[The effects of night breaks on flowering of sinapis alba L]

The induction of flowering in mustard (Sinapis alba L.) was studied by means of night-breaks ("Störlicht"). The plants were cultivated under fully controlled conditions: 8000 Lux white light (mixed fluorescent and incandescent) 18°C, 80% relative humidity. Raised under our conditions in short days (8 hours of white light) mustard behaved as a quantitative long-day plant (Fig. 2). Flowering can be promoted by long-day treatment (Fig. 3). The long day (16 hours of white light) can be replaced by a short day plus a night-break. The highest effectiveness of the night-break is found near the middle of the dark period (Figs. 4, 5). -The spectral dependence of flower induction was studied with blue, green, yellow, red (Fig. 1) and far-red light using a 2-hour break near the middle of the dark period. The dose response curves (Fig. 6) and the action spectrum (Fig. 7) indicate a very strong effectiveness in the blue part of the spectrum, a small response in red and yellow light and no response at all in green and far-red light. The participation of phytochrome is indicated (Table 1), but no far-red reversibility could be detected (Table 2). Simultaneous irradiation with red and far-red light yielded significant enhancement effects (Fig. 8). In view of the strong shadowing in the leaves (Figs. 9, 10) these data are interpretable on the basis of phytochrome.