EFFECTS OF PLANT SIZE AND MORPHOLOGICAL CONSTRAINTS ON VARIATION IN REPRODUCTIVE COMPONENTS IN 2 RELATED SPECIES OF EPILOBIUM

1 This study explored the effects of plant size, position of fruits wi thin inflorescences, and position of seeds within fruits on the regula tion of fecundity in the two outcrossing perennial herbs Epilobium dod onaei and E. fleischeri which replace each other along an altitudinal gradient. 2 Almost all reproductive components were affected by size. The largest difference between small and large individuals of both spe cies concerned the number of mature seeds per stem, followed by the nu mber of fruits per stem, flower buds per stem, seeds per fruit, ovules per fruit and individual seed mass. 3 The increase in the number of s eeds per stem from small to large plants was not only due to the incre asing number of flower buds per stem and ovules per fruit, but also a result of a decreasing abortion rate of flower buds and seeds. 4 The f ruit/flower ratio was not affected by plant size, but the fruit/flower -bud ratio increased from 53% in small plants to 68% in large plants o f E. fleischeri and from 22 to 62% in E. dodonaei, respectively. Early abortion of flower buds determined the abortion pattern, abortions af ter flowering were rare. The pattern of abortion in both species was s trongly affected by the position of flower buds within the inflorescen ces. Buds from intermediate parts of the inflorescence had the smalles t abortion frequency. 5 Seed/ovule ratio was 50% in E. fleischeri and 40% in E. dodonaei. The fate of ovules was not affected significantly by the size of plants. However, positional effects of fruits within in florescences and of ovules within fruits on abortions were highly sign ificant. 6 The overwhelming part of the variation in individual seed m ass in both species was found within fruits and among fruits within a single inflorescence. Only in E. fleischeri was individual seed mass a ffected by the size of plants. 7 Our results suggest that abortion pat terns in reproductive components are strongly affected by changes in r esource availability during plant development, but also by morphologic al constraints. 8 Although the alpine E. fleischeri produced fewer fru its than E. dodonaei, a tradeoff between seed number and seed mass con tributed to a similar seed number per shoot in both species. We conclu de that the pattern and regulation capacity in reproductive components are tightly linked to specific life-history traits of the two species .