Observations on nectar secretion in fireweed, Epilobium angustifolium L. (Onagraceae)
J. P. Michaud
Capillary rubes were used t0 extract nectar accumulating in protected flowers of fireweed at various sites in coastal British Columbia. Mean sugar yields collected from individual flowers ranged from 0.66 mg/flower/day to 4-11 mg/flower/day. Sixty four percent of me variation in daily yield was shared by different sites, suggesting that weather factors were of primary importance to nectar secretion patterns. Sunshine and mean temperature demonstrated the best correlations with nectar yield, explaining 62% and 63% of the variation, respectively. Flowers on plants defoliated 900.4 secreted an average of 68% of the sugar secreted by control flowers over a 3 day period, suggesting that scored photosynthate was mobilized for secretion as nectar. Measurements of nectar secretion in a controlled environment under conditions of saturated photosynthesis revealed that variations in temperature alone could influence yield significantly. Mean yields of sugar collected under conditions of constant temperature ranged from S·2 mg/flower at 14° to 8·7 mg/flower at 24°C. Further increases in temperature depressed yield. The thermal threshold for nectar secretion was found [0 be dependent on the intensity of illumination and ranged from < 12°C at high intensity illumination to c. 19°C at low light intensity. Flower longevity ranged from 2-3 days at 32°C to 5-6 days at 14°C.