Vergara - 1976 - Physiological and morphological adaptability of ri

202 crux-tars AND RICE
At such low mean temperatures. rice plants in the field may have many chances
of chill injury and frost, because of daily variation in temperature and rapid fall
of night temperature. This seems to explain why the optimum temperature for
ripening is higher in the field than in the pliyttatrtin.
The time course 0f optimum temperature can be obtained from the temperature—ripening
grade curves in each stage (Fig. 10). With 6 hours of sunshine.
the optimum temperatures are 230° at 1-20], 26.0°C at heading. and 225°C
at [ +20] stage.
Relationships between light and ripening grade
The effect of light on ripening grade may vary depending upon stage and
temperature. Figure 11 indicates that the effect of sunshine hours is positive
under high temperature conditions. irrespective of stage, while negligible or even
negative under low temperature conditions, especially at [+ 10] stage. Wada et
a1. (1972) reported that the effect of light intensity on the ripening grade (percentage
of sterile grains) W35 negligible at 18°C during the meiosis stage. Similar
results were obtained by Kiyosawa (1962). However. there is no report about
the negative effect of light on ripening under low temperature. Negative effect of
sunshine hours under low temperature conditions may be attributable to low
night temperatures associated with long sunshine hours. The correlation between
sunshine hours and diurnal temperature was 0.74 (p < 0.1%) in Kitami in the
cool region.
Munakata (1968) ObSBTVCd that ripening grade of the rice plant tvas decreased
by strong solar radiation (over 450 cal -em-= - day-l at high temperature above
27°C). Such a tendency was scarcely found in these statistical data.
Estimation 0f ripening grade
The accuracy of estimation of ripening grade was considered to be increased
by the equation which included climatic data at three stages. i.e. at [-20]
(spikelet differentiation stage), [0] (heading stage) and [— 20] (ripening stage).
The model used was the type IV modified eq. (2) (type III, including six terms).
In this model, five terms were used for each stage and the total was 15, because
of limitation of memory size in our mid-computer. The regression formula of
ripening grade was as follows:
(I/Yaa: = _ — 4.33 - 101. T,"~-‘-.S';= + 2.34-10-1-T,'1 - S,"
+ 6.97-101-T,°-S‘;1 + 1.80 - 10" T5“ S,“
+ 1.33-10**T;*-‘ S,“ + 1.75-101T+* S‘
— 1.30 - 10 ' TJS,“ - 1.84- 10 7982-1 — 6.08 - 10-’ T,“ 5'9“
— 1.88-10‘7',-’.S',°-“ + 1.47 ~ 101 7'," 5}“
— 4.27 - 10’ T98," + 2.34 - 10 =T;' S," + 1.8716 (3)
where T,
and
I}; ripening grade (Ti/Y; -mg).