LOW NECTAR CONCENTRATIONS IN A DRY ATMOSPHERE - A STUDY OF GREVILLEA-ROBUSTA (PROTEACEAE) AND CALLISTEMON-VIMINALIS (MYRTACEAE)

Factors affecting nectar concentration have been studied for the inflo rescences of two Australian plants, Grevillea robusta (Proteaceae) and Callistemon viminalis (Myrtaceae), growing in Cape Town. Both species produced large volumes of nectar which remained dilute (10-25%) throu ghout the day and in varying ambient humidities, in spite of being exp osed to the atmosphere. Nectar volumes in G. robusta remained constant (mean of 16 mu l per flower) throughout the day. Microclimatic effect s maintaining a low nectar concentration could be ruled out on the bas is of flower morphology. The presence of surface-active material on th e nectar was demonstrated, and the lipid concentration of the nectars of both species was about 2 mu g mu l(-1) No gradients in sugar concen tration could be demonstrated in the nectar drop of G. robusta. Repeat ed sampling from individual flowers of G. robusta showed that nectar w as secreted continuously at about 5.6 mu l h(-1) and at a concentratio n of 16.6%. Secretion apparently ceased when a critical volume was rea ched. Continuous secretion and perhaps also surface-active material se rve to maintain the low concentrations of these exposed nectars.