Compartmentation in Vicia faba Leaves. III. Photosynthesis in the Spongy and Palisade Parenchyma

Abstract

Relative rates of photosynthesis in the palisade and spongy parenchyma were found to be dependent upon the intensity of incident photosynthetically active radiation. The rate of photosynthesis in the spongy parenchyma relative to the rate in the palisade parenchyma increased with increasing light intensity. We attribute this behaviour to shading of the spongy parenchyma by palisade parenchyma.

The kinetics of photosynthetic carbon metabolism were qualitatively similar in the two tissues. Immediately after a 6-s ¹⁴CO₂ pulse, phosphoglyceric acid plus sugar phosphates represented about 85 % of the ¹⁴C in the water-soluble compounds in both the palisade and spongy parenchyma, and were the only compounds in which radioactivity declined. Sucrose was the main end-product of photosynthesis and represented 30% of the water-soluble radioactivity after 104 s in both these tissues. Other compounds (phosphoglycollic acid plus phosphoenolpyruvic acid, malic acid plus glyceric acid, and alanine) represented smaller, similar amounts of radioactivity in each tissue. A higher proportion of activity due to ¹⁴C-labelled (glycine plus serine) in the water-soluble fraction of the spongy parenchyma may reflect a difference in the proportion of photosynthate being respired there.