RELATIONSHIP BETWEEN PHOTOSYNTHESIS AND THICKNESS OF COTYLEDONS FOR TROPICAL TREE SPECIES

1. Maximum photosynthesis and dark respiration by epigeal cotyledons of various thickness (0.2-5.0 mm at the centre) for 10 tropical tree species were measured with an oxygen electrode. 2. The balance of photosynthesis and respiration in cotyledons of each species was closely related to cotyledon thickness of the species. Per unit dry mass, both gross and net photosynthetic rates were strongly correlated with the inverse of cotyledon thickness (1/cotyledon thickness). Thin cotyledons (e.g. 0.2 mm) had very high rates of gross photosynthesis (e.g. 0.6 mumol O2 g-1 s-1), while cotyledons thicker than 1 mm had photosynthetic rates just high enough to balance respiration and achieve a small but positive net photosynthesis. 3. There was no significant correlation between dark respiration rate per unit mass and cotyledon thickness, indicating similar respiration rates for storage cells and photosynthetic cells. 4. Per unit area, gross photosynthetic rates were high (e.g. 16 mumol O2 m-2 s-1) and did not differ significantly among species with various cotyledon thicknesses, indicating a similar total amount of photosynthetic machinery per unit area. 5. Stomatal frequencies on the abaxial surface of cotyledons were positively and linearly correlated with maximum photosynthetic rates and dark respiration rates on a dry mass basis, but no such correlation existed on an area basis. 6. Unlike true leaves, there was no relationship between nitrogen content and photosynthetic rate for cotyledons of the 10 species. 7. The study demonstrated higher photosynthetic capacities in both thin and thick cotyledons than previously reported. Photosynthesis by thick epigeal cotyledons can supply all the energy required for non-photosynthetic function, e.g. modification and export of stored reserves.