Carbon Reduction and Photosystem II Deficiency in Leaves of C₄ Plants

Abstract

The Hill reaction and photosystem I activity of chloroplasts isolated from mesophyll and bundle sheath cells of Atviplex spongiosa and Sorghum bicolov was measured using narrow-band, high-intensity illumination of 646, 712, and 730 nm. The photosystem I reactions (methylviologen Mehler reaction and diphenylcarbazone reduction) were equally active in 646 and 712 nm light, whereas Hill activity was reduced by 70% in 712 nm light relative to 646 nm.

Intact leaves were illuminated with 646 nm light and exposed to a pulse of ¹⁴CO₂. The pulse was followed by a chase in 646,712, or 730 nm light or in darkness. In A. spongiosa leaves the movement of carbon from C₄ acids to carbohydrates during the chase period only occurred to a significant extent in 646 nm light. In S. bicolor leaves, 712 nm light was almost as effective as 646 nm light in inducing the movement of carbon from C₄ acids to carbohydrate. Evidently in this species carbon reduction is not entirely dependent on concurrent photosystem I + II activity for the provision of reducing potential. The data are discussed in relation to the deficiency of photosystem II activity in bundle sheath cells of S. bicolor.