Pattern of respiration by intact inflorescences of the thermogenic arum lily Philodendron selloum

Abstract

odours that are attractive to insect pollen vectors (Meeuse and Raskin, 1988). Some species are able to raise inflor- Inflorescences of the neotropical arum lily, escence temperature as much as 35°C above the temper- Philodendron selloum, are strongly thermogenic for ature of the air, and consume oxygen at prodigious rates 2 d during anthesis. Continuous measurements of (Seymour, 1997; Seymour and Schultze-Motel, 1997). A spadix temperature (T s ) and rate of oxygen consump- few species are extraordinary because they regulate tion (VO 2 ) were made outdoors in whole inflorescences inflorescence temperature by varying the rate of heat attached to the plants. Some inflorescences were production inversely with ambient air temperature. exposed to uncontrolled ambient temperature (T a ) The first documented thermoregulating plant was the while others were enclosed in clear water-jackets arum lily, Philodendron selloum, a native of Brazil (Nagy that produced nearly constant ambient conditions. et al., 1972). This study involved measuring temperature A repeatable, diphasic pattern of heat production and rate of oxygen consumption of the spadix after it appeared, most clearly in water-jacketed inflores- had been cut from the plant and placed into a cabinet at cences, and it comprised a short peak phase at sunset selected ambient temperatures. Maximum spadix temper- followed by a plateau phase that lasted until the follow- atures varied little (38-46°C ) within a broad ambient ing sunset. Regulation of T s occurred in both phases, temperature range (4-39°C ). However, each severed but at different levels. Peak phase T s was regulated in spadix produced only one intense thermogenic episode the region of 38-42?C, but plateau phase T s was usu- that ended in less than 2 h and never achieved steady- ally in the range of 25-36?C. Both VO 2 and total heat state. A subsequent study showed that thermogenesis by produced throughout anthesis increased at lower T a . intact (uncut) inflorescences lasted almost 2 d and was The data imply that the short peak phase is related to characterized by two episodes of heating—the first pro- the enhancement of odour production that attracts a duced a large temperature elevation that was associated single species of large scarabaeid beetle in its native with receptivity of female (pistillate) florets at the base Brazil, and regulation of maximum T s may prevent over- of the spadix, and the second produced a smaller elevation heating. Thermoregulation in the long plateau phase that occurred before pollen was shed from male (stamin- produces equable temperatures inside the inflores- ate) florets at the top (Seymour et al., 1983). However, cence that may facilitate the resident beetles' activit- neither study included measurements of oxygen consump- ies as a direct energetic reward. tion of inflorescences attached to the plants, and there were no respiratory measurements during the second