INFLUENCE OF SEX-RATIO AND SEXUAL LABILITY ON SEED PRODUCTION IN THE DIOECIOUS PERENNIAL SHRUB ATRIPLEX AMNICOLA (CHENOPODIACEAE)

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 w idely spaced field plots at Tammin, Western Australia. After I season, plants were examined for fruit fill and fruit germination. The optima l male:female ratio for Atriplex amnicola seed production was between 1:8 (11% males) and 1:1 (50% males). These ratios resulted in mean fru it fills of 24 and 36%, respectively. Genotype had a significant effec t 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 obtaine d from reducing the percentage of males from 50 to 11%. A population o f 700 seedlings of A. amnicola examined over 5 years was shown to be s exually labile, with a slight decrease over time in males and an incre ase in females and monoecious plants. Sexual lability appeared to be l inked to plant aging rather than to environmental stimuli. It was conc luded that plantations of river saltbush set up for seed production sh ould be planted with selected genotypes at a male:female ratio of betw een 1:8 and 1:1 and that plantations need to be monitored for change i n sex ratio with time, to ratios suboptimal for seed quality and produ ction.