Vergara - 1976 - Physiological and morphological adaptability of ri

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crux-LATE AND RICE
Although the rice plant evolved from a semi-aquatic progenitor, the range in
field resistance to drought found in rice eultivars is surprisingly large and promising
for genetic improvement (Chang et al.. I972. 1974).
Reeoverj; after the termination of drought is probably more important to
grain production than resistance or tolerance. The lowland types and semidvvarfs
have better recovery ability than the traditional upland varieties (Chang
et a1. 1972. 1974). We need to combine both drought resistance and recovery’
capacity in eultivars intended for rainfed culture.
In Japanese varieties. resistance to drought and tolerance to low temperatures
appeared to be correlated in the seedling stage (Hasegativa, 1963). At IRRI the
traditional upland varieties of the tropics showed slightly higher levels of tolerance
to low water temperatures in the seedling stage than IR8 (IRRI. 19B). In
upland varieties of the tropics. field resistance to drought and heat tolerance of
the leaves were found t0 be associated in 12 varieties (IRRI. 1973). Floating
ability’ did not appear to be associated with resistance to desiccation (Morishima
et al., I962). Findings from field screening at IRR1 indicate that the floating
varieties of tropical Asia have only moderate levels of field resistance to drought.
Similarly. strains of O. glaberrilzia did not have the same level of resistance as
found in the resistant O. sariva varieties (Chang, and Loresto, unpublished).
ADAPTATION AND YIELD STABILITY
The general adaptive range of crop varieties can be evaluated on two bases.
mean yield and yield stability. when they are tested under a large number of
environments (Finlay and Wilkinson, 1963). The yield stability of a variety can
be measured by the standard deviation of its yields due to environments or by
the regression of its yields on environmental means. It is an overall estimate of
responses to numerous and unspecified environmental conditions.
Extensive variety testing in widely distributed rice-growing areas has established
the wide adaptive range of Taichung Native 1. 1R8. and several ponlai
varieties for year-round cultivation in the tropics (Chang. 1967a). In recent
adaptation trials. with two fertilizer levels. sponsored by the IBP/UM at eight
sites in tropical Africa and Asia. 1R8 led a group of 22 varieties in both _vielding
performance and region-wide stability’. Tainan 3. a ponlai variety. produced top
yields at stable levels in three series of trials at 13 sites. ranging from central
Japan to central Africa (Evelyn et a1. 1971). 1R8 has also produced top yiields
in many countries spanning from South Korea (37°N) to southern Brazil (32°S)
(Chang and Vergara. 1972). The photoperiodinsensitivity of 1R8. TNl. and
Tainan 3 is a major contributory factor to their yield stability‘ (Oka, 1966;
Chang. 1967a). Other attributes could be a moderately long BVP, moderately’
high tillering ability. low response to variations in temperature. and high panicle
fertility:
Analysis of the data from the 3-year Inteniational Rice Adaptation Experiments
of lBP/UM has fully" demonstrated that yield stability is a genotyipieallv