Vergara - 1976 - Physiological and morphological adaptability of ri

88 CLIMATE AND RICE
spectrum ofecotypes. some of which became adapted to temperate environments
also. The water regimes of the plantis substrate trary‘ from meters of standing
water to aerobic conditions in a mesic soil. The highest yields on fanns are
generally obtained from high-latitude areas such as Stfllmtt/BSMH] Australia,
northern Japan. and Spain. The high yields from such areas in one aspect and
the great diversity existing in the cultivated riccs in another illustrate the
effectiveness of human and natural selection in developing numerous etiltivars,
many of which have become specifically adapted to different physical environmerits.
Among the climatic factors, photoperiod. solar radiation, air and ‘water
temperatures. precipitation. and wind are undoubtedly the principal forces in
forging the eeologie diversity’ found in rice eultivars of the present. This paper
aims to assess differences in varietal responses to the climatic factors. to examine
the evolutionary trend in the development of different ceotypes in relation to the
climatic factor concerned. to review available information on the genetic control
of plant response to variations in a climatic factor. and to re-examine the genetic
aspect of climatic adaptation in relation to maximum grain production.
P1 IOTOPERIOD RESPONSE
Sensitivity to photoperiod is one Of the major factors that restrict the agronomic
usefulness of a day length-sensitive variety at another location which differs in
latitude from the native habitat of the variety. Photopeiiod response affects not
Only the growth duration but also the plant height. tiller number, panicle number
paniele length. and grain size (see Chandraratna, 1964). The strong sensitivity‘.
found in many tropical varieties reduces the suitability‘ of such varieties for
planting in the subtropics and at higher latitudes (Chang, 1961; Parthasarathy.
1964}. Low levels of photoperiod sensitivity are also essential to the wide adaptation
of varieties within a tropical country that covers a span of 1O degrees in
latitude (Dasananda. 1963).
Rice varieties can be broadly classified into photoperiod-sensitive and
photoperiod-insensitive
types (Wada. 1954; Oka, 1954a; Vergara et a1, I965). When
several controlled photopcriods are used in experiments. photoperiod response
can be further resolved into strongly sensitive. weakly sensitive essentially
insensitive, and completely insensitive types (Fig. l) (Chang and Vergara, 1971).
The principal components of the highly variable vegetative growth period from
germination to panicle initiation have been shown to be the basic vegetative
phase (BVP) and the photoperiod-sensitive phase (PSP) (Suenaga. 1936;
Vergara et a1, 1965'. Chang ct a1, 1969). The PSP is in turn determined by
the optimum photoperiod and the critical photoperiod (Li. 1970).
Nearly all wild relatives of the common rice as well as the majority of the
traditional varieties grown in the vvet season of the tropics are strongly or
w=eakly' photoperiod sensitive (Oka, 1954a; Oka and Chang. 1960'. Katayama.
1964: Vergara et a1. 1965). insensitivity; is found in a small proportion of tradi-