DIRECT AND INDIRECT CALORIMETRY OF THERMOGENIC FLOWERS OF THE SACRED LOTUS, NELUMBO-NUCIFERA

Direct and indirect calorimetric experiments were performed on flowers of the sacred lotus, Nelumbo nucifera, and compared with temperature measurements. To this end, a simple, light and cheap heat-flow calorim eter of the twin type was developed to monitor the heat output of lotu s flowers in an outdoor pond. Each side of the calorimeter consisted o f a water jacket as a heat sink surrounding a 730 ml concentric can as a calorimetric vessel. The vessel and heat sink were thermally connec ted via a Peltier element but otherwise thermally isolated. Both water jackets were housed in a styrofoam box and connected in parallel to a thermostated water bath. The calorimeter exhibited a mean sensitivity of 25.8 mV W-1, a time constant of 8 min and a 24 h baseline stabilit y better than 18 of the chosen range. This differential calorimeter wa s placed around lotus flowers approximate to 1 m above the water level . Direct calorimetry was accompanied with indirect calorimetry by meas uring oxygen consumption rates of the flowers with open-how respiromet ry, and the patterns of temperature change were recorded with thermoco uples. Flowers maintained mean temperatures of ca. 30.7 degrees and 34 .2 degrees C at mean calorimeter temperatures of 18.4 degrees and 30.4 degrees C, respectively, demonstrating good thermoregulatory ability. Metabolic heat production averaged ca. 0.51 W at the low temperature and 0.25 W at the high temperature. Dry heat loss to the calorimeter a veraged -0.62 W and -0.17 W, respectively, which indicated that there was a small condensation of atmospheric water vapor inside the calorim eter at the low temperature, but net evaporation from the flower at a level of ca. 33% of heat production occurred at the high temperature. In a set of laboratory experiments on cut lotus flowers, a heat-flux b udget was constructed from measurements of heat production (open-Row r espirometry), heat loss (gradient-layer calorimeter of the Benzinger/K itzinger type), and evaporative heat loss (gravimetric). Heat producti on rate was ca. 0.3 W and was balanced almost completely by evaporativ e heat loss into the calorimeter air (25 degrees C; 37% relative humid ity). Therefore, total heat Bur by convection, conduction and radiatio n was essentially zero, despite the flower's heat-producing receptacle prevailing ca. 5 degrees C higher than the calorimeter air. Heat from the receptacle was apparently transferred to the petals which, in tur n, lost it mainly through evaporation. Equivalence of direct and indir ect calorimetry substantiated the assumed caloric equivalent of oxygen consumption of 21.1 J ml(-1) and indicated that there was no conserva tion of energy in metabolic processes during thermogenesis. (C) 1998 E lsevier Science B.V.