Abstract: It was conducted with ‘Guoqing 4’Satsuma mandarin young tree whose rootstocks were Trifoliate, Sexual hybrid Troyer Citrange, Swingle Citrumelo and somatic hybrid Red tangerine + Trifoliate. Studing on the effects of rootstocks on the net photosynthesis rate ( Pn) through potted experiment in the green house. The results showed that: The diurnal variation of Pn in young tree with different rootstocks was seminar and presented bimodal carve, the first peak occurred at 10: 00, the second peak occurred after 13: 00 and the
value was lower than the first one. With the analysis of Pn, transp iration rate ( Tr) , water use efficiency(WUE) , apparent quantum yield (AQY) and carboxylation efficiency (CE) , the results were the young tree—Troyer Citrange had a weaker photosynthesis, Trifoliateps photosynthesiswas the best, red tangerin + Trifoliate and Swingle Citrumelo was between the above two; Trifoliate had a higher physiological value in soluble protein, specific leaf weight and photosynthetic pigment, the other three didnpt have remarkable difference. Under protected condition, Pn had obvious positive relationship with stomatal conductance (Gs) , Tr and relative humidity (RH) , Pn had obvious negative relationship with intercellular CO2 concentration (Ci) , there were comp lex relation between Pn and air temperature ( Tair) , photosynthesis availability radiation ( PAR) and vapor p ressure deficit at the leaf surface (Vpdl).

Key words: Rootstock, Citrus L., C. reticuiata Blanco, Photosynthesis