Nectar accumulation in flowers of Fireweed, Epilobium Angustifolium (Onagraceae), in response to simulated defoliation

The amount of nectar accumulating in inflorescences of defoliated and intact shoots of fireweed from which pollinators were excluded for a 14- to 16-h period was assayed by timing the visits of bees and wording the numbers of flowers visited. The day following a defoliation of c. 80% of leaf area, there was a 42% reduction in the mean time spent per flower by bees, relative to undamaged controls. No further reductions were evident, 2, 3 or 5 days postdefoliation; defoliated shoots maintained a nectar secretion rate of c. 58% that of controls. Although under normal circumstances most photosynthate for nectar secretion is provided by adjacent leaves, these results suggest that the sink effect of open flowers is sufficient to mobilize the translocation of photosynthate from distant plant parts. Defoliated plants did not significantly reduce the numbers of flowers open until day 3 postdefoliation, and even then, there were no differences in the numbers of flowers visited by bees. Parallel fluctuations in flower quality appeared in both control and defoliated inflorescences across the experimental timeframe and presumably reflect variation in environmental factors that influenced the rate of nectar production and/or secretion.