Objectives We studied the effects of top pruning and P application rate on female cone density, branch tip development and nutrient uptake of Pinus massoniana, in order to improve seed yield and management level of Pinus massoniana.

Methods Two clones of P. massoniana with high-yielding (209) and low-yielding (225) were selected as test materials for a pot experiment. The saplings were divided into two groups without and with top pruning. Calcium magnesium phosphate fertilizer was used as P fertilizer and applied 0, 200, and 400 g/plant (denoted as P₀, P₂₀₀, and P₄₀₀), and ¹⁵N-urea 10 g/plant was basal applied with P fertilizer at the same time. At the florigenic base formation period, the branch samples of saplings were collected from the first whorl (bottom, H₁), second whorl (H₂) and top whorl (H₃) for the analysis of C, N, and P contents, and the nitrogen derived from fertilizer (Ndff) was calculated. At full blossom period, partitioning rates of N and P in various organs were analyzed.

Results Compared with not-pruning, top pruning increased the average branch length, branch diameter, and female cone density of the clone 209 by 11.06%, 40.14%, and 9.18% at H₁, and by 7.75%, 65.81%, and 65.27% on H₂; increased the branch length and thickness of clone 225 on H₁ by 21.94% and 28.99%, and on H₂ by 17.03% and 41.01%, but did not change the number of female cones obviously. Regardless of top pruning, P fertilizer did not cause significant differences in female cone density, branch length and branch thickness growth. For clone 209, top pruning increased the leaf C, N, P contents and Ndff by 25.05%, 9.15%, 4.12%, and 37.72% on H₁, and by 20.57%, 8.37%, 3.96%, and 17.09% on H₂; and for the clone 225, the increment were 2.51%, 19.87%, 8.84%, and 30.09% on H₁, and 20.82%, 12.40%, 4.39%, and 19.85% on H₂. Top pruning increased the leaf C∶N, N∶P, and C∶P ratios of both clones at H₁ and H₂, especially in clone 209 the C∶N on H₁ was increased by 7.91%. In the not pruning plot, P application significantly increased the C, N, and P content of needle leaves, with the highest increment under P₄₀₀ treatment. In the top pruning plot, P application merely increased the C and P content but did not affect N content significantly. Regardless of top pruning, both P₂₀₀ and P₄₀₀ treatments enhanced Ndff values of needle leaves, compared with P₀, indicating that P fertilization improved the uptake of fertilizer N. At flowering period, top pruning did not change the N and P allocation in the organs of clone 225, but stimulated the allocation to female cones in clone 209.

Conclusions Top pruning could stimulate the needle leaf C, N, P content, and the growth and development of lower branches in P. massoniana, thus promote the formation of female cones in lower branches and lower the fruit bearing layer of P. massoniana, the effect in high-yielding clone is more obvious than in low-yielding clone. Phosphorus fertilization could enhance the ability of nitrogen uptake by P. massoniana, especially from the nitrogen fertilizer.