Seed morphological screening improves germination efficiency but poorly predicts selfing in Eucalyptus hybrids

Abstract

Breeding strategies in commercial Eucalyptus plantations have largely relied on crosses among hybrid genotypes, resulting in the recurrent selection of a limited number of superior clones. In this context, the development of inbred lines represents a complementary approach to broaden breeding schemes and improve genetic control. However, obtaining self-fertilized seeds in predominantly allogamous species is labor-intensive, technically challenging, and costly. In this study, we evaluated whether seed morphological screening, combined with seed collection from the central areas of monoclonal plantations, could improve germination efficiency, seedling uniformity, and the identification of self-fertilized individuals. Although widely adopted in breeding programs, this strategy does not guarantee the exclusive production of selfed seeds. Seeds from four commercial Eucalyptus urophylla × E. grandis clones were collected from the center of monoclonal plantations. The seeds were screened using granulometric sieves and classified according to size and uniformity, followed by weight-based separation using a seed blower. Germination rate was assessed two weeks after sowing, seedling height three months after germination, and tree height and circumference at breast height (CBH) 18 months after planting. Genetic parameters included selfing rate, observed heterozygosity, fixation index, and allelic profiles obtained from 16 SSR markers. Germination rates varied among genitors and morphological categories, with consistently higher values observed for larger and heavier seeds. Seed screening significantly increased the overall germination efficiency and promoted greater seedling uniformity. However, substantial variation in selfing rates was detected among genitors, and the morphological categories showed a limited ability to discriminate selfed individuals, except for one genitor. Seedling height was influenced by seed morphology, whereas associations between individual fixation index and growth were trait-, stage-, and progeny-dependent. Overall, seed morphological screening represents a practical and potentially valuable strategy to enhance seedling production from seeds, reduce operational costs and support breeding programs aimed at increasing genetic diversification in commercial Eucalyptus plantations.