Abstract: In this paper,the responses of larch seedling's photosynthetic characteristics to N and P deficiency were studied in greenhouse from April to September, 2002.On April 10,one-year-old larch seedlings,after their roots washed and sterilized,were transplanted into a plastic pot (diameter 30 cm,height 27 cm) containing sterilized and washed quartz sand,and supplied with complete nutrient solution (4 mmol NH₄NO₃·L^-1,1 mmol KH₂PO₄·L^-1,1 mmol KCl·L^-1, 1 mmol CaCl₂·L^-1,0.6 mmol MgSO₄·L^-1,0.02 mmol FeCl₃·L^-1,6 μmol MnCl₂·L^-1,0.016 mmol H₃BO₃·L^-1,0.3 μmol ZnCl₂·L^-1,0.3 μmol CuCl₂·L^-1,and 0.3 μmol NaMoO₄·L^-1).On May 20,the seedlings were treated with four different combinations of N and P supply,i.e.,control (CK,8 mmol N·L^-1 and 1 mmol P·L^-1),low nitrogen supply (LN,1 mmol N·L^-1 and 1 mmol P·L^-1),low phosphorus supply (LP,8 mmol N·L^-1and 1/8 mmol P·L^-1),and low nitrogen with low phosphorus supply (LN-LP,1 mmol N·L^-1 and 1/8 mmol P·L^-1).In treatment LN,the other nutrients were given as CK,while in treatment LP,the K level was adjusted by KCl,with the other nutrients kept as CK.On September 4,the net photosynthetic rate,chlorophyll fluorescence parameters, chlorophyll content,total soluble protein content,and N and P contents of the needle were determined.The results showed that in treatment LN,the N content,chlorophyll content,net photosynthetic rate (Pn),maximal PSII efficiency (Fv/Fm) and potential PSII activity (Fv/Fo) of the needle declined significantly by 37%,31%,58%,22% and 57%,respectively, while in treatment LP,the N and P contents,chlorophyll content,total soluble protein content and Fv/Fm had no significant differences with CK,but the Pn and Fv/Fo was 13% lower than CK.The variation tendency and extent of Pn,chlorophyll fluorescence parameters,and other physiological indices in treatment LN-LP were similar to those in treatment LN.

Key words: Semi-desert algal crust, Species composition, Distribution