Vergara - 1976 - Physiological and morphological adaptability of ri

GENETIC VARIOIISNESS 1N ("LINIATTF ADAPTATION 99
In broad terms. plant responses to different water regimes may be grouped
under: (1) tolerance to rapidly rising water (submergence), (2) steadily rising and
deep water (floating ability), and (3) resistance to drought.
Genetic information on floating ability is limited to a few" studies made in
India. Japan. and Thailand. Ramiah and REIHHSWHIDI (1941) used the prostrate
growth habit as an indication of floating ability and postulated that it was
controlled by two duplicate genes ofa reeessibe nature (div, and dw, ). One of the
genes appeared to be linked with a dominant gene for late maturity’. Workers in
Japan suggested that the floating habit might be investigated on the basis of
three components: (1) rate of intemode elongation under normal conditions,
(2) capacity to elongate in response to rise in water. and (3) the maximum rate of
elongation (Kihara ct a1, 1962). Studies made in Thailand indicate that internode
elongation in cultivars under continuously rising water appears to be a
recessive trait controlled by several genes, but elongation ability conferred by a
wild strain appeared to be a dominant trait (B.R. Jackson. personal commun.).
Morishima et al. (1962) pointed out that while cultivars of O. sativa differ
markedly in floating ability, the Asian and American wild relatives generally
have moderately strong to strong floating ability’. Floating ability is therefore
a primitive trait which enables the wild and semiwild taxa to adapt themselves
to haphazard V9316!’ regimes.
For all types of rainfed rice culture, resistance to drought is essential. Drought
resistance is a complex trait and includes escape, avoidance, and tolerance inechanisms
(Sullivan et al., 1971; Levitt. 1972). Limited studies have been made on
resistance to and recovery from desiccation (Oka and Ru, 1957; Morishima et
a1. 1962'. Chang et a1, 1972), avoidance in terms of leaf rolling and root growth
(Chang et al, I972. I974; IRRI, 1974), and stomatal and cuticular resistance
(Chen, 1973', IRRI, I973, I974), tolerance to desiccation (IRRI, I974), and
heat tolerance (IRRI. I973). Workers in Japan and Taiu/an found that the
indica varieties were generally more resistant to extreme stress than the japonica
varieties and that the Asian wild relative excelled the Asian cultivars. In a cross
between a weed race and a japonica variety, resistance to desiccation in pot
experiments appeared to be a quantitative trait, Wlllll resistance being dominant
over susceptibility’ (Oka and Ru, 1957; Morishima cl a1, 1962). In the extensive
field screening program at IRRI. resistance to drought in the field and recovery
from desiccation in pots were not positively correlated (Chang et al, I972).
Outstanding sources of field resistance were identified from a small proportion
of the traditional upland varieties or hill rices and these came from both the
indica and japonica groups. Moderately resistant genotypes ‘were frequently
found among varieties adapted to the rainfed-lowland culture or from dualpurpose
varieties which are grown under both rainfed-lowland and upland cultures
(Chang et a1. 1973; IRRI. 1975). Resistant progenies were obtained only
from those crosses which involved one or more resistant parents having tall to
moderately tall plant stature. plasticity in leaf rolling and unfolding, and a deep
root system containing many thick roots (Chang and Loresto. unpublished).