Influence of sex ratio and sexual lability on seed production in the dioecious perennial shrub Atriplex amnicola (Chenopodiaceae)

Abstract

Summary. Pollen supply can limit seed set in dioecious species such as Atriplex amnicola Paul G. Wilson (river saltbush). Clonal populations, with 4 different male : female ratios (0 : 1, 1 : 8, 1 : 1 and 5 : 1), were set up in widely spaced field plots at Tammin, Western Australia. After 1 season, plants were examined for fruit fill and fruit germination. The optimal male : female ratio for Atriplex amnicola seed production was between 1 : 8 (11% males) and 1 : 1 (50% males). These ratios resulted in mean fruit fills of 24 and 36%, respectively.

Genotype had a significant effect on fruit fill. The 3 clones tested had fruit fills of 40, 36 and 31%. These differences in fruit fill were as significant as those obtained from reducing the percentage of males from 50 to 11%.

A population of 700 seedlings of A. amnicola examined over 5 years was shown to be sexually labile, with a slight decrease over time in males and an increase in females and monoecious plants. Sexual lability appeared to be linked to plant aging rather than to environmental stimuli. It was concluded that plantations of river saltbush set up for seed production should be planted with selected genotypes at a male : female ratio of between 1 : 8 and 1 : 1 and that plantations need to be monitored for change in sex ratio with time, to ratios suboptimal for seed quality and production.