The amount of pectin in the cell wall is considered a critical element regulating cell wall porosity. Thus, it is likely to influence the diffusional processes particularly that from the substomatal cavities to the carboxylation sites in the chloroplast stroma (i.e. mesophyll conductance, gm). We analyzed in four deciduous species (Acer campestre, Crateaugus monogyna, Corylus avellana, Quercus robur) the correlation between pectin and gm in two phase of leaves development (i.e. mature green

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... es and senescent leaves). Results showed that in A. campestre, C. monogyna and Q. robur to higher value of pectin in mature green leaves corresponded the higher gm value. Taking into account this result, we can assume that a higher amount of pectins is associated to an increased cell wall hydrophilicity and elasticity, thus increasing gm because CO2 molecules cross the wall dissolved in water. An opposite behavior was observed in C. avellana.