Photosynthesis and dry matter production in Tibet of China, and the effect of CO_2 enrichment for them

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Climatic condition of Tibet plateau (3500-4000 m in sea level) holds a unique position for agriculture. Contents of CO_2 and O_2 are 60 to 70 % relative to flats of 0 m sea level, the solar radiation in summer is very high usually beyond 2000 μ mol m^<-2> sec^<-1> and the temperature difference between day and night is often beyond 20℃. The aim of the present research was to clarify these particular conditions for the photosynthesis and dry matter production of crops cultivated in this area in the present and the future. The experiment was conducted at the experimental farm in Lhasa (3700 m in sea level), located by Chinese Academy of Science, with open top chambers of enriched (1.55 times higher) and control (normal air) CO2 content growing a local variety of spring wheat. Mean net photosynthetic rate during the growing season was increased by CO2 enrichment, 19.5μ mol m^<-2> sec^<-1> in the control chamber and 26.9μ mol m^<-2> sec^<-1> in the enriched chamber. However, the total dry weight at the heading stage and the grain yield at the harvesting stage were almost the same in the two chambers, 1401〜1472 g m^<-2> and 610〜702 g m^<-2>, respectively. It was mainly due to higher rate of blighted tillers before heading stage in the enriched CO_2 plants relative to the control plants. The results indicated that main limiting factor for grain yield in this region was not low CO_2 content, but low soil nutrient condition.