Phenological pattern, breeding behaviour, and estimation of outcrossing rate in Grevillea robusta (Proteaceae)

Grevillea robusta (Silver Oak) is an evergreen tree valued for its eco-protection and commercial attributes in its native region. The species was introduced in Africa and the Indian subcontinent to reap similar benefits. In the present study, tree stands of G. robusta were selected and grouped into three DBH classes, to record phenological events and assess the breeding system. Further, simple sequence repeats (SSRs) markers were used to investigate the reproductive attributes encompassing outcrossing-rate, fixation index, paternity correlation, and correlation of selfing among families. The study demonstrated that flowering in the species occurs with moderate asynchrony at the population level. The reproductive phase at the individual tree level was completed in a shorter duration (similar to 65 days) than that at the population level (similar to 90 days). Flowers exhibited protandry. Experimental pollinations revealed significantly higher seed-set through outcrossing (similar to 52%) than either through natural (similar to 35%) or self-pollination (similar to 20%); and there was similar to 60% inbreeding depression. A greater amount of seed-set through outcrossing indicates that the tree species probably suffers from pollen/pollination limitation or inbreeding depression in nature. The outcrossing rate through open-pollination was considerably high (t(m) = 0.96), which clearly demonstrated that in G. robusta trees a significant amount of seed formation occurs through outcrossing. High outcrossing may facilitate greater amounts of genetic variation, local adaptations and speciation. Although partial selfing is possible in the species, dichogamy and inbreeding depression (similar to 60%) prevent autogamy, which may happen through geitonogamy. Overall, the phenological analysis and molecular marker-based assessment of the mating-system opens new avenues in ascertaining the performance of exotic tree species selected for plantation forestry in the non-native regions. Speciation, geographical variation, and tree spacing in the multilocation trial could be easily succeeded with the assessment of reproductive behaviour of trees.