Vergara - 1976 - Physiological and morphological adaptability of ri

STERlLE-TYPE coor INJURY IN PADDY RICE PLANTS 285
observations ofmicroscopical abnormalities support the idea that the stage most
sensitive to coolness is the meiotic division stage. Of these abnormalities, Sakai
(l949a) noticed the hypertrophy’ of tapetal cells causing degeneration of anther
contents. resulting in high sterility. The significance of tapetal abnormalities to
sterility will be discussed later in detail.
EFFECTIVE METHODS OF STUDYING
COOL INJURIES IN THE PHYTOTRON
Reproducibility 0f cool injury
In the earlier studies. cool injury of rice plants was induced mainly by running
cold ivater through paddy fields. The percentage ofsterility caused by this method
differed with years and locations because of uncontrolled water temperatures
under the natural conditions. So this method is unsuitable for precise
physiological studies. but it has been practised to select cool-tolerant lines in
breeding work. In order to make physiological experiments effectively, we need
a facility which could be used to create and control weather conditions with
precision. Since World War II, cool injuries have been induced by controlling
the cool temperature in a phytotron or in a growth cabinet. Kondo (I952) found
that the percentage of sterility yvas different in every yiear even when the same
variety’ was cooled to the same temperature at the same susceptible stage. He
thought that these differences resulted from the vatyiing climatic conditions
before or after the cooling treatment. Ito (1971) shmved the percentage of sterility
was significantly increased by slightly cool temperature before or after the
cooling treatment. during the young microspore stage.
These facts suggest that precise and reproducible results may be expected only
when rice plants have grown under controlled climatic conditions both during
cooling treatment and also before or after it. Consequently’. to obtain reproducible
results it is necessary to grow rice plants through their whole life under the
controlled climatic conditions of the phytotron. However, the space in the phytotron
is limited. So we had to devise a method of growing many uniform panicles
within the available space of the phytotron.
Culturing uniform panicles
Terao et al. (I940). in precise experiments. used only panicles of main stems of
two hills planted in a pot. Even if uniform panicles could be obtained. this
method required too many pols.
We devised circular dense-culture in pots as an effective method of obtaining
unifonn and Iiealthy‘ panicles (Satake et at, I969; Satake. 1972b). Twenty seeds
were soxvn in a circle in each I/iOUO-a Wangeris pot and were grown through
their whole life in natural-light rooms of the phyrtotron under the regime of 24°C
day'-I9°C night. Pots were moved in tum every day in order to minimize growth
difference due to microelimate in the room. We used only main stems as materials.
Under such cultivation. only ttvo tillers developed from every plant. These