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The altitude-related responses to the increased application of CO2, N, and their combination were investigated in two Abies faxoniana populations, which originated from a subalpine coniferous forest at elevations of 2,580 and 3,200 m using closed-top chambers. The two contrasting populations were subjected to two CO2 regimes (350 and 700 lmol mol-1) and two N levels (0 and 5 g N m-2 year-1). Their net photosynthetic rate, nonstructural carbohydrate concentration, and photosynthetic N use efficiency (PNUE) increased under elevated CO2. However, the increases detected in the high-elevation (HE) population were significantly greater than those found in the low-elevation (LE) population. Under elevated CO2 and N application, the maximal carboxylation rate (Vcmax) increased in HE population, whereas no effects were found on Vcmax in LE population. The C to N ratio decreased under N application in both populations. N application also induced the HE population to show greater increases in free amino acids, soluble proteins, N concentration, and PNUE than LE population. These results suggested that the population from HE was more sensitive to elevated CO2 and (or) N application than LE population. Results of this study provided valuable knowledge for predicting forest development under increased atmospheric CO2 concentration and (or) N deposition.
To ultimately determine whether different levels of soil nitrogen (N) deposition can modify the detrimental effects of cadmium (Cd), the seedlings of Toxicodendron vernicifluum (Strokes) F. A. Barkley were exposed to soil Cd stress (0, 5 and 15 mg kg-1 dry soil), N deposition (0, 13 and 40 mg kg-1 dry soil) and their combinations. Soil Cd stress caused damage in plant growth, photosynthesis and other physiological indexes, and in the ultrastructure of mesophyll cells. The effects of N deposition on growth, lipid peroxidation and enzyme activities depended on the relative amounts of N supplied. The combination of low N deposition and Cd stress was positive to plant growth, photosynthesis and enzyme activities, and it caused lower levels of Cd accumulation and lipid peroxidation compared with the effect of Cd stress alone. The combination of high N deposition and Cd stress led to a higher Cd accumulation and lipid peroxidation, and to lower enzyme activities, as compared with the effect of Cd stress alone. T. vernicifluum was found to be sensitive to soil Cd stress. Soil Cd had detrimental effects on T. vernicifluum seedlings, but the tolerance of T. vernicifluum to Cd increased under low N deposition.
We investigated the effects of leaf color change in the fall on photosynthetic production and nitrogen resorption. Seedlings of Acer platanoides L. and A. saccharum Marsh. were grown in a shade house for 5 months in either 21 % (intermediate light, M) or 4.9 % (low light, L) of incident irradiance. After this period, a subset of the intermediate-light grown seedlings was transferred to a high-light stress treatment (H). Gas exchange, chlorophyll fluorescence, pigments, antioxidant activity, and nitrogen (N) resorption were examined at three leaf senescence stages during September and October. Our results show that plants of both species produce more anthocyanins in the H treatment. In comparison with plants grown in the L and M treatments, plants of both species in the H treatments had lower chlorophyll, carotenoid and chlorophyll fluorescence parameters (Fv/Fm, ΦPSII, NPQ and ETR) at the third sampling date (October 12–18), and indicating higher levels of photoinhibition in the seedlings exposed to high light. Our results imply that autumn leaf redness is inducible and closely linked to photo-oxidative stress. However, anthocyanins did not enhance antioxidant capacity in red leaves in either species, when exposed to high light. For both species, our results showed a higher N-resorption for high-light stressed plants. We also observed that the number of abscised leaves at the second sampling dates (September 10) was higher than at the third sampling dates. The intra-leaf distribution of anthocyanin, the association between anthocyanin production and the high-light environments, the retention of red leaves, the substantial physiological gain of photosynthetic activity, as well as the links between anthocyanins and increased N resorption led us to assume that one primary role of autumn anthocyanin could be to protect the photosynthetic apparatus from photo-oxidative damage as light filters rather than as antioxidant. Another major role is to extend carbon capture and help supply the energy needed for N resorption from senescing leaves in both A. saccharum and A. Platanoides during highlight stress. Nevertheless, photoprotective capacity of anthocyanins was not able to fully compensate for photoinhibitory stress as the anthocyanins are not optimally located to efficiently reduce light within the leaves.
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