SUPPLEMENTARY LIGHTING OF YEAR-ROUND CHRYSANTHEMUMS

In preliminary experiments, plants of the chrysanthemum cultivar 'Bright Golden Anne', obtained from Messrs. Framptons Nurseries Ltd., were grown in controlled environment cabinets lit by 'warm white' fluorescent lamps following a normal schedule for the production of pot plants in winter. The plants were grown singly in 10 cm square plastic pots in a soilless rooting medium of vermiculite/sand/Cornish river grit (2:1:2 by volume) which was flooded daily by subirrigation with a complete nutrient solution. Within each cabinet the day temperature was 18.3°C, the night temperature 15.6°C and the carbon dioxide concentration 325 ppm. The main light period lasted 8 hours and for the first six days of growth the 16 hour dark period was interrupted about its mid point by a 5 hour night break given with incandescent lamps. This treatment inhibits flower initiation in the chrysanthemums. After 6 days the plants received uninterrupted nights of 16 hours duration (short days), a treatment which favours flower initiation and development. After one week of this shortday treatment the terminal growing point was removed from the main axis to encourage the production of lateral branches ('pinching') and after a further three weeks of growth, weak lateral branches were removed. All the axillary flower buds were removed as they developed, to leave a single terminal flower on each lateral branch.

In the first experiment we examined the effects on flowering of various constant light levels given throughout the period of growth. The four levels we chose (table 1) covered the range of daily totals of visible radiation received in glasshouses in the South of England during the winter period from September to April.

Flower development was assessed against the scale of developmental stages outlined in table 2 (Cockshull and Hughes, 1967), and the results from this experiment (figure 1) showed that flower initiation and development proceeded normally at the two higher light levels of 60 and 30 cal/cm2/day and the plants were in flower (stage 8) after 10 weeks of shortday treatment, whereas flower development at the other two light levels was delayed. The plants grown at 15 cal/cm2/day, for example, reached our flower stage 8 after 13 weeks of short-day treatment.

In the next experiment, plants were transferred to the 'very low light level' for short periods at different stages in their development. We adopted a light régime of 30 ca1/cm2/day with a carbon dioxide level of 900 ppm CO₂ as our 'standard' and sample plants were transferred from this to a 'very low light' régime of 7.5 ca1/cm2/day also at 900 ppm CO₂ at weekly intervals. The plants remained in the transfer conditions