The effect of irradiance and temperature on the growth and flowering of Sinningia cardinalis

• Flowering was the earliest plants were grown under a continuous 30% irradiance reduction (SH).
• The number of flowers was the highest under 30% SH during the first 40 days.
• Flowering was delayed as day temperatures were increased from 18° to 30 °C.
• Sinningia cardinalis can be considered as a day-neutral plant for flower bud initiation.

The effects of irradiance and temperature on growth and flowering in seed propagated Sinningia cardinalis especially to increase the number of flowers were investigated. To understand the effect of irradiance on flowering and the number of flowers, seedlings were grown under 30% shade (30% SH), natural irradiance (ND), and ND supplemented with 14 h of high irradiance at 14 W/m2 (ND + 14HID) at 3 stages (ST) lasted for 40 days (d) at each ST (Expt. 1). Flowering was the earliest (205 d) when 20-d old plants were grown under 30% SH during the entire three stages (July 1 through October 31). Longer than 215 d to flower was required when plants received ND + 14HID during stage 3 (ST 3) (September 21–October 31). The number of flowers was highest (22) when grown under 30% SH during ST 1 (July 1–August 10) followed by ND or ND + 14HID during the ST 2 (August 12–September 20) or ST 3. To investigate the effect of temperature on flowering and the number of flowers, 42 d of 15/13 °C (15 °C), 19/17 °C (19 °C), 23/21 °C (23 °C), and 27/25 °C (27 °C) and under ND + 10HDI were given to 45-d old plants (Expt. 2). The number of days to flower was only significantly accelerated at 23° and 27 °C as compared to 15 °C and 19 °C, but not by irradiance treatments. The highest number of flowers (24) was produced when plants were grown at 15 °C under ND + 10HID given prior to formation of three pairs of leaves (<3L). To further refine the level of reduced of light irradiance on the number of flowers, plants were grown under 45% shade (45% SH), 30% shade (30% SH) from 08:00 to 20:00 h, and ND were given to 128-d old plants between days 0 and 20 (0 and 20), 21 and 40, and 0 and 40 (Expt. 3). The number of flowers (17.6) was significantly increased when grown under 45% SH, especially during the 21–40 d period. To investigate the effect of temperature on plants at visible buds stages, plants were transferred to growth chambers maintained at 18, 21, 24, 27, and 30 °C during the day (0800–1600 h; 35 W/m2) and at 16 °C at night for 42 d (Expt. 4). Flowering was delayed from 100 to 115 d as day temperatures were increased from 18° to 30 °C, and the number of flowers was the highest number (26.5 flowers) at 24 °C. Therefore, to promote flowering and to increase the flower numbers, S. cardinalis should be grown under 30% SH or under ND photoperiod during May–October. At a later stage of flower bud development, temperature rather than irradiance controls the speed of flowering. The differences in flowering time for only 16 d from sowing to flowering may suggest that S. cardinalis can be considered as a day-neutral plant for flower bud initiation.