FLORAL COLOR-CHANGE IN ERRAZURIZIA-MEGACARPA, A SONORAN DESERT SHRUB

Flowers of many species undergo partial or complete color changes in t he course of their development. These changes are thought to cue polli nators to receptive flowers. In the Lower Sonoran Desert of northern M exico, individuals of the long-lived perennial shrub, Errazurizia mega carpa (S. Watts) I.M. Johnston 1924 (Leguminosae), produce many hundre ds of racemose inflorescences in which the terminal, younger flowers h ave yellow corollas, while lower, older flowers have red ones. I studi ed the response of visitor species to the floral display of E. megacar pa by direct observation of visitor activity and by manipulating the f loral display to remove yellow or red flowers. Equal-sized plants with similar sized floral displays containing only red flowers, only yello w flowers, and control plants with both red and yellow flowers, attrac t roughly equal numbers of putative pollinating insects. However, once pollinators have approached a plant, they discriminate against red fl owers, visiting these extremely rarely. Floral color change in this sp ecies provides a cue to direct pollinator activity to receptive and re warding flowers.