Yoshida - 1981 - Fundamentals of Rice Crop Science

CLIMATIC ENVIRONMENT AND ITS INFLUENCE 77
ment. Leaf elongation and plant height growth, however, are affected by both air
and water temperatures, presumably because they are completed in an aerial
environment.
As the growing panicles reach above the water surface around reduction division
stage and, thereafter, the effects of water temperature decrease; eventually air
temperature becomes dominant in controlling panicle growth and ripening
(Tsunoda and Matsushima 1962, Matsushima et al 1964c). Thus, the effects of air
and water temperatures on grain yield and yield components vary with growth
stage (Matsushima et al 1964b). At early growth stages, the water temperature
affects yield by affecting the panicle number per plant, spikelet number per
panicle, and the percentage of ripened grains. At later stages air temperatures
affect yield by affecting the percentages of unfertilized spikelets and percentages
of ripened grains.
The effects of water temperature are determined by both the magnitude of
temperature and water depth. Under most conditions, water temperature is higher
than air temperature, and increasing the water depth extends the duration during
which water temperature controls panicle growth. Thus, when air temperature
goes down below the critical level, increasing the water depth about 15–20 cm at
the reduction division stage is an efficient method of protecting the rice plant
against sterility caused by low air temperature (Nishiyama et al 1969).
2.3.5. Low-temperature stress
a. Occurrence of low-temperature problems. Depending on growth stages, injury
to rice may occur when the daily mean temperature drops below 20°C. Cool injury
can occur not only in the temperate regions but also at high altitudes and in dry
season crops in the tropics. Countries reporting cool injury on rice include
Australia, Bangladesh, China, Colombia, Cuba, India, Indonesia. Iran, Japan,
Korea, Nepal, Pakistan, Peru, Sri Lanka, USA, and USSR.
b. Types of cool injury. Common cool injuries are failure to germinate, delayed
seedling emergence, stunting, leaf discoloration, panicle tip degeneration, incomplete
panicle exsertion, delayed flowering, high spikelet sterility, and irregular
maturity. Among them, high spikelet sterility, delayed heading, and irregular
maturity are common in many countries.
c. Stunting at seedling stage. Stunting, a reduction in plant height, is a common
symptom of cool injury in seedlings and is highly correlated with weight growth of
both shoot and root. In flooded fields, stunting is caused by cold water brought
about by either cool weather conditions or cold reservoirs. Tolerance for cold
water is particularly important in areas such as California, USA, where rice seeds
are broadcast into about 15 cm of cold standing water.
A wide range of varietal differences is found in cold water tolerance at the
seedling stage. When shoot length and percentage of survival were taken as
measures for cold tolerance and when those of a U.S. variety, Caloro, were taken
as 100, the relative performance of 19 U.S. varieties ranged from 35 to 110 for
shoot length and from 26 to 110 for percentage of survival (Table 2.6). A further