Photoperiodism was studied in 417 rice varieties of which 28, selected by differences in photoperiodic response, were studied in more detail. The plants were usually grown under 16 different photoperiods ranging from 5 to 24 h. In detailed experiments the types of response to short and long days were studied and each developmental stage of the plant from germination until grain maturation was examined separately. Particular attention was given to the juvenile phase, photoperiodic induction, reproductive development, varietal classification, and the construction of response curves. Response was plotted as time from sowing to floral initiation on the ordinate against photoperiod used on the abscissa. These curves gave much information on photoperiodic response of the varieties used.Much detailed work was devoted to specific light effects such as: the perception of the photoperiod, the effects of alternating, increasing and decreasing photoperiods, light intensities, wave-length dependence, interrupted light and dark periods. The experimental data were used to interpret the photoperiodic response of rice plants grown in natural daylengths. Seasonal changes in cloudiness (monsoons) shortened twilight and daylength, thereby markedly affecting the time of flowering in very sensitive varieties.
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