Changes in apparent photosynthetic quantum efficiency (Φi), carboxylation efficiency (CE), and chlorophyll fluorescence parameters of flag leaves of wheat grown in a phytotron during development were observed by a portable photosynthetic gas analysis system (CI 301) and a portable fluorescence measurement system (PAM 2000). The expansion of the flag leaves stopped about 8 days after their emergence. Φi in the flag leaves increased gradually at first and reached its maximum about 12 d after their emergence, then declined gradually. The trend of change in CE was very similar to that of Φi (Fig.1). There were no remarkable changes in the initial fluorescence level (Fo) and the photochemical efficiency of photosystem Ⅱ (PSⅡ) (Fv/Fm and ΔF/Fm’ for fully dark adapted and illuminated leaves, respectively) in the 20 days after emergence, but a trend of decline in the coefficient of non photochemical quenching of chlorophyll fluorescence (qN) was observed. About 20 d after emergence Fv/Fm and ΔF/Fm’ declined rapidly, while Fo and qN increased fast (Fig. 2). Moreover, the level of correlation was higher between Φi and CE than that between Φi and Fv/Fm or ΔF/Fm’ during the development of flag leaves (Fig.3). It appears that the quantum yield of photosynthetic carbon assimilation in flag leaves is not ultimately limited by the photochemical efficiency of PSⅡ. At the late stage of leaf development the excess of assimilation power originated from the decline in the activity of photosynthetic carbon assimilation due to the degradation of Rubisco causes the photodamage of PSⅡ, accelerating the senescence of leaves.
