Regulation of Flower, Fruit and Seed Production by a Monocarpic Perennial, Yucca Whipplei

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Abstract
Flower, fruit and seed production of the monocarpic perennial, Y. whipplei, were measured in a population in southern California [USA]. The size of rosette, number of flowers reaching anthesis, number of fruits initiated, number of fruits aborted and number of fruits matured were determined for all plants flowering at 2 sites in 1979 and 1 site in 1980. Estimates of the average number of ovules per capsule and average seed weight were obtained for plants flowering in 1979. The number of flowers reaching anthesis was directly proportional to the size of the rosette. The number of fruits initiated on a plant was much less than, but proportional to, the number of flowers produced. Many fruits were aborted and abscised prematurely in both years. Mature fruit production was determined by the size of the rosette (or inflorescence) and the number of fruits initiated. Mature fruit set (mature fruits/flowers) averaged 9.1% in 1979 and 10.0% in 1980. The number of ovules per capsule did not vary with plant size but average seed weight was positively correlated with rosette size at 1 site, but not at the other. Y. whipplei regulates its maternal expenditure at the stages of flower determination and fruit maturation. Unpredictable pollinator availability within and between seasons appears to favor this pattern of reproduction. A major difference in rainfall between the 2 yr of the study had no significant effect on percentage mature fruit set. The plants may respond to changes in moisture availability by adjusting the size of the inflorescence rather than the proportion of flowers maturing to fruits.