Determination of chlorophyll a fluorescence
Chlorophyll a fluorescence in the stems and leaves was measured by using a chlorophyll fluorescence imaging (CFI) system (Technologica, UK). Plants were adapted to the dark for 40-50 min, after which the stems of the third internode were detached and placed in the measurement chamber of the CFI system. First, the minimum fluorescence (F o) and the maximum fluorescence (F m) stimulated by a 6000 μmol m−2 s−1 saturating pulse were measured, and the ratio of variable fluorescence to maximum fluorescence (F v/F m, calculated as 1–F o/F m), which represents the maximum efficiency of photosystem II (Oxborough & Baker, 1997), was calculated. Light curves of chlorophyll a fluorescence were measured under the following PAR amounts: 50, 100, 200, 400, 600, 800, 1000 and 1200 μmol m−2 s−1. The stems were adapted for 90 s at each irradiance, after which the steady fluorescence (F ) and maximum fluorescence (F m′) in the light-adapted state were measured. The effective quantum yield of PSII (Φ PSII) was calculated as Φ PSII = ΔF /F m′ = (F m′ −F )/F m′ (Genty et al., 1989). In addition, the electron transport rate (ETR) was calculated asΦ PSII×PAR×0.85×0.5, where the coefficient 0.85 is the PAR absorptivity by plant photosynthetic tissues and where the coefficient 0.5 indicates that the absorbed PAR was equally allocated between PSI and PSII (Krall & Edwards, 1992).