Effects Of N Form On Photosynthetic Electron Allocation And Xanthophyll Cycle Activity Of Cucumber And Rice

Nitrogen plays an important role throughout the whole life of all living organisms. Nitrate and ammonium are the major forms of nitrogen available to plants. Though both forms can be utilized, great differences in plant growth are found among various plants. Since 1950s many research efforts have been directed toward unraveling the causes and mechanisms of NH₄⁺ toxicity as well as the relationship of nitrate and photoprotection in plants, while present knowledge is far from complete. It is known that most of the energy for nitrate assimilation in a cell derives from photosynthesis. Recently, nitrate reduction has been suggested to be involved in the photoprotection mechanism. However, our present understanding of this role of nitrate is poor. This research focuses on the difference in allocation. The results are as follows:1) The effects of different nitrogen forms on biomass and gas exchange of cucumber and rice were studied. Ammonium significantly decreased both shoot biomass and root biomass of cucumber, especially root biomass. Photosynthetic rate (Pn) of cucumber under ammonium condition was also significantly lower than those under nitrate condition while stomatal conductance and intracellular CO₂ concentration were higher. Thus, decreased Pn could be attributed to stomatal limitation. In comparison, there were no significant differences in biomass and gas exchange between rice cultured under nitrate or ammonium condition.2) The effects of different nitrogen forms on photosynthetic electron allocation and nitrate reductase activity were investigated in cucumber and rice plants. When cultured with ammonium as nitrogen source, cucumber plants had lower Je(PSâ…¡), Je(PCR), Je(PCO), Ja(O₂-independent) and higher Ja(O₂-dependent) than the plants cultured with nitrate as nitrogen source. In comparison, Je(PSâ…¡), Je(PCR), Je(PCO)andJa(O₂-dependent) for plants with ammonium as N souces were not different from those with nitrate as N source while Ja(O₂-independent) in plants with ammonium as N souces was significantly lower than that with nitrate as N source in rice.The decreased Ja(O₂-independent) under ammonium condition in both cucumber and rice is mainly due to incaesed nitrate reduction. Both cucumber and rice have higher NR activity when they cultured with nitrate as nitrogen source.3) The effects of different nitrogen forms on chlorophyll fluorescence and xanthophyll cycle activity were studied in cucumber and rice plants. Compared with nitrate-feed plants, ammonium-feed plants had lower quantum yield of PSâ…¡electron transport (Φ_PSâ…¡), which was mainly due to decreasess in the photochemical quanching (qP). Simultaneity, ammonium increases xanthophyll cycle activity evidently. There was no significant difference in either chlorophyll fluorescence or xanthophyll cycle activity between nitrate and ammonium feed rice plants.