Fruit abortion and the regulation of fruit number in Yucca whipplei.

Flowering and fruiting patterns of Yucca whipplei (Agavaceae), a monocarpic perennial of the southern California chaparral, were studied to determine the role of fruit abortion in the regulation of seed yield. Data on reproductive expenditure, fruit abortion and fruit maturation were obtained for 108 flowering individuals at two sites. The proportion of flowers maturing fruits was less than 10% at both sites. Inflorescences produced an average of 2,045±111 and 1,743±77 flowers at the two sites, initiated an average of 439±48 and 304±21 fruits, and matured an average of 194±16 and 162±10 fruits, respectively. Overall, 51% of the fruits which initiated development were aborted and abscized in an early stage of development.The hypothesis that fruit abortion serves to regulate seed yeild, matching fruit production to the available resources, yields three predictions which are borne out by our findings: (1) the proportion of initiated fruits which abort is an increasing function of the number of fruits initiated, (2) the number of fruits matured is an increasing function of estimates of the amount of resources available to support fruits, and (3) on any inflorescence, fruits initiated early have a lower probability of aborting than fruits initiated late.These results suggest that low fruit set in Yucca whipplei is caused by shortage of resources to support developing fruits, rather than by a shortage of pollinators. Possible explanations of the adaptive significance of regulating maternal investment via fruit abortion are mentioned, but further experimentation is required before any conclusions can be drawn.