Effects of Decapitation on Chlorophyll Metabolism, Endogenous Hormones, and Tillering Ability in Pinus yunnanensis Seedlings of Different Ages

<i>Pinus yunnanensis</i> is an essential tree species in southwest China. However, its genetic degeneration problem urgently needs to be addressed. Decapitation promotes seedling propagation primarily by disrupting apical dominance, triggering hormonal changes that stimulate lateral bud growth. To investigate the response of hormones and photosynthetic pigments in <i>P. yunnanensis</i> to decapitation at different seedling ages, seedlings aged 6, 10, 14, 18, and 30 months were used as materials to carry out unified decapitation treatment, and the dynamics of photosynthetic pigments, changes in endogenous hormones, and their relationship with tillering ability were analyzed. The results showed that the photosynthetic pigments were higher in younger decapitated seedlings than in older ones, and carotenoids showed an upward trend with time. Additionally, decapitation significantly altered the balance of endogenous hormones, including the contents of GAs, ABA, SA, JA, JA-Ile, and ACC. The GA₃ and ABA contents in the middle-aged decapitated seedlings of <i>P. yunnanensis</i> were higher. The seedlings with older decapitation ages showed higher contents of IAA, SA, and JA. Overall, seedlings with different decapitation ages exhibit significant differences in their responses to decapitation, as indicated by variations in photosynthetic pigments and hormones. This research elucidated the optimal decapitation age for <i>P. yunnanensis</i>, providing a theoretical foundation for establishing efficient decapitation nurseries and promoting near-natural propagation.