Responses Of Photosynthesis And Photosynthate Allocation Of Chinese Fir Seedling To Soil Water And Nutrients

Changing the allocation of photosynthate to the various organs is a central mechanism by which plants cope with temporally or spatially varying environments. Hence, a primary objective of ecophysiological research is to understand when and how this process is affected by specific external conditions. In particular when the global climate has changed with elevated CO2, the carbon allocation and partitioning of forests became the focus of ecophysiologieal research because the trees constitute a major carbon reservoir of total plant carbon is found in forests-their ability to sequester carbon is a key determinant of future global change problems. To investigate the carbon allocation in artificial forests, the responses of photosynthesis and photosynthate allocation to soil water and nutrients in 2-year-old Chinese fir (Cunninghamia lanceolata) seedlings, the dominant species in artificial forests of south of China, were studied. The experiment was conducted in Subtropical Forestry Experimental center of The Chinese Academy of Forestry. About 240 potting 2-year-old Chinese fir (Cunninghamia lanceolatd} seedlings were grown in a glasshouse for one growing season and treated with 6 varies of moisture and nutrient availability. The quantity of water-controlled treatments is one third of normal watering treatments. The approach of N or P application is that the fertilizer are weighed and diluted then applied into the soil of pots at three times, which are the end of March, the middle of April and ten days before experiment commencing. The net photosynthetic rates (Pn) in response to photosynthetic photo flux density (PPFD) were measured by LI-6400 portable photosynthesis system in spring. The seedlings assimilate higher I3C02 than the seedling without labeling by I3C pulselabeling technique, an approach that the plants assimilate higher density 13CO2 forshort period. The current-needle,1-year-old needle, branch-and-stem, fine roots and coarse roots were sampled, dried, and grinded to powder at 1 day, 3 day, 7 day and 21 day after labeling. The samples were combusted and fined then were put into isotope ratio mass spectrometers to analyze the changes of the rules and the patterns of photosynthate allocation within the seedlings.Our studies showed that soil moisture can impacts photosynthetic rate of Chinese fir seedlings to some extent. For treatments without nutrient application, the maximum net photosynthetic rate (Pmax) and light saturation point (LSP) of the seedlings under water-controlled treatment are lower than those under normal watering treatment. The net photosynthetic rate of seedlings under water control declined sharply while those under normol watering treatment are still high when photosynthetic photo flux density (PPFD) is higher than 1500 mol.m's"1. For treatments having nutrient application, Pn, Pmax and LSP of the seedling under normal water conditions are higher than those of water-controlled treatments.N and P applications can increase net photosynthetic rate of the seedlings under the normal water conditions. Pn of the seedlings with N and P application are higher than those without nutrient applications when PPFD is over 600umol.m'2.s~'. Pmax and LSP of the seedlings with N or P applications are also higher than those without nutrient application. The effects of N and P on seedlings photosynthesis are different. Pmaxand LSP of the seedlings with P applications are 9.63% and 37.18% respectively higher than the seedlings with N application under normal water conditions. There are no significant differences between the effects of N and P on photosynthetic rates of seedlings under water-controlled conditions.The stable carbon isotope ratios ( 6 13C) are different among the 6 treatmentsand the 5 organs within each treatment. However the differences among12treatments are not significant, while they are significant among different organs. The 8 13C values of the organs under water-controlled conditions are higher than those seedlings under normal water conditions except for the branch an...