Vergara - 1976 - Physiological and morphological adaptability of ri

284 CLIh-IATE AND RICE
Susceptible stage t0 coolness
For many years it has generally’ been assumed that sterility resulted from cool
summer temperature at anthcsis. Sakai ( 1937a) reported first that sterility of rice
plants could be induced by cool temperature at the meiotic stage. After a year,
Kakizaki and Kido (1938) cooled rice plants at the successive stages of paniele
development and clearly’ demonstrated that the percentage of sterility was the
greatest when rice plants were cooled at the meiotic stage of PMC"s (10-11 day's
before heading). Their results have been confirmed by many later researchers
(Terao et al. 194th Kondo, I952: Shimazaki et a1, 1960; Play-arse et al. 1969).
Recently, Satakc and l-layase (1970) found that the most sensitive stage to coolness
is the young microspore stage after the meiotic division. This most sensitive
stage will be discussed later in detail.
Critical cool temperature
Tcrao ct al. (1940. 1941) induced remarkably high sterility by cooling at 17°C for
6 days and 10w sterility at 20°C for 10 days. Sakai (1949a) showed that abnormalities
of meiotic division and hypertrophy of tapetal cells, being considered the
major causes of sterility‘. occurred below 15°C. Nishijrama et a1. (1969) showed
that the critical cool temperature for inducing sterilitjv was l5°—l'i°C in the highly
cool-tolerant varieties and 17°—l9°C in the cool-sensitive varieties. Judging from
these studies. high sterility’ seems to occur below the critical temperature from
15°C to 20°C.
The above-mentioned studies had been conducted under constant day and
night temperatures during the cooling period, but constant temperatures do not
occur under natural climatic conditions. Matsushima et a1. (1958) investigated
the effect of temperature on sterility’ at the meiotic division stage by using a
combination of regimes. controlling different day and night temperatures. lligh
sterility occurred at continuous temperature, while very‘ low sterility occurred
when warm temperature during the day" was combined with cool temperature
at night. A similar effect ofdaytime warm temperature in decreasing sterility; was
reported by Shimazaki. Satake. Watanabe. and Ito (1964). Shibata ct al. (1970)
studied the relation between sterility’ and the same average tifdaily temperatures,
differing in day and night temperatures. The_y obtained the lowest sterility' when
a certain difference existed between day" and night temperatures.
Cytological abnormalities
Sakai (1943) made cytological observations on the reproductive organs of rice
plants in 1941 when they were severely damaged due to summer cool temperature.
Ile found various abnormalities: (l) injuries in differentiation of pollenand
embryo sac-mother cells. (2) uncoupling of chromosomes during meiosis.
(3) interruption of cell wall formation after meiosis. and (4) hypertrophy of
tapetal tissues in the injured anthers. The interruption ofccll wall formation in
pollen- and embryo sac-mother cells produced large cells u-ith two or four nuclei
(Sakai. 1937b. 1939). They had no function or they degenerated later. These