Abstract
Robinia pseudoacacia L. has been widely planted worldwide for a variety of purposes, but it is a nonindigenous species currently invading the central part of Japanese river terraces. To understand and control this invasion, we investigated how this species invests nitrogen resources in different functions depending on the leaf location, and how these resources are used in physiological reactions such as photosynthesis. The Tama river terrace was examined in Tokyo, Japan. The leaf nitrogen (N) concentration, chlorophyll (Chl) concentration, Chl a/b ratio, leaf mass per unit area (LMA) and ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCo) concentration were all significantly lower in shade leaves than in leaves exposed to the sun. Conversely, the net photosynthetic rate in saturated light conditions (Pmax), the net photosynthetic rate under enhanced CO2 concentration and light saturation (Amax), the maximum carboxylation rate of RuBisCo (Vcmax) and the maximum rate of electron transport driving RUBP regeneration (Jmax) were all significantly lower in shade leaves than in leaves exposed to the sun. We also found that RuBisCo/N and Chl/N were significantly less in shade leaves, and values of Jmax/N, Vcmax/N less in shade leaves than in sun leaves, but not significantly. Allocation of nitrogen in leaves to photosynthetic proteins, RuBisCo (NR) was broadly less in shade leaves, and NL (light-harvesting complex: LHC, photosystem I and II: PSI and PSII) and NE (electron transport) were also lower. The N remaining was much greater in shade leaves than in sun leaves. We suggest that N remobilization from RuBisCo is more efficient than remobilization from proteins of NE, and from NL. This study shows that R. pseudoacacia has an enhanced ability to adapt to environmental changes via characteristic changes in N allocation trade-offs and physiological traits in its sun and shade leaves.
Highlights
Robinia pseudoacacia L. is a deciduous legume and is a native tree species in North America
The purpose of this study is to investigate the physiological responses of sun and shade leaves in R. pseudoacacia in the Tama river basin, depending on irradiance
This study focuses on the physiological traits of sun leaves and shade leaves in R. pseudoacacia, looking at leaf mass per unit area (LMA), leaf N concentration, photosynthetic parameters relating to RuBisCo, chlorophyll and N concentration, and N allocation patterns in the whole single tree
Summary
Robinia pseudoacacia L. (the black locust) is a deciduous legume and is a native tree species in North America. R. pseudoacacia is a nitrogen (N) fixing tree, which is able to fix N2 from the atmosphere with the help of N-fixing bacteria. In forest ecosystems it can increase the soil N concentration and N cycling by producing N-rich litter [3,4,5]. For these reasons, this species has been frequently planted in Japan in degraded areas, roadsides, seacoasts and riverbanks for greening since 1873, when Japanese encountered it at the 1873 Vienna International. It is used today as the main source of nectar, providing half of the domestic honey production in Japan
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