Abstract: A pot experiment was conducted to study the effects of water stress on the photosynthetic characteristics, biomass accumulation, and principal medicinal compositions of Tribulus terrestri. Water stress changed the diurnal variation of T. terrestri photosynthesis, decreased the net photosynthetic rate and transpiration rate significantly, weakened the plant photosynthetic and assimilative capability, and accordingly, hindered the dry matter accumulation and decreased the biomass. However, water stress promoted the accumulation of gross saponins in T. terrestri. In the treatments with irrigation amounts 150 and 450 mm, the gross saponins content was 13.26 and 12.47 mg·g^-1, respectively, being significantly different from that in the treatments with irrigation amounts 250 and 350 mm. The dynamic accumulation of gross flavonoids and gross saponins had an opposite tendency with that of biomass, namely, the contents of these principal medicinal compositions were the lowest when the biomass was the maximal. Correlation analysis showed that the biomass accumulation of T. terrestri was significantly correlated with outside environmental factors, while the contents of secondary metabolism products were affected not only by the outside environmental factors, but also by the inside factors such as biomass accumulation.

Key words: Tribulus terrestri, photosynthetic characteristic, biomass accumulation, flavonoid, saponin, secondary metabolism, alpine timberline ecotone, decomposition rate, litter, initial quality, lignin/N.