Abstract
Soil nitrogen (N) deficiencies can affect the photosynthetic N-use efficiency (PNUE), mesophyll conductance (gm), and leaf N allocation. However, lack of information about how these physiological characteristics in N-fixing trees could be affected by soil N deficiency and the difference between N-fixing and non-N-fixing trees. In this study, we chose seedlings of two N-fixing (Dalbergia odorifera and Erythrophleum fordii) and two non-N-fixing trees (Castanopsis hystrix and Betula alnoides) as study objects, and we conducted a pot experiment with three levels of soil N treatments (high nitrogen, set as Control; medium nitrogen, MN; and low nitrogen, LN). Our results showed that soil N deficiency significantly decreased the leaf N concentration and photosynthesis ability of the two non-N-fixing trees, but it had less influence on two N-fixing trees. The LN treatment had lower gm in D. odorifera and lower leaf N allocated to Rubisco (PR), leaf N allocated to bioenergetics (PB), and gm in B. alnoides, eventually resulting in low PNUE values. Our findings suggested that the D. odorifera and E. fordii seedlings could grow well in N-deficient soil, and adding N may increase the growth rates of B. alnoides and C. hystrix seedlings and promote the growth of artificial forests.
Highlights
Nitrogen (N) is one of the most important biological elements for plants because it is a component of amino acids, proteins, genetic materials, pigments, and other key organic molecules[1,2,3]
We investigated the photosynthetic N-use efficiency (PNUE), photosynthesis, leaf N allocation and mesophyll conductance to CO2 in D. odorifera, E. fordii, B. alnoides and C. hystrix seedling leaves that were exposed to different soil N treatments
The seedling leaf Narea and N content per mass (Nmass) values were significant higher in D. odorifera and E. Fordii than they were in C. hystrix and B. alnoides under all the soil N treatments, and the PNUE was significantly lower in D. odorifera and E. fordii than it was in C. hystrix and B. alnoides (Fig. 1)
Summary
Nitrogen (N) is one of the most important biological elements for plants because it is a component of amino acids, proteins, genetic materials, pigments, and other key organic molecules[1,2,3]. Mesophyll conductance to CO2 and N allocation in the photosynthetic apparatus of a leaf cell are important factors that explain the differences in the PNUE9,10. N is involved in carbonic anhydrases and aquaporins[22], with carbonic anhydrases accounting for 0.5–2% of the total soluble leaf protein[23] These proteins play a role in mesophyll conductance (gm) by changing the nature of the diffusing molecule[24] and facilitating CO2 diffusion through membranes[25]. Soil N deficiency could affect the leaf N content, photosynthesis, PNUE, gm, and leaf N allocation in many species. Adding N to the soil could improve the leaf N content in the Rubisco, bioenergetics, and light-harvesting components[7,33,34,35], but the changes in the proportion of N in these components were unclear[1,11]
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