Cone-bearing effects on photosynthetic traits do not change with needle age in Pinus koraiensis trees

Needle experience large variations in photosynthetic rates in response to cone presence and needle aging. Exploring the interaction effect of these factors on photosynthetic traits is helpful to understand the source-sink relationship between needles and cones. Here, we examined the impact of needle age on needle CO2 assimilation rate (Pn), needle mass per area (LMA), and needle concentration of soluble sugars, starch, total non-structural carbohydrates ([NSC]), nitrogen ([N]) and phosphorus ([P]) in cone‐bearing branches (reproductive branches) and branches without cones (vegetative branches) of Pinus koraiensis. We also compared instantaneous photosynthetic N-use efficiency (PNUE) and P-use efficiency (PPUE) in reproductive and vegetative branches. Our results showed that needles in reproductive branches had lower Pn, [N], [P], PNUE and PPUE than the needles in vegetative branches, but higher [NSC] and LMA. The differences between branch types were similar across needle ages, despite a clear effect of aging on photosynthetic traits: Pn, [N], [P], PNUE and PPUE decreased with increasing needle age, whereas LMA increased and [NSC] did not change with needle age. Unexpectedly, these results suggest that Pn in needles of reproductive branches is limited by NSC accumulation, and that N and P needed for cone development come from adjacent needles/reproductive branches while NSC come from more distant organs. These results shed light on the source-sink transport of carbon, N and P between needles and cones and are helpful for eventually predicting and improving crop size in Pinus koraiensis.