Abstract: A two-year (2007-2008) pot experiment with cotton varieties Sumian 12 (salinity-sensitive) and Zhongmiansuo 44 (salinity-tolerance) was conducted at the Pailou experimental station of Nanjing Agricultural University to study the diurnal variation of the gas exchange and chlorophyll fluorescence parameters of cotton functional leaves under five levels (0, 0.35%, 0.60%, 0.85%, and 1.00%) of soil salinity. With the increase of soil salinity, the concentrations of Na⁺, Cl^-, and Mg²⁺ in functional leaves increased, whereas the concentrations of K⁺ and Ca²⁺ decreased. The salinity level <0.35% had little effects on the gas exchange and chlorophyll fluorescence parameters, but that >0.35% depressed the net photosynthetic rate (P_n) dramatically. At the salinity level >0.35%, the sensitivity of functional leaves to daytime photon flux density (PFD) and air temperature (T_a) enhanced, which in turn resulted in more severe photo- and temperature inhibition, and changed the diurnal variation patterns of P_n and stomatal conductance (G_s) from a one-peak curve to a constantly decreasing one. Along with the variations of daytime PED and T_a, the diurnal variation patterns of the maximum photochemical efficiency (F_v/F_m), quantum yield of electron transport (Φ_PSⅡ）), and photochemical quenching coefficient (q_P) of functional leaves presented a V-shaped curve, with the minimum value appeared at 12:00-13:00, while the non-photochemical quenching coefficient (q_N) showed a single-peak curve. Soil salinity decreased the F_v/F_m, Φ_PSⅡ）,and q_P significantly, but increased the q_N and enlarged its change trend. The comparatively low concentrations of Na⁺ and Cl^- and the relatively high concentrations of K⁺and Ca²⁺ in salt-tolerant Zhongmiansuo 44 functional leaves benefited the relative stability of PSⅡ, and the maintenance of a relatively high thermal dissipation capacity could be one of the reasons for a high level of P_n at high salinity level.

Key words: cotton (Gossypium hirsutum), soil salinity, functional leaf, gas exchange parameter, chlorophyll fluorescence parameter, diurnal variation, natural regeneration, Chinese fir pure plantation, mixed broadleaf-conifer forest, seed rain