Photosynthetic responses of Ulva lactuca at very low light

Abstract

Changes in photosynthetic variables and dark respiration were measured in Ulva lactuca acchmated to continuous photon f l u densities between 0 and 55prnol n ~ ~ s ' at ?OC for up to 80d. Photosynthetic light harvesting efficiency (a ) was lughest in U. lactuca acclimated to intermedate light levels of 8.8 and 25 prnol m-2s-1 and was associated with maximum chlorophyll density and light absorptance. Below 2.5 pm01 m-'s-', energy capture was insufficient to maintan the maximum light harvesting apparatus and at 55 pm01 m-2s-i, light was no longer fully lirnihng to photosynthesis. The light harvesting efficiency (m) was linearly related to light absorptance (A). The quantum efficiency (0, = a ~ ' ) was, therefore, relatively constant and independent of the light acclimation level (0.045 to 0.059 m01 C [m01 photon absorbed]-'). The light compensation point for photosynthesis (I,) is given by the ratio between dark respiration (r) and light harvesting efficiency (a). Dark respiration rates increased with incident light level. As a result of combined changes in r and a, I, had minimum values (1.5 to 2.3 pm01 m-2s') in plants acclimated to 8.8 vmol m-2s' and it increased in the dark and particularly in the high light acclimated plants (5.3 to 7.6 pm01 m-2s' ) . Photosynthetic capacity (P,) was lmearly related to light harvesting efficiency (cc) , suggesting that photoadaptation mainly occurred by changes in number of photosynthetic units. U. lactuca growing in continuous low light possessed a photosynthetic capacity in excess of photosynthetic performance and would benefit from short-term exposure to high light. This may be of selective advantage, considering that natural light levels are widely fluctuating.