CONTRIBUTION OF INTERCELLULAR REFLECTANCE TO PHOTOSYNTHESIS IN SHADE LEAVES

The potential contribution of intercellular light reflectance to photo synthesis was investigated by infiltrating shade leaves with mineral o il, Infiltration of leaves of Hydrophyllum canadense and Asarum canade nse with mineral oil decreased adaxial leaf reflectance but increased transmittance, As a result of the large increase in transmittance, inf iltration caused a decrease in absorptance of 25% and 30% at 550 and 7 50 nm, respectively, Thus, intercellular reflectance increased absorpt ance in these species by this amount, In a comparison of sun and shade leaves of Acer saccharum and Parthenocissus quinquefolia, oil infiltr ation decreased absorptance more in shade than in sun leaves, This dif ference suggests that the higher proportion of spongy mesophyll in sha de leaves may increase internal light scattering and thus absorptance, The importance of the spongy mesophyll in increasing internal reflect ance was also evident in comparisons of the optics of Populus leaves a nd in the fluorescence yield of oil-infiltrated leaves of several sun and shade species, Oil infiltration decreased the quantum yield of flu orescence (F-o) by 39-52% for shade leaves but only 21-25% for sun lea ves, We conclude that the greater proportion of spongy parenchyma in s hade leaves increased intercellular light scattering and thus absorpta nce, Direct measurements with fibre-optic light probes of the distribu tion of light inside leaves of Hydrophyllum canadense confirmed that o il infiltration decreased the amount of back-scattered light and that most of the light scattering for this species occurred from the middle of the palisade layer to the middle of the spongy mesophyll, We were not, however, able to assess the potential contribution of reflectance from the internal abaxial epidermis to total internal light scatterin g in these experiments, Using a mathematical model to compare the resp onse of net photosynthesis (O-2 flux) to incident irradiance for contr ol leaves of H. canadense and theoretical leaves with no intercellular reflectance, we calculated that intercellular reflectance caused a 1. 97-fold increase in photosynthesis at 20 mu mol m(-2) s(-1) (incident photon flux density), This enhancement of absorption and photosynthesi s by inter-cellular reflectance, without additional production and mai ntenance of photosynthetic pigments, may maintain shade leaves above t he photosynthetic light compensation point between sunflecks and maint ain the light induction state during protracted periods of low diffuse light.