Contrasting Effects of Carbon Dioxide and Irradiance on the Acclimation of Photosynthesis in Developing Soybean Leaves

Leaves developed at high irradiance (I) often have higher photosynthetic capacity than those developed at low I, while leaves developed at elevated CO₂ concentration CO₂] often have reduced photosynthetic capacity compared with leaves developed at lower CO₂]. Because both high I and elevated CO₂] stimulate photosynthesis of developing leaves, their contrasting effects on photosynthetic capacity at maturity suggest that the extra photosynthate may be utilized differently depending on whether I or CO₂] stimulates photosynthesis. These experiments were designed to test whether relationships between photosynthetic income and the net accumulation of soluble protein in developing leaves, or relationships between soluble protein and photosynthetic capacity at full expansion differed depending on whether I or CO₂] was varied during leaf development. Soybean plants were grown initially with a photosynthetic photon flux density (PPFD) of 950 µmol m^–2 s^–1 and 350 µmol CO₂] mol^–1, then exposed to CO₂] ranging from 135 to 1400 µmol mol^–1 for the last 3 d of expansion of third trifoliolate leaves. These results were compared with experiments in which I was varied at a constant CO₂] of 350 µmol mol^–1 over the same developmental period. Increases in area and dry mass over the 3 d were determined along with daily photosynthesis and respiration. Photosynthetic CO₂ exchange characteristics and soluble protein content of leaves were determined at the end of the treatment periods. The increase in leaflet mass was about 28 % of the dry mass income from photosynthesis minus respiration, regardless of whether CO₂] or I was varied, except that very low I or CO₂] increased this percentage. Leaflet soluble protein per unit of area at full expansion had the same positive linear relationship to photosynthetic income whether CO₂] or I was varied. For variation in I, photosynthetic capacity varied directly with soluble protein per unit area. This was not the case for variation in CO₂]. Increasing CO₂] reduced photosynthetic capacity per unit of soluble protein by up to a factor of 2.5, and photosynthetic capacity exhibited an optimum with respect to growth CO₂]. Thus CO₂ did not alter the relationship between photosynthetic income and the utilization of photosynthate in the net accumulation of soluble protein, but did alter the relationship between soluble protein content and photosynthetic characteristics in this species.