Vergara - 1976 - Physiological and morphological adaptability of ri

78 tiLIMATE AND RICE
yield potential, as shown in IR442-2-58 (De Datta and Abilayfi, 1974). This rice
line is adapted to upland conditions as well as deep water conditions (90 em of
xvater).
In tropical Asia, the rice plant is often subjected to submergence in water for
one to several days during the monsoon season as a result ofheavy precipitation.
Varietal differences have been noted in the resistance of the rice plant to submergence.
Resistance is associated with the amount of carbohydrate aeey mulated
in the plant and utith the factors affecting carbohydrate accumulation
(Palada and Vergara, 1972). Varieties with higher carbohydrate content have
greater resistance to submergenee.
Rice is often subjected to moisture stress, especially rainfed lowland rice and
upland rice. Numerous experiments, using different techniques, varieties, and
definitions of drought resistance, have shown differences in varietal response to
moisture stress (IRRI, 1975). Morphologically, moisture stress in rice results
in reduced plant height, reduced leaf area and tiller number per hill, and a conspicuous
rolling of leaves. Physiologieallyg plants generally have higher temperatures
during moisture stress, since the stomata are closed and photosynthesis
is low; proline content also increases (Yoshida et ali, 1974).
Due to plant height reduction, the upland rice varieties were generally more
resistant to drought than most lou-‘land improved varieties (Yoshida et al., 1974;
Krishnainurthy’, unpublished). Using cuticular resistance, Yoshida et a1. (1974),
found several upland varieties with low resistance, although the highest values
were measured in the upland nee varieties. Still. sorghum and com had much
higher cuticular resistance than rice. It has been shown that moisture stress
increases the prolinc content of rice plants. Varietal differences were also noted
in proline accumulation (Yoshida et al.. I974).
Chang et a1. (1972) also reported varietal differences in root development and
recovery from desiccation. Presumably such characters as high stomatal resistance.
long roots, and rapid rectiveryx to name a few. are important characters
for drought resistance. The problem at present is incorporating these characters
to develop a variety with high specific adaptability‘ to moisture stress. without
decreasing its potential yield.
The rice plant is still a long way from becoming a drought-resistant type comparable
to sorghum. This is not the real aim of the rice plant breeder however,
because upland rice is grown in areas with much higher precipitation than that
required by’ sorghum.
INCREASING AND TESTING FOR GENERAL ADAPTABILITY
The wide adaptability of the new rice varieties coming from the International
Rice Research Institute is the result. in part. of selecting the highest yielding
segregates grown in different seasons, locations, and fertility‘ levels; and eliminating
the tall, photoperiod-sensitive segregates. There has been a considerable
improvement in yielding ability" of the rice without a marked loss in stability of