Densely packed needles along the shoots of evergreen conifers exhibit shade-acclimated photosynthetic characteristics even under full sunlight

Background and Aims Evergreen conifer shoots that develop under full sunlight eventually move to the shade within the canopy as the tree grows. We hypothesized that densely packed needles develop shade characteristics even under full sunlight due to mutual shading, allowing them to survive longer in the canopy shade. Methods To test this hypothesis, we measured photosynthetic light responses at the shoot and needle levels in four species: Picea glehnii (shade-intolerant), P. jezoensis (intermediate shade-tolerant), Abies sachalinensis (shade-tolerant) and Taxus cuspidata (highly shade-tolerant), grown under open (100 % sunlight) and shade (10 % sunlight) conditions. Key Results In P. glehnii, P. jezoensis and A. sachalinensis, needle-level photosynthetic and morphological traits showed minimal differences between light conditions, indicating the development of shade-characteristic needles even under full sunlight. In contrast, open-grown needles of T. cuspidata exhibited sun-acclimated traits, including higher photosynthetic capacity and thicker mesophyll tissue than shade-grown counterparts. Shoot-level photosynthetic capacity was higher in more shade-intolerant species with denser needle packing, and in open-grown seedlings across species. Conclusions Shoot-level acclimation to light environments in P. glehnii, P. jezoensis and A. sachalinensis was governed primarily by needle configuration. In T. cuspidata, it was influenced by both needle-level photosynthesis and shoot morphology. The presence of shade-characteristic needles along with reduced needle packing from age-related stem thickening may contribute to the longevity of evergreen conifer shoots under canopy shading.