Adaptation of Chloroplast-Ultrastructure and of Chlorophyll- Protein Levels to High-Light and Low-Light Growth Conditions*

In saturating light radish seedlings grown in high-light growth conditions (90 W • n r 2) possess a much higher photosynthetic capacity on a chlorophyll and leaf area basis than the low-light grown plants (10 W • m-2). The higher C 0 2-fixation rate of HL-plants is due to the presence of HL-chloroplasts which possess a different ultrastructure and also different levels of the individual chlorophyll-carotenoid-proteins than the LL-chloroplasts of LL-seedlings. 1. Ultrastructure: The high-light adapted chloroplasts are characterized by fewer photo­ synthetic membranes per chloroplast section, by low grana stacks (only few thylakoids per granum), a lower stacking degree of thylakoids, a higher proportion of non-appressed mem­ branes (stroma thylakoids + end grana membranes) and a high starch content. The LL-chloro­ plasts possess no starch, their grana stacks are higher (up to 17 thylakoids per granum) and also significantly broader than that of HL-chloroplasts. 2. Chlorophyll-proteins: The photosynthetic apparatus of HL-chloroplasts contains a larger proportion of chlorophyll a-proteins of photosystem I (CPIa + CPI) and of photosystem II (CPa, the presumable reaction center of PS II) than the LL-chloroplasts which possess a higher propor­ tion of light-harvesting chlorophyll a/fc-proteins (LHCP,, LHCP2, LHCP3, LHCPy). The higher levels of LHCPs in LL-plants are associated with a higher ground fluorescence fo and maximum fluorescence fp of the in vivo chlorophyll. 3. Chlorophyll and carotenoid ratios: The chloroplasts of HL-plants possess a higher proportion of chlorophyll a and /2-carotene (higher values for the ratios chlorophyll a /b and lower values for a/c and x /c ) which reflect the increased level of the chlorophyll a//?-carotene-proteins CPIa, CPI and CPa. The higher level of light-harvesting chlorophyll a/6-xanthophyll-proteins (LHCPs) in LL-plants is also indicated by an increased content of xanthophylls and chlorophyll b as seen from lower a/b and higher x /c and a/c ratios. 4. The results indicate that plants possess the capacity for an ontogenetic adaptation of their photosynthetic apparatus to the incident light intensity. The HL-chloroplasts of HL-plants which contain less antenna chlorophyll, are adapted for a more efficient photosynthetic quantum conversion at light saturation than the LL-chloroplasts with high grana stacks. The correlation between higher levels of light-harvesting chlorophyll ö/6-proteins (LHCPs) and a higher stacking degree of thylakoids, and the involvement of LHCPs in stacking is discussed.