Dynamics of heavy metals during the development and decomposition of leaves of Avicennia marina and Kandelia obovata in a subtropical mangrove swamp.

In mangrove wetlands, leaves make up a high proportion of the plant biomass and can accumulate heavy metals from contaminated sediment. Despite this, it is still unclear how heavy metal concentrations in leaves change as they develop and how metals in senescence leaves are recycled back into the mangrove ecosystems during decomposition. The present study aims to investigate the dynamics of six heavy metals (Cu, Zn, Cr, Ni, Cd, and Pb) in leaves of two common mangrove plants, Avicennia marina and Kandelia obovata, at different stages of development (young, mature, and senescent) and leaf litter decomposition (from 0 to 20 weeks). Based on litterbag experiments in a subtropical mangrove swamp, both plant species showed similar trends in alternations of the six heavy metals during leaf development, that was, decreased in Cu and Zn but increased in Pb, while Cr, Ni, and Cd remained steady. All heavy metals in litter gradually increased in concentration during decomposition. By the end of the 20-weeks decomposition, the concentrations of Cu, Zn, and Cd in decayed leaves were comparable to those in sediment, with Cu, Zn, and Cd at approximately 18, 75, and 0.2 mg·kg-1, respectively, while Cr (66 mg·kg-1), Ni (65 mg·kg-1), and Pb (55 mg·kg-1) were lower than those in sediment, indicating that metals were not retained in litter but recycled back to the sediment. Tannins in mangrove leaf litter might chelate heavy metals, affecting their migration and transformation of heavy metals in estuarine mangrove wetlands. The findings of our study provide insight into the interactions between toxic heavy metals and mangrove plant species during leaf development, representing the first example of how most metals would be retained in leaf litter during decomposition, thereby reducing their release to estuarine and marine ecosystems.

Elevated CO2 Enhances Dynamic Photosynthesis in Rice and Wheat.

Crops developed under elevated carbon dioxide (eCO2) exhibit enhanced leaf photosynthesis under steady states. However, little is known about the effect of eCO2 on dynamic photosynthesis and the relative contribution of the short-term (substrate) and long-term (acclimation) effects of eCO2. We grew an Oryza sativa japonica cultivar and a Triticum aestivum cultivar under 400 µmol CO2 mol-1 air (ambient, A) and 600 µmol CO2 mol-1 air (elevated, E). Regardless of growth [CO2], the photosynthetic responses to the sudden increase and decrease in light intensity were characterized under 400 (a) or 600 µmol CO2 mol-1 air (e). The Aa, Ae, Ea, and Ee treatments were employed to quantify the acclimation effect (Ae vs. Ee and Aa vs. Ea) and substrate effect (Aa vs. Ae and Ea vs. Ee). In comparison with the Aa treatment, both the steady-state photosynthetic rate (P N) and induction state (IS) were higher under the Ae and Ee treatments but lower under the Ea treatment in both species. However, IS reached at the 60 sec after the increase in light intensity, the time required for photosynthetic induction, and induction efficiency under Ae and Ee treatment did not differ significantly from those under Aa treatment. The substrate effect increased the accumulative carbon gain (ACG) during photosynthetic induction by 45.5% in rice and by 39.3% in wheat, whereas the acclimation effect decreased the ACG by 18.3% in rice but increased it by 7.5% in wheat. Thus, eCO2, either during growth or at measurement, enhances the dynamic photosynthetic carbon gain in both crop species. This indicates that photosynthetic carbon loss due to an induction limitation may be reduced in the future, under a high-CO2 world.