Chlorophyll content and the light response curve of shade-adapted conifer needles

Rates of photosynthesis were measured for shade-adapted conifer needles Picea abies (L.) Karst., Picea glauca (Moench) Voss, Picea mariana (Mill.) B. S. P., Picea sitchensis (Bong.) Carr., Pinus sylvestris L.] illuminated on i) their top surface, ii) their top and bottom surfaces simultaneously, or iii) uniformly in a light integrating sphere. The rate of bending of the light response curve of photosynthesis was strongly correlated with the amount of chlorophyll per unit of illuminated leaf area, when the amounts of chlorophyll were above 200 mg m⁻². Below 200 mg m⁻² only small differences in the rate of bending occurred and the light response curves took the form of a quasi-Blackman response. At chlorophyll contents below 200 mg m⁻² the rate of bending of the light response curves was insignificantly different from a mean published value for enzymatically isolated cells of angiosperm leaves (I. Terashima and T. Saeki. Ann. Bot. 56: 489–499, 1985). The apparent quantum yield of photosynthesis (based on incident light) also showed a response to chlorophyll content not different from the pattern reported for angiosperm leaves (E. K. Gabrielsen. Physiol. Plantarum 1: 5–37, 1948). Neither CO₂ nor O₂ concentration significantly affected the rate of bending of the light response curves. It is concluded that the light response curve of individual chlorenchyma cells is a quasi-Blackman response and that the rate of bending of the light response curve of intact leaves is a strong function of the development of gradients of light and thus the chlorophyll content of leaves.