Yoshida - 1981 - Fundamentals of Rice Crop Science

GROWTH AND DEVELOPMENT OF THE RICE PLANT 43
Optimum photoperiod and critical photoperiod are terms commonly used to
characterize the PSP. Optimum photoperiod is the day length at which the duration
from sowing to flowering is at a minimum. The optimum photoperiod of most
varieties is about 9–10 hours. A longer or shorter photoperiod delays flowering,
the delay depending upon the sensitivity of the variety. Critical photoperiod is the
longest photoperiod at which the plant will flower or the photoperiod beyond
which it cannot flower. The critical photoperiod of most varieties ranges from 12 to
14 hours.
The length of the PSP is usually determined by subtracting the shortest growth
duration (growth duration at the optimum photoperiod) from the longest growth
duration (growth duration at the critical photoperiod) of the variety. Since many
varieties remain vegetative for a long time if grown under long-day conditions,
experiments are usually terminated after 200 days, and the PSP of the variety is
given the value of 200 + . Under continually long photoperiods, some varieties have
been reported to remain vegetative even after 12 years of growth.
The length of the PSP is a measure of the variety’s sensitivity to photoperiod.
The shorter the critical photoperiod, the longer the PSP. A short optimum photoperiod
is also associated with long PSP (Vergara and Chang 1976).
d. Classification of rice varieties by photoperiodic reactions. Agronomic terms
such as early, medium, late, aus, or aman are useful only in particular regions
Table 1.11. Photoperiodic reaction of some selected rice varieties and breeding
lines. a
Days to flowering at
Sensitivity photoperiod b of
to Variety PSP BVP Date of
photoperiod 10 12 14 16 (days) (days) sowing c
Insensitive lR12-178 89 89 d 91 2 54 Dec. 1968
Fujisaka 5 67 64 71
d 7 29 July 1962
Norin 20 49 48 48 59 11 13
IR8 101 100 109 112 12 65 March 1970
Tainan 3 89 89 97 114 25 54 Jan. 1964
Weakly IR22 69 73 101 102 33 34 March 1970
sensitive IR20 76 76 103 112 36 41 March 1970
IR5 93 101 130 134 41 58 March 1970
C4-63 72 81 110 123 51 37 July 1967
Peta 70 85 125 145 75 35 May 1963
Strongly Siam 29 61 112 e e 200 + 26 Jan. 1964
sensitive Latisail 55 84
e e 200 + 20 March 1972
Podiwi A-8 76 136 e d 200 + 41 July 1962
Puang Nahk 16 68 90 e e 200 + 33 May 1963
Raminad St. 3 69 121
e e 200 + 34 May 1964
a Adapted from Vergara and Chang (1976). b The plants received 10 hours of natural light
from 0700 to 1700 hours. The light intensity in the darkrooms was 400 Ix and the
temperature, 20°C. c Date of test. Variations in growth duration can occur because of
differences in greenhouse temperature during growth. d Not tested. e No panicle initiation
after 200 days of growth.