Vergara - 1976 - Physiological and morphological adaptability of ri

GENETIC \';—\RIOLTS?\TESS IN FLINIATTC ADAPTATION 93
However. the growth and yield of the plant are more readily‘ affected by the
lower temperatures than by the high extremes (see papers by Vergara. Nishiyiama,
a11d Satake in this Symposium).
The complexity of the temperature responses is the distinct interaction among
eultivars as to the effect of temperature at a particular stage of growth or development.
Extreme levels at both the high and the low limits adversely affect
rice growth and reproduction. Moreover. the experimental conditions also
influence the temperature response. Therefore. one might expect to find frequent
exceptions to any generalized statements.
The lower limit for seed germination and seedling growth is about 15°C
(Sasaki and Takahashi. 1970). When the rate of seed germination was expressed
by the "temperature constant" (regression of germination rate on air temperature
on a logarithmic scale). the value varied from 1.8 to 3.6 among 70 cultivars.
The indiea varieties of the tropics generally have a higher temperature constant
than the japonica varieties of the temperate zone (Oka. 1954b). One might then
expect the indiea varieties to have a higher minimum germination temperatuie.
However, 1R8 has been shown to germinate faster than Japanese varieties at
levels of temperature lower than 20°C (IRRI, 1971; Sato, 1973).
Japanese eultivars and genetic testers differed appreciably in the rate at which
seed germination proceeded at 15°C. Sasaki et al. (1974) estimated that four or
more loci controlled the total variation in the number ofdays for seed germination
in one cross.
Although stunted seedling growth and leaf discoloration may occasionally
occur during an unusually cool period in the subtropics (Kaneda and Beaehell.
1974), the effect of low temperatures is more frequently expressed by the slower
vegetative growth and delayed floral initiation of the susceptible varieties. When
the responses in vegetative and floral initiation stages were separated by the use of
partial regression of number of days to heading on the mean temperatures in a
given period of growth, the indiea varieties were more sensitive than thejaponica
varieties to low temperatures during ytegetatiy-"e growth (Oka, 1955, 1958b, 1959).
It l1as been shown that the groitvth durations of 1R8 at Dacca. Darwin, and
Kathmandu xvere negatively correlated with the minimum daily temperatures
(Chang and Vergara, 1971). The optimum night temperature for floral initiation
and development was between 20° and 25°C. while 15°C prevented floral
initiation (Ovren, 1972).
At high elevations in tropical areas or during the spring months in the subtropics.
cool tnteather prior to panicle initiation generally resulted in profuse
tillering and reduced plant height of tropical semidwarfs such as IR8 (Vergara
and Visperas. unpublished). Effects of low water temperatures (l3-25°C) on
seedlings range from killing to inhibited growth or tillering (Kondo. 1952;
Onnrod a11d Buntcr, 1961". IRRI, 1973).
On the other hand. warm day temperatures ranging from 25° and above frequently
reduce the tillering period in varieties coming from high latitudes and
lead to accelerated paniele initiation and emergence. In monthly seedings at