Vergara - 1976 - Physiological and morphological adaptability of ri

GENTETTIZT \".=\RIOLTS.\IFSS IN CLIlvLATIC ADAPTUKTTON 89
Daylluhnntinq
1. Four types of photoperiod response in rice cultivars.
tional varieties grown in the dry season or the summer season (aus varieties of
the Indian subcontinent). the bulu varieties of Indonesia, many upland varieties,
and more recently in the high-yielding ponlai and semidwart‘ varieties (Matsuo,
1952; Oka. 1954a; Vergara et al. I965; Chang and Vergara. 197]).
Among both wild and cultivated sensitive strains, the requirement for a
critical day length shows a latitudinal clinc (Hara. 1930'. Uka. 1954a; Oka and
Chang. 1960). When one scans rice strains from the northern temperate region
southvsard, the proportion of sensitive varieties increases as the latitude decreases.
The critical day length also becomes shorter in this direction. In insensilive
genotypes, the vegetative growth period becomes progressively longer
toward the equator (Oka. 1958a). The finding of such latitudinal clines indicates
that growth duration is an important feature of climatic adaptation and is subject
to strong selection pressure.
Among tropical and subtropical cultivars. the length of the BVP appears to be
inversely proportional to the length of the PSP (IRRI. [Undatedjzu Chang et al..
1969'. Lin. F. I—l.. 1972). Such a relationship is not so obvious in Japanese and
Korean varieties. however (Wada, 1952, 1954; Toriyama et aL. 1969).
In photoperiod-sensitive varieties, the vegetative-lag phase or PSP accounts
for much of the late maturity. While the lag phase contributes to a taller plant
stature and a lower nitrogen response, it also serves as a safety’ mechanism when
the plants are grown in an unfavorable environment under a rainfed culture
without water control. The protracted vegetative growth enables the plants to