Abstract:

Objective: This study monitored the photosynthetic characteristics of Populus tomentosa and the non-structural carbohydrates (NSC) of each organ at different stages after thinning and water and nitrogen addition, and elucidated the response of tree organ-scale NSC content to light, water, and nutrient resources under different treatments, aiming to provide a solid theoretical basis for the precise cultivation and efficient utilization of resources in the P. tomentosa plantations in the region. Method: The triploid P. tomentosa S86 plantation in the North China Plain was taken as the research object, and a two-factor completely randomized block design was adopted. Two thinning intensities were set: row thinning (50% thinning, T50) and row and tree thinning (75% thinning, T75). Three water and nitrogen treatments were implemented: water and nitrogen addition (WN), water addition (W), and no water and no nitrogen addition (CK). The photosynthetic characteristics of forest trees were monitored, and leaves, branches, trunks, and roots were sampled to determine their non-structural carbohydrate, thereby clarifying the response of photosynthetic characteristics and non-structural carbohydrates to thinning and water-nitrogen interactions of P. tomentosa at different stages, and further exploring the coupling relationship between non-structural carbohydrates and photosynthetic characteristics in different periods. Result: 1) From the dry season to the rainy season, the net photosynthetic rate and instantaneous water use efficiency of P. tomentosa leaves significantly decreased. Water and nitrogen treatment mainly regulated the net photosynthetic rate of the leaves, which was also affected by the thinning intensity. In the dry season, low-intensity thinning combined with irrigation plus fertilization, and irrigation was able to increase the net photosynthetic rate of P. tomentosa by 103.51% and 45.16%, respectively. In the rainy season, only high-intensity thinning followed by irrigation was able to increase the net photosynthetic rate by 30.31%. 2) The alternation of dry and rainy seasons led to an increase in the content of soluble sugars, starch, and NSC in leaves, branches, trunks, and roots. However, there were differences in the responses of different organs to thinning and water-nitrogen regulation. Irrigation and fertigation significantly promoted the increase in soluble sugar and NSC contents in leaves and branches during the dry season, but significantly decreased the soluble sugar and NSC contents in roots. 3) Soluble sugars during the growing season were the main NSC storage form in P. tomentosa. The content of soluble sugar and NSC in different organs changed in the same pattern, showing a trend of leaves>branches, roots>stems. The starch formed by the conversion of soluble sugars was mainly concentrated in the branches and roots. 4) There was a significant positive correlation between the soluble sugar content of leaves, branches, and trunks and NSC, while the NSC of roots was jointly affected by soluble sugars and starch. In terms of different organs, there was a high correlation between NSC and its component contents in leaves and roots. 5) In the dry season, transpiration rate was the key factor explaining the variation in NSC content in various organs, with a contribution rate as high as 47.6%. The NSC content of leaves and roots in the dry season was positively regulated by photosynthetic rate, transpiration rate, and stomatal conductance, while the NSC content of leaves and roots in the rainy season was negatively regulated by leaf photosynthetic characteristics. Conclusion: From the dry season to the rainy season, the net photosynthetic rate and instantaneous water use efficiency of P. tomentosa leaves decrease. The regulation of net photosynthetic rate by water and nitrogen treatment is affected by thinning intensity. When the dry and rainy seasons alternate, NSC content of each organ increases, but there are differences in the response of different organs to thinning and water-nitrogen regulation.

Key words: Populus tomentosa, thinning effect, water and nitrogen regulation, photosynthetic properties, non-structural carbohydrates