). In this case too, it is clearly a matter of
the added Ca ion eliminating the uptake anomaly, but not inhibiting the passive K ion
efflux.
3.3. Investigations of cold-resistance of thermophilic plants on the basis of measurement
of K iOIl influx at low temperature
Investigations into the effect of a sudden fall in temperature on the initial uptake of K
ion by different cold-sensitive plants showed that the critical temperature below which
cold shock occurs (as shown by anomalous uptake) varies considerably for different
plants (fig. 5). Thus, for example, it is 5-6° C in sorghum, maize and tomato, about 9° C
in rice, /4_15° C ill cllcumber and 16_17° C i/1 musk melon.
Accordingly, the K ion uptake minimum of a plant may be higher than 0° C, to an extent
depending on its temperature-sensitivity. The experimentally observed differences
for these plants are in agreement with field experience of temperature-sensitivity. For
cold-resistant plants such as wheat the decrease in uptake rate with decreasing temperature
occurs down to near 0° C without anomaly.
Two important deductions can be made from these experiments: (i) the more cold-sensitive
a plant, the higher the critical value on the temperature scale; (ii) the more coldresistant
the plant, the less the uptake anomaly at 0° C. The recognition of this served
as a basis for further investigations connected with the cold-resistance of thermophilic
plants.
Figure 6 shows K ion uptakes at low temperature by roots of different rice varieties.
There is a considerable difference between rice varieties in the ion uptake at 0° C,
where a passive influx is known to take place. It can be seen from the graphs that the
K ion uptakes of cold-resistant Dunghan Shali, G-70 and Ktikai 203 rice varieties were
. always lower than that of the Dubovsky variety, which is less cold-resistant, as already
proved by practical observations.
These results convincingly supported our supposition that the measurement of K ion
uptake anomaly (passive influx) at 0° C can be used to determine differences in
cold-resistance not only between species, but also between varieties of thermophilic
plants. Indeed, our experiments with other rice varieties shown in the graphs are very
encouraging. On the basis of earlier results, the varieties Dunghan Shali, Ktikai 203 and
G-70 appear to be the most cold-resistant, while No. 83 is the least cold-resistant. Our
laboratory results were confirmed by field control experiments conducted at the Irrigatioll
Research Institule at Szarvas (Hungary). The method is already in use in the rice
screening work.
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