Vergara - 1976 - Physiological and morphological adaptability of ri

GENETIC VARIOUSNESS IN CLIMATTC ADAPTr-‘XTTON 97
ratio of carbohydrates translocated from the vegetative parts to the grain during
the second crop (Lin. TF2 1972). These features along with slow leaf senescence
may explain the popularity of Tainan 5 in the sectind crop season of Taiwan
(Huang et a1. 1972).
However, the higher photosynthetic capacity and translocation ratio indicated
by Tainan 5 in the second crop season did not lead to a yield higher than
that ofthe first season (Lin_ T.F.. 1972'. Chen, 1973).
The identification of cultivars with high photosynthetic capacity under the
low radiation intensity of the monsoon crop season is worthy’ of further investigation.
Preliminary investigations indicate that rice eultivars differ in the gross
photosynthesis rate at different light and temperature levels and that broad
sense heritability estimates range from 55 to 75 percent (McDonald. 1971).
Leaf thickness and chlorophyll content of leaves have been mentioned as
varietal characteristics of probable association with photosynthetic efficiency:
(Paulsen et a1., I972; Tsunoda. 1972). This is another area where further research
is needed.
In the course of domestication. the rice cultivars acquired faster leaf growth.
larger leaves. and more leaves than their ‘wild progenitor (Chang, 1976)‘ HOWever,
the modem eultivars have not shown significant improvement in photosynthetic
efficiency" over the wild taxa (IRRI, 1973). On the other hand, it is
interesting to note that during the vegetative phase. a late maturing variety has
a higher photosynthetic rate than an early one. With added nitrogen. a nitrogenresponsive
variety has a higher rate than a non-responsive one (Osada. 1964).
This may explain why a leafy and long-duration variety’ such as Raminad Strain
3 is highly competitive with weeds and tolerant of many adverse conditions and
yet it is most unresponsive to nitrogen fertilization and iveed control (Vergara
et a1, 1967).
PRECIPITATION AND WIND
For rice eultixtars ripening under the monsoon weather. grain dormancy is
essential. Most tropical varieties TIZIXIB moderately strong to strong levels of
donnancy’, while varieties of temperate areas have “teak or no dormancy (Oka
and Tsai, 1955; Jennings and dc Jesus, 1964). A number of early maturing
tropical varieties lack dtirmancyg however (Nair and Pillai. 1964; Chang and
Tagumpay‘, 1973). The wild relatives have an extremely strong level and a long
period of grain dormancy (Oka and Chang, 1962). Frequent ShOWCfS during the
grain-ripening period enhance the expression of dormancy (Ghosh, 1962)‘
especially in weakly} dormant varieties such as 1R8 (Chang and Yen, 1969).
Studies at lRRl have shown that all three levels of grain donnancy—strong.
moderately strong, and wcak—arc controlled by several anisomcric genes having
dominance. The strength of dormancy’ is largely indicated by the rate at which
dormancy disappears after harvest. The identification of two or more germination-inhibitors
in the hulls of donnant eultivars (Takahashi. 1968; Hayashi and