Vergara - 1976 - Physiological and morphological adaptability of ri

PHYSlOLt')GItT.=\I. AND l\tlORPHt')LOGICAL ADAPTABILITY 73
elongating at the time of low temperature will be affected; leaves formed subsequently
at normal temperatures would have normal sizes. Low temperatures may
also induce an irreversible strain. and the plant suffers either permanent injury}
or death. Pennanent injury comes in the form of sterile spikelets. while death is
the indirect result of the severe yellowing of the leaves and possibly of the
inability of the roots to take up nutrients.
Some rice varieties are more resistant to low temperatures. and this is reflected
in their higher rate of photosynthesis under low temperature and low light
intensity (Yamada et a1. 1955). The decrease in photosynthetic rate was most
remarkable in indica varieties such as 1R8 and Taichung Native I, while Calrose.
a temperate variety. showed the least effect ofltiv.’ temperature (Tsunoda, 1972).
However. under high light intensity. temperature (l8 to 33.5°C) has almost no
effect on photosynthesis even with different varieties over the whole growing
period (Yamada et al.. 1955; Ormrod, 1961).
Osada (1964). using a different set of varieties, reported that the optimum
temperature range for photosynthesis of indica rice varieties was 25 to 35°C.
which is higher than that of japonica (l8 to 33°C). Although maximum temperatures
in the temperate areas where rice is grown may be as high as in the
tropics. the mean temperature or thc accumulated average temperature for thc
entire growing season is generally ltvw in the temperate areas. hence the need for
varieties tvith lower optimum temperature for photosynthesis.
In studying the riee varieties adapted to various climates and locations. Shen
and Wey (1970) used a multivariate linear regression analysis in which agronomic
characteristics such as growth duration. number of panicles per hill. and
grain yield were used as regressants. while climatic factors such
and hours of sunlight were used as regressors. They found that in general the
indiea rice is more sensitive to climatic changes. especially to temperature and
sunlight.
Tsunoda and Khan (1968) found differences in the photosynthetic tissue of
indica and japonica types. The indica types have sparsely arranged chlorenchyma
cells in the mesophyjll". the adaxial surface of the mesophytll is flatter. The
japonica types have chlorenchyma cells more compactly} arranged and the adaxial
surface of the mesophyll is very wavyt Ilowever, these differences were
found only at the seedling stage and not at later stages of growth. The experiment
was conducted to find an explanation for the higher photosynthetic rates
of japonica types (Tsunoda. 1972). The data. however. can be interpreted differently.
Since light is not limiting at the seedling stage, these differences may
not he an adaptation directly concerned with photosynthesis; rather an adaptation
developed as a result of low temperature. The waw, compact mesophyll
can absorb and conserve heat better during the seedling stage. a very’ important
adaptive character needed by the japoniea types. Strueturally; the japonica
plants are llltffln0dylliilfilfiilllfi" adapted to low temperature at seedling stage.
Low temperature during the reproductive and ripening phase of the rice plant
usually results in high shattering percentage of the grains in several of the high