Is acclimation required for success in high light environments? A case study using Mycelis muralis (L.) Dumort...

In the field significant differences in maximum photosynthetic O₂ -exchange rate (P_m ) were found between leaves of Mycelis muralis (L.) Dumort (Asteraceae) collected from woodland and exposed habitats, with the highest values in the exposed sites- However, there were no differences in the P_m of leaves collected from plants growing in grikes (fissures in the limestone pavement), of exposed limestone pavement, despite a greater than four-fold difference in the integrated daily irradiance. Leaves of plants from the open pavement had lower photon yields (ø₁ ) and higher dark respiration rates and light compensation points, in comparison to shaded plants. Under controlled environmental conditions the highest P_m of leaves from plants subjected to variations in irradiance were found at the intermediate (8-6 mol photon m^-2 d^-1 growth light level used. At the highest growth irradiance 17.3 mol photon m^-2 d^-1 used in the laboratory both P_m and ø_l were reduced, although the latest plant biomass was found at this irradiance. No changes were found in the chlorophyll a:b ratio over the same range of irradiances. Examination of plant populations of M. muralis, collected from open or shaded habitats and exposed to growth irradiances that covered the range over which increases in photosynthesis were, observed in the laboratory (0.86-8.6 mol photon m^-2 d^-1 ), resulted in changes in leaf structure and pigment composition. The chlorophyll a:b ratio was low and largely independent of irradiance or the origin of the plant population. Differences in total chlorophyll content were small with the lowest values m the Durrow woodland populations at both irradiances. No variations were found in a number of chloroplast thylakoid structural features. In particular, the ratio of oppressed to non-appressed membranes was unchanged by growth at the two irradiances, consistent with an invariant chlorophyll a:b ratio. Based on peaks in the difference spectra the woodland populations had mi enhanced in vivo absorption at λlD= 650 and 706 nm when grown at low irradiance. These peaks were absent from the population collected from the open limestone pavement. The significance of the enhanced absorption at low irradiance and the possibility that these peaks represent long-wavelength forms of chlorophyll a (λlD = 706) and b (λlD = 650) is discussed. A particular feature of plants grown at high irradiance was an enhanced anthocyanin content in comparison to those grown at low irradiance. This was associated with an increase in absorptance. particularly in the green region (λlD = 550 nm) of the visible spectrum. Overall these results suggest that complete acclimation of photosynthesis and an ability to modulate light-harvesting is not a prerequisite, for success in a high light environment.