Photosynthesis in Trees

Chloroplasts have been isolated in high yield from several gymnosperms and from two deciduous trees. The organization of chlorophyll in the chloroplasts of these woody species is basically similar to that in angiosperm crop plants and green algae. The tree chloroplasts contain two chlorophyll proteins, the P700-chlorophyll a-protein and the major light-harvesting chlorophyll a/b-protein, the size, spectral characteristics, and function of which are the same as the equivalent complexes previously isolated from other classes of green plants. All the gymnosperms have chlorophyll/P700 ratios (photosynthetic unit sizes) 1.6 to 3.8 times larger than that typically found in crop plants; the deciduous trees have units of intermediary size. The presence of fewer but larger photosynthetic units in the woody species can partially account for their lower photosynthetic rate and explains why their photosynthetic processes saturate at lower light intensities. Chloroplasts of shade needles have large units containing a greater proportion of the light-harvesting chlorophyll a/b-protein than those of sun needles.