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  • Author or Editor: Will Healy x
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Petunia × hybrida Villm. `Red Flash' plants received either 10 or 20 mol·day-1 photosynthetic photon flux (PPF) in growth chambers at: 175 μmol·m-2·s-1 for 16 hours, 350 μmol·m-2·s-1 for 8 or 16 hours, or 350 μmol·m-2 s-1 for 8 hours plus 8 hours of incandescent photoperiod extension (5 μmol·m-2·s-1 PPF). The irradiation components of peak, total, and duration were examined. Doubling total PPF increased total carbohydrate (CHO) production by 60%, seedling dry weight (DW) by 30%, rate of seedling growth by 25%, and acid invertase activity by 50% compared to the other treatments, once the seedlings had reached the two-leaf stage. Seedlings receiving 20 mol·day-1 PPF partitioned 14% more CHO into ethanol soluble sugars rather than starch, which may explain the increase in relative growth rate observed with supplemental irradiance treatments. Extending the photoperiod for 8 hours with 5 μmol·m-2·s-1 PPF reduced total CHO production by 50% compared to the same treatment without photoperiodic lighting. Treatment with 350 μmol·m-2·s-1 for 8 hours resulted in the highest O2 evolution (8.8 μmol O2/min per dm2). Increasing the photoperiod from 8 to 16 hours gave the lowest rate of O2 evolution (4.5 μmol O2/min per dm2). Previous reports of the importance of photosynthetic period in controlling partitioning between starch and sugars may have simply observed a decreasing rate of starch accumulation due to increased total PPF.

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Stock plants of Aeschynanthus `Koral' were grown with irradiances of 120 or 240 μmol·s-1·m-2 at 18/17, 24/17, or 30/17C (day/night) under 12-hour thermo- and photoperiods. Tip cuttings from stock plants grown at 18/17C flowered earlier than those from stock plants grown at 24/17 or 30/17C when cuttings were forced in a glasshouse under natural days (23/18 C). No cuttings from stock plants grown at 30/17C reached the visible bud stage after 86 days, while 93% of the cuttings forced at 18/17C did reach the visible bud stage. A. `Koral' plants were grown at 18, 24, or 30C in a factorial combination of temperatures at 12-hour thermo- and photoperiods (100 μmol·s-1·m-2). After 8 weeks, only plants grown at 18/18C had visible buds. After 18 weeks, plants grown at 24/24 or 24/18C had visible buds after having unfolded =2.5 times as many leaves as plants grown at 18/18C. Rapid flowering of A. `Koral' is promoted by constant 18C under a 12-hour photoperiod.

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