Among-shoot variability and leaf-specific absorptance characteristics affect diel estimates of in situ electron transport of Posidonia australis

Electron transport rate (ETR) calculations require values of irradiance, effective quantum yield of chlorophyll fluorescence (DF=F0 m), the distribution of energy between PSII and PSI and leaf-specific photosynthetic absorptance. We conducted virtually simultaneous replicated measurements of diel changes in DF=F0 m of Posidonia australis in situ, and determined leaf-specific photosynthetic absorptance in vivo using integrating spheres. Leaf-specific non-photosynthetic and photosynthetic absorptance, corrected for spectral reflectance, comprised 16 and 68% of total spectral absorptance (84%); about 7% of photosynthetically active radiation incident on the leaves was reflected. Virtually simultaneous quantum-yield measurements of eight short shoots provided an estimate of amongsample variability in DF=F0 m: generally the standard deviation was 10% of the mean except for a few hours around midday when the variability rose to 25% of the mean. Our results indicate that leaf-specific non-photosynthetic absorptance, spectral reflectance and shoot-to-shoot variability need to be taken into account when estimating ETR in the field. # 2004 Published by Elsevier B.V.