Vergara - 1976 - Physiological and morphological adaptability of ri

166 cunt-ms AND RICE
mersion at 25°C for 20 hours before stiwing (Inouye ct al. 1973). The growth of
the plumules in japonica with high temperature treatment tvas rnuch slower than
in indica (Inouye et al.. 1973). This was due to the difference between them in
the rate of mesocotyl elongation. Furthermore. multiplication of the parenchymal
cells in the indica type was more active, and occurred through the period
of rnesoeotyrl elongation. Thus. these authors suggested the necessity of introducing
genes controlling mesocotyl elongation from indica varieties to japonica
ones. to improve emergence ability in direct-sown culture.
lnouyte and Anayama (1971) studied the effect of temperature on the strength
of plumule elongation. Plurnule elongation was vigorous in seedlings which ‘were
cultured under warm temperatures from 20° to 30°C; seedlings which were cultured
at 17°C had much lower ability to elongate (about one-fourth as compared
with that at 30°C). This may be a primary factor in low emergence ability“ at
lOWCf temperatures (lnouye et al.. 1967).
Initial growth of germinated seeds was accelerated by germination to a degree
of pigeon-breast shape. then kept at 5°—l0°C for 10-20 days and dried in air
(Yamada et al. 1963).
Seedling establishment under water
The temperature of irrigation water into which seeds are
sown is sometimes
twerly’ high in the tropical region. On the other hand. it is sometimes tn-'erl_y' low
in temperate and subarctic regions.
Chapman and Peterson (1962) reported that water temperatures between 25°
and 30°C were most favorable for seedling establishment of pregerminated rice
seeds sown directly into water. Emergence of shoots from the water was most
rapid at 30°C. but the optimal temperature range for primary root elongation
and penetration of the soil was 20°—25°C. At 35°C the roots grew mainly in the
water layer. so that the seedlings floated free or were poorly attached to the soil.
The elongation of primary roots was inhibited more strongly at 35°C than at
20°C. Contrarnvise. the elongation of shoots and secondary roots was inhibited
more strongly at 20°C than at 35°C. Thus, the root is more sensitive to high
temperature than the shoot. A tyater temperature of 40°C was lethal to pregenninated
rice seeds.
Herath and (fJr-ntrod (1965) reported that “tanner temperatures (24°, 27°, and
32°C) favored growth of both shoots and roots as compared to a cooler one
(l6°C) in direct sowing into water. However. the adverse influence of low
temperature was much smaller in the roots. Chapman (1969) reported that
seedling establishment decreased linearly with increasing mean temperature
over the range from 25° to 35°C near the soil-water interface for the first l0
day's. The turbidity of water contributed to a decrease in the mean water ternperature
(Chapman. 1969). Hoxvever. it generally did not improve seedling
establishment. because of some factor such as less firm rooting in the soil.
Onnrod and Bunter (l96la) estimated the cold water tolerance (at 155°C) of
pregerminated rice seedlings sown directly into water. California varieties