It was shown previously that the convexity (curvature or rate of bending) of the photosynthetic light response curve was strongly correlated with chlorophyll content in shade acclimated conifer needles (Leverenz 1987, Physiol. Plant. 71: 20–29), in agreement with an hypothesis that gradients of light within leaves affect the convexity. In the present study it is shown that the convexity at any given chlorophyll content can be altered when leaves of Pinus sylvestris L. Picea glauca (Muench), Picea mariana (M.II.) BS.P. and Picea abies (L.) Karst pre-treated with less shade. This probably induced a differential acclimation of cells on the top and bottom sides of the leaves to their local light environment. Leaves were illuminated on i) their top surface, ii) their bottom surface, or iii) uniformly in a light integrating sphere during measurements of photosynthesis. After shoots had been transferred from their growth environment to a new measuring environment, the convexity increased from the first to the second day towards a maximum of 0.97. The rate of increase towards this maximum was 55 to 62% per day and probably is the result of re-acclimation of cells within the leaves. The data shown that the act of measuring photosynthesis induces a significant alteration in the experimental material when measurements are made for more than one day.
The convexity of the light response curve of photosynthesis, was independent of whether the steady state measurements were made beginning in the dark and sequentially increasing photon flux density or beginning at high light and sequentially lowering photon flux density. Neither variation of CO2 concentration from 35 to 200 Pa, nor of temperature from 5° to 32°C affected the convexity.