Characteristics of secretion of nectar in Pyrostegia venusta (Ker‐Gawl.) Miers (Bignoniaceae)

summary Secretion patterns and characteristics of nectar of an Argentinian population of Pyrostegia venusta at different times of the day are described with respect to flower age. Flowers are protandrous and hummingbird‐pollinated, producing exudate before and after anthesis. There is a concomitant increase in both concentration and volume in 1‐d‐old flowers, followed by a sharp increase in concentration and decrease in nectar volume in 2‐d‐old flowers. Removal of nectar throughout the flower lifetime increases the volume produced but reduces the concentration. Flowers with nectar removed three times produced 73% more nectar volume than the controls, and those with nectar removed twice produced 20% more. However, the total mass of sugar produced was comparable in all three sets. Two experiments were designed to determine the effect of changes in volume and concentration on nectar production: (a) the concentration was initially maintained while the volume was changed (halved, doubled and untouched as a control), and (b) the concentration was reduced to a minimum replacing nectar by different volumes of distilled water as in the previous experiment. These concentration‐volume changes do not essentially affect total nectar production irrespective of treatment. A powerful homeostatic mechanism would operate by which the plant maintains both these variables in the nectar. The differences observed in the nectar production rate among treatments would indicate its existence.