Abstract:

Under the ambient atmospheric CO₂(350±10)μL·L^-1,70%±5% relative humidity and PPFD of 400、800 and 1200μmol·m^-2s^-1,the temperature dependence of net photosynthetic rate (Pn),stomatal conductance (g_s),transpiration rate (E) and dark respiration (Rd) of 3 age classes of 18-year-old Chinese fir shoots were measured in 3 growing seasons of 1998 in Fenyi County of Jiangxi Province, one of the central distribution area of Chinese fir. With the increase of temperature the Pn, g_s and E showed sharp parabolic responses while R d responded exponentially. However the maximum g_s reached at much lower temperature than Pn and E. The optimum temperature of Pn was around 25℃ in fast-growing season (late May) and late season (middle October) while it was around 30℃ in middle season (late July) except for the current shoot in the late May with the optimum temperature between 22℃ and 23℃ because it was not fully developed then.The optimum temperature of Pn for mature shoots was more relevant to the average highest temperature of 5 days before measuring date. The impact of temperature on g_s may result from the changes of saturated vapour pressure deficiency (VPD) and the intercellular CO₂ concentration (Ci). The response of E to temperature may be related to the changes of g_s and VPD. The Rd of old shoot was lower than current shoot due to the lower physiological activity and poor light environment. However the responses of Pn,E and g_s to temperature discussed in this paper are instantaneous and the long-term responses and acclimation need to be studied further.

Key words: Net photosynthetic rate, Stomatal conductance, Transpiration rate, Dark respiration, Temperature, 18-year-old Chinese fir