Vergara - 1976 - Physiological and morphological adaptability of ri

286 cusutrs AhD RICE
tillers were so weak that the plants were considered as a population consisting
only of main stems. Culms and panicles ofmain stems tvere shorter than those of
the usual thin-spacing culture. l-ltivvever, the growth of main stems was unifonn
and healthy, resulting in good fertility under the growing conditions. Thus, it
was possible to sample about l0 unifomi particles per pot within l day’ of their
most susceptible stage. The efficiency’ of sampling the unifonn particles. in this
culture is much greater than that in the usual thin-spacing culture.
We used thcrmo-sensitive. early maturing varieties to speed the cy-‘clc of experiments.
For example, a stool-tolerant variety’. Hayiayjuki. attained heading
time about 47 dayts after sowing. We tvere able to induce 50-60 percent sterility
by a cooling treatment at 12°C for 4 days at the young microspore stage and
to complete a series ofexperiments every’ 6O days.
Sampling susceptible spikelets
A new culture method for obtaining uniform panicles in the limited space ofthe
phytotron ‘was thus established. Ilowever, one more problem remained to be
solved for making reproducible experiments. That is to select the spikelets strictly
at the most sensitive stage. Paniele development can be estimated by panicle
length (Terao et al., 1940), but panieles at the most sensitive stage are not seen
from the outside because they are within the leaf sheath. We used the distance
between the last two auricles (auricle distance) as an excellent criterion for
estimating panicle development (Sakai, 1949c).
The age of spikelets in a panicle. however. usually differs by '7 days The susceptible
period ofa spikelet may be 2 day's at most (Satake and Hayase. 1970).
So tvc used spikelets located third. fourth. and fifth from the top on the three
upper primary branches (9 spikelets per panicle). to reduce the difference in
spikelet age. Thus. the difference in spikelet age was reduced to less than 2 days.
When physiologically’ more homogenous materials were needed. we used spikelets
of the same palea length. In some cases we investigated the response to cool
temperature at every differently located spikelet on the upper primary branches in
the panieles grouped by auricle distance at intervals of I cm. These experimental
procedures have been very" useful in making precise. reproducible experiments
and have contributed greatly to rapid advances in our studies.
SUSCEFTIBILYIY OF ORGANS TO COOLING 'I'EMPERA'I‘URE
Panicle
In the cool-injury] yfiear of 194]. Sakai (l949b) found that sterility of rice plants
varied ivith the depth of irrigation water, and sterility was significantly less in
l5 cm depth than in the usual 5 cm. IIe suggested that the location of panicle-s
at the susceptible stage was under the surface of l5 cm deep water and this
protected the panicles from the cool-injury‘ temperature, resulting in a significant
decrease in sterility. The validity of Sakafs suggestion was demonstrated by
several later precise experiments with controlled air and “rater temperatures