Yoshida - 1981 - Fundamentals of Rice Crop Science

GROWTH AND DEVELOPMENT OF THE RICE PLANT 41
primordia initiation to heading is slightly affected by variety and weather conditions,
it is considered almost constant, about 30 days under most conditions.
Variations in length of time from germination to heading are attributed to variations
in duration from germination to panicle primordia initiation. Thus, earlymaturing
varieties initiate panicle primordia earlier than late-maturing varieties.
1.8.1. Response to photoperiod
a. Introduction, The length of a day, defined as the interval between sunrise and
sunset, is known as photoperiod. The response of the plant to photoperiod is called
photoperiodism.
Rice is basically a short-day plant. It initiates panicle primordia in response to
short photoperiods. Panicle primordia may be initiated late or they may not
develop when the plant is subjected to long photoperiods. Strictly speaking,
however, the rice plant responds to nyctoperiods or the length of night. Under
natural conditions, a short day implies a long night because a day cycles every 24
hours. Thus, the term short day can still be used without any practical confusion.
Rice is now being grown in northeastern China at 53°N, in Central Sumatra on
the equator, and in New South Wales, Australia, at 35°S. Day length varies widely
with latitude and with season for a given latitude (Fig. 1.38). At a latitude of 44°N,
which is close to the practical northern limit for rice cultivation in Japan, day
length varies from 9 hours to 15 hours 30 minutes. At the equator, however, it
varies from only 12 hours 6 minutes to 12 hours 8 minutes, the difference being
only 2 minutes.
At the early stages of research on photoperiodism, the small seasonal differences
in day length in the tropics were thought to be unimportant in controlling
plant behavior. Later work, however, demonstrated that tropical plants might be
more sensitive than temperate plants to small differences in day length (Chang,
J.H. 1968). Rice provides a good example. In Malacca, Malaysia, at a latitude of
2°N, the difference between the maximum and the minimum day lengths is only 14
minutes: The variety Siam 29 differentiates this small difference in day length: it
takes 329 days to flower when planted in January and only 161 days when planted
in September (Dore 1959). Historically, the northerly movement of rice cultivation
in the temperate regions has been made possible by selecting varieties that are
early maturing, more tolerant of low temperatures, and less sensitive to day length.
Light as low as several to 100 lx may delay initiation of flower primordia.
Electric lights installed near the fields may delay flowering. There is some
evidence that civil twilight in the morning can delay flowering, while that in the
evening may or may not. Civil twilight is defined as the interval between sunrise or
sunset and the time when the true position of the center of the sun is 6° below the
horizon. At this time stars and planets of the first magnitude are just visible and
darkness forces the suspension of normal outdoor activities. The duration of civil
twilight varies with locality and time of year; it is about 21-32 minutes long at
latitudes from 0° to 40°N (Smithsonian Meteorological Tables 1963). The intensity
of light during this time ranges between about 10 and 4 lx.