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Bud Temperature in Relation to Nyctinastic Leaf Movement in an Andean Giant Rosette Plant

1974; Wiley; Volume: 6; Issue: 4 Linguagem: Inglês

10.2307/2989670

1744-7429

Alan P. Smith,

Plant Water Relations and Carbon Dynamics

Espeletia schultzii Wedd. (Compositae), a large caulescent rosette species of the paramo zone of the Venezuelan Andes, has strongly nyctinastic leaves. At night, the rosette leaves close around the apical bud, protecting the bud from freezing temperatures, and also preventing rapid heating of young leaves just after sunrise, when water transport to leaves is probably inefficient. If nyctinastic leaf movement is prevented, leaf wilting and death occur. Espeletia schultzii Wedd. (Compositae) is a common caulescent rosette species in the pairamo zone of the Venezuelan Andes. Its large pubescent leaves show distinct nyctinastic movements; they close around the single apical bud at night, and open during the day (figure 1). This species is highly polymorphic with respect to leaf movement; nyctinasty seems to be most pronounced in valley-floor populations subject to nighttime cold-air drainage. Similar growth forms, large rosettes supported by unbranched stems, have evolved independently in the high elevation zones of Hawaii (Argyroxiphium) and East Africa (Lobelia and Senecio). The East African giant rosettes show similar diurnal leaf movements. Hedberg (1964) suggested that night buds formed by nyctinastic leaves in the East African plants protect the apical meristem from freezing damage. However, it appears that detailed experimental studies of temperature relations have not been made previously in giant rosette plants of this kind. This paper reports such data in relation to nyctinastic opening and closing of the rosette. MATERIALS AND METHODS Field measurements were made at Mucubaji, Estado Merida, Venezuela (80 45' N.), at an elevation of 3600 m. Four sets of seven plants were located on a level valley floor. Plants were selected which showed particularly strong nyctinastic leaf movement. Average plant height to bud base was 25.2? 1.1 cm; average rosette diameter was 55.2?+1.5 cm. For each plant, one thermocouple was placed inside the apical btud (bud core) and another in the tissue of an outer bud leaf on the north-facing side of the bud (bud surface). The first set of seven plants was used as a control. In the second set of plants, rosette leaves were immobilized in the fully opened position (the position reached in mid-afternoon on a clear day) using stiff wire rings. Comparison of bud temperatures of these plants with those of control plants permitted study of the effects of rosette closure. In the third set of plants, rosette leaves were immobilized in the fully closed position (the position reached just before sunrise after a clear cold night), again using stiff wire rings. Comparison of temperatures of these plants with those of controls permitted study of the effects of rosette expansion during the day. In the fourth set of plants, the rosette leaves were removed, leaving only the apical bud. Comparison of these plant temperatures with those for set number two (rosettes held open) permitted study of the effects of non-nyctinastic leaves on bud temper-