Résumé
Low light is a characteristic of coffee agroforestry systems, yet the optimal shade level for establishing such systems remains unclear. Furthermore, the effects of reduced light on the growth and physiology of juvenile Coffea arabica F1 hybrids have not been fully explored. This study examined gene-by-environment interactions among coffee hybrids and their parental lines across a controlled shade gradient in a greenhouse. The tested C. arabica included landrace accessions (i.e. 'ET531', 'MS', 'T4905', and 'Rume Sudan'), pure lines (i.e. 'Caturra','T5296', 'Marsellesa' and 'IAPAR59'), and F1 hybrids (i.e. 'H1', 'H3', 'Mariana', and 'Starmaya'). Plants were grown under five shade treatments intercepting 0, 35, 58, 73, and 88% of ambient light (ranging from ~10 to 800 μmol m−2 s−1) to simulate agroforestry light environments. Shade significantly influenced all measured traits, including photosynthetic parameters, chlorophyll a fluorescence, aboveground biomass, specific leaf area, total leaf area, growth rate and leaf count. Notably, the line varieties 'Caturra' and 'Marsellesa' and hybrid 'Starmaya' demonstrated comparatively enhanced vegetative performance when grown under shade (i.e, shade level of 35 – 88%), particularly in leaf count (ca. 75 % more), plant height (ca. 75 % taller), and stem dry mass (50 % greater). Wild Ethiopian accessions 'ET531' and 'Rume Sudan' exhibited distinctive plant height responses, reflecting their unique genetic adaptation patterns to shade, with plant height and apical dominance increasing, as a photomorphogenetic response to avoid the shade. These findings offer preliminary insights to inform the selection of C. arabica genotypes suitable for coffee agroforestry systems based on juvenile plants.